EPA R2-72-039
August 1972 Environmental Protection Technology Series
The Appearance and Visibility
of Thin Oil Films on Water
National Environmental Research Center
Office of Research and Monitoring
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series describes
research performed to develop and demonstrate
instrumentation, equipment and methodology to
repair or prevent environmental degradation from
point and nonpoint sources of pollution. This work
provides the new or improved technology required
for the control and treatment of pollution sources
to meet environmental quality standards.
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EPA-R2-72-039
August 1972
THE APPEARANCE AND VISIBILITY OF
THIN OIL FILMS ON WATER
Bernard Horstein
Edison Water Quality Research Laboratory
Edison, New Jersey 08817
Program Element 1B2041
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO ^5268
a U. S. GOVERNMENT PRINTING OFFICE : 1972 O - 477-049
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.r. 204D2 - Price $2.50
Stock Number 5501-00420
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ABSTRACT
When oil films of controlled thickness (up to 3000 nanometers)
were formed upon water surfaces in the laboratory, an inherent
and orderly thickness-appearance relationship was confirmed —
a relationship that is independent of oil type and water type.
The laboratory studies also investigated the effects of viewing
conditions upon the ase with which these thin films were visible.
Out-of-doors observations were made; these and the observations
reported by other sources corresponded with the laboratory re-
sults. The visibility of a thin oil film depends not only upon
its thickness-dependent inherent appearance, but also upon con-
ditions external to the film: the nature of illumination and sky
conditions, sun angle, color and depth of water, color of bottom,
and viewing angle.
Color photographs illustrate the points discussed.
iii
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CONTENTS
Section Page
1. CONCLUSIONS 1
2. INTRODUCTION 3
2.1 Reasons for this Investigation 3
2.2 Background and Approach 6
3. THEORY 11
3.1 Applicable Phenomena 11
3.2 Reflectivity 11
3.3 Interference 13
3.4 Fluorescence not Significant 19
4. EXPERIMENTAL 21
4.1 General 21
4.2 Physical Procedure 22
4.3 Photographic Procedure 27
5. RESULTS 31
5.1 General 31
5.2 Spreading on Different Waters 31
5.3 Photographs of Laboratory Oil Films 31
5.4 Field Observations 37
6. DISCUSSION 87
6.1 Inherent Optical Properties and Visibility 87
6.2 Results are General, not Unique 87
6.3 Field Application of Results 89
6.4 A Modified Thickness-Appearance Table 90
7. GLOSSARY 93
8. REFERENCES 95
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FIGURES
Page
1. Relative Sensitivity of the Human Eye to Light of Various 5
Wavelengths
2. Oil Film Appearance and Effective Thickness vs. Coverage 10
3. Reflection and Refraction of Light by a Thin Film 12
4. Arrangement for Photographing Oil Film in Tray 28
5. Light Arabian Crude (1 yl). Thickness is 15 nm. 41
6. Light Arabian Crude (2.5 yl). Thickness is 38 nm. 43
7. Light Arabian Crude (5 yl). Thickness is 75 nm. 45
8. Light Arabian Crude (10 yl). Thickness is 150 nm. 47
9. Light Arabian Crude (20 yl). Thickness is 300 nm. 49
10. Light Arabian Crude (50 yl). Thickness is 750 nm. 51
11. Light Arabian Crude (100 yl). Thickness is 1500 nm. 53
12a. Light Arabian Crude (200 yl). Thickness is 3000 nm. 55
12b. Light Arabian Crude - Enlarged view of 3000 nm film,
second from top of Figure 12a. 57
13. South Louisiana Crude (1 yl). Thickness would be 15 nm
for full coverage. 59
14. South Louisiana Crude (2.5 yl). Thickness is 38 nm. 61
15. South Louisiana Crude (5 yl). Thickness is 75 nm. 63
16. South Louisiana Crude (10 yl). Thickness is 150 nm. 65
17. South Louisiana Crude (20 yl). Thickness is 300 nm. 67
18. No. 2 Fuel Oil (1 yl). Thickness would be 15 nm with
full coverage. 69
19. No. 2 Fuel Oil (10 yl). Thickness is 150 nm. 71
VI
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Page
20. Agha Jari Crude (1 yl). Thickness is 15 nm. 73
21. Agha Jari Crude (10 yl). Thickness is 150 nm. 75
22. Effect of Background Brightness of Visibility (a), (b),
(c): Bright and Medium Background, (d): Dark Back-
ground. 77
23. No. 2 Diesel Fuel on Ran'tan River 79
24. No. 2 Diesel Fuel on Ran tan River (iridescence) 79
25. No. 2 Diesel Fuel, with bare water exposed. 79
26. No. 2 Fuel Oil on Arthur Kill (iridescence) 79
27. Diminishing visibility as viewing angle departs from
vertical. 81
28. Visibility of 50 gpm, 100 ppm oil-water discharge, from
the surface (a) and from the air (b). 83
29. Visibility of 25 gpm, 10 ppm discharge, from the surface
(a) and from the air (b). 83
30. Undimini shed visibility of driveway oil film in low
intensity light. 85
31. Reduced visibility in direct sunlight compared to shadow. 85
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TABLES
Page
IA Oil Film Thickness vs. Visible Color 7
IB Oil Film Thickness vs. Surface Coverage 8
II Theoretically Implied Color vs. Wavelength 16
III Wavelengths Subject to Interference vs. Thickness 18
IV Comparison of Oil Spread on Sea Water and Distilled Water 32
V Schematic Basis for Thickness-Appearance Relationship 91
vm
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1. CONCLUSIONS
1. Oil films up to approximately 3000 nanometers (3 microns)
thick show characteristic inherent color effects that are
governed by interference phenomena.
2. Within this range (up to 3000 nanometers), these inherent
color effects have, in theory and in fact, an orderly re-
lationship to film thickness and are independent of the
type of oil forming the film.
3. A table such as that published by the American Petroleum
Institute and included in this report or an equivalent
table assembled from the EPA tray photographs, reliably
indicates film thickness in this range.
4. The visibility of an oil film is not constant but depends
upon conditions of observation as well as upon its inherent
characteristics. Optimum conditions include the absence of
direct sunlight, a white overcast, a high viewing angle
(approaching vertical), and low background brightness from
the underlying water.
5. As viewing conditions deviate from the optimum, the visi-
bility of a given film is lessened.
6. Under sufficiently adverse conditions, visibility becomes
low or nil for films less than 3 microns thick.
7. For films too thin to show color, an adjacent area of bare
water for comparison is usually necessary to discern the
higher reflectivity of the film. This is not usually re-
quired for films that show definite color.
8. For films too thin to show color, waves or chop tend to
obscure the higher reflectivity of the film.
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2. INTRODUCTION
2.1 Reasons for this Investigation
Present Federal regulations (Ref. 1) prohibit harmful
discharges of oil. Harmful discharges are stated by
the regulation to be those which:
11 cause a film or sheen upon or
discoloration of the surface of the
water "
The prohibition applies not only to oil per se, but also
to oil mixed with water. The latter includes, for ex-
ample, oily ballast or bilge water from a vessel, and
waste water from an offshore drilling platform.
The term "sheen" is defined in the regulation as:
"an iridescent appearance on the surface
of the water",
and the American Heritage Dictionary definition of
"iridescence" is:
"producing a display of lustrous,
rainbow-like colors".
For convenience, this regulation will be referred to as
the "sheen regulation".
2.1.1 Visibility as a Criterion
The sheen regulation, if read literally, prohibits the
existence of an oil film, sheen, or discoloration; proof
of existence depends upon detectability. In the absence
of devices on the water surface for chemical or physical
sampling and analysis, potential detection is by remote
sensing devices. These devices usually respond to the
electromagnetic spectrum, from the ultraviolet through
the visible and infrared, and into the microwave region.
-------
In this context, the human eye is a remote sensing
device, responding to the visible spectrum in the
400 nanometer (nm) to 750 nm wavelength band (Fig.l).
Compared with other presently developed remote sensing
systems, the eye has greater sensitivity, it is stan-
dard equipment with observers, and it is served by a
data processing center which interprets and correlates
the eye's output with excellent reliability and repro-
ducibility.
These attributes, together with the fact that the
definition of "sheen" invokes the visible spectrum,
leads to the stipulation that detection by eye, or
visibility, is the primary criterion for the exis-
tence of a film, sheen, etc. Accordingly, this
report is concerned with visual properties of thin
oil films and with the visibility of oil films.
2.1.2 Problem and Scope
Limiting cases of visibility involve very thin films
which produce rainbow-like colors and films which are
even thinner and show no color. Films too thick to
show bright color are assumed readily visible either
by virtue of their self-color (browns and blacks for
crudes and residuals) or by virtue of their calming of
the water. These thin films range in thickness from
about 15 nm, about 1/30 the wavelength of visible light,
to about 3000 nm (3 microns), about 6 times the wave-
length of visible light.
The following questions developed in our examination of
the sheen regulation:
(a) Once thin oil films have been formed, what are
their inherent optical characteristics and
what are the corresponding inherent visual
effects?
(b) Do these inherent visual effects imply an
inherent, or constant, visibility or are
there other factors that make visibility
variable? Factors to consider include
oil type, illumination, and viewing position.
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100 i-
50
>
t—
<
400
500
600
700
WAVELENGTHS IN NANOMETERS (nm)
Figure 1. Relative sensitivity of the human eye to light of
various wavelengths (from Ref. 7).
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(c) If we can establish relationships concerning
the visibility of oil films per se, can we
also establish relationships between the con-
ditions of oily water discharge and the for-
mation of surface films?
(d) Can we then relate conditions of oily water
discharge and their consequent visibility?
We undertook a two-part study. This report addresses itself
to the first two questions that concern the oil film per se.
The second part, concerning the latter two questions, is the
subject of a separate report.
2.2 Background and Approach
2.2.1 Background Information
With the exception of some crude oils and processed products
that form discrete "lenses" on the water's surface, most
crudes and products spread spontaneously because of surface-
active components (Ref. 2). This spreading can continue
until extremely thin films are formed. The American Petro-
leum Institute (API) (Ref. 3) indicates a relationship be-
tween film thickness and visual effect. Table I contains
this information, with thickness values in various metric
units added for convenience.
In Table I, a film 38 nm thick is listed as "barely visible
under most favorable light conditions". This suggests that
there is a limiting thickness below which a film is not
visible. It also suggests that this limiting thickness can
vary with lighting conditions. The other entries indicate
an orderly trend between appearance and thickness, in three
main groupings:
Up to 150 nm - colorless film
300 - 1000 nm - rainbow effects
(iridescence)
Greater than 1000 nm - progressively re-
duced iridescence.
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TABLE IA *
OIL FILM THICKNESS VS. VISIBLE COLOR
Appearance
Barely visible under most favorable light
conditions
Visible as a silvery sheen on surface
First trace of color may be observed
Bright bands of color are visible
Colors begin to turn dull
Collors are much darker
Film Thickness
1.
1
in.
5xlO-6
3xlO-6
6x10-6
2x10-6
40x10-6
80x1 0~b
3
7
1
3
1
2
cm
.8x10-6
.6x10-6
.5xlO-5
.IxlO-5
.OxlO-4
.OxlO-4
3.
7.
1.
3.
1.
2.
V
8x1
6x1
5x1
1x1
0
0
0-2
0-?
o-1
o-i
3.
7.
1.
3.
1.
2.
nm
8x1 01
6x1 01
5x1 02,
1x10^
Oxl O3
Oxl O3
u
Angstrom
3
7
1
3
1
2
.8xl02
.6xl02
.5xl03
.IxlO3
.OxlO4.
.OxlO4
*Tables IA and IB are based upon Ref. 3
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TABLE IB
OIL FILM THICKNESS VS. SURFACE COVERAGE
00
Film Thickness, nm
3.8xlO]
7.6xlO]
1.5x10?
S.lxlO2
1.0x103
2.0xl03
Coverage
gal/acre
0.04
0.08
0.16
0.32
1.08
2.16
gal/mile2
25
50
100
200
666
1332
mg/m2 *
38
76
150
310
1000 (1 gm)
2000 (2 gm)
*Computed values assuming film specific gravity =1.0
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The API report (Ref. 3) does not indicate the type or
types of oil used or the range of lighting investigated.
The General Research Corporation (Ref. 4) reports an
appearance - thickness relationship (Fig. 2) based upon
a Santa Barbara crude that agrees substantially with the
API data. This agreement suggests, but does not estab-
lish, that appearance is not dependent upon the type of
oil.
Neither set of data mentions the effect of other variables
such as water depth, bottom characteristics, or viewing
angle upon visibility. Experience, however, has shown
that visibility is best under an overcast sky (Ref. 5 and
6). We thus start out with the central notion that ap-
pearance is related to thickness, but not oil type, and
that visibility is variable. The identity and influence
of other factors affecting visibility was not known.
Additionally, we found that the existing verbal descriptions
used in the appearance relationships were vague enough that
different observers could classify the same film into dif-
ferent appearance categories.
2.2.2 Approach
In view of the foregoing, we set out to form oil films of
known thickness under laboratory conditions and to vary
factors of interest in a controlled manner. These factors,
besides film thickness, included type of oil, surface state
of the water, water salinity, water opacity and character
of bottom. Along with these controlled experiments, random
or contrived observations out-of-doors were made to corre-
late the laboratory observations with those in the outdoor
environment.
To avoid the inescapable difficulty of describing in words
the complicated visual effects, color photography was se-
lected for primary documentation.
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1x10
-3
— 3000
— 2000
— 1000
o
>
1x10
-5 .
300
200
100
50
1x10
-6
0.001
DULL GRAY/BROWN
IRIDESCENCE
SILVERY SHEEN
-I—I I I I
OIL SAMPLED, gm/ff'
DULL IRIDESCENCE
WITH BROWN STREAKS
1.0
Figure 2.
Oil film appearance and effective thickness vs. coverage
(from Ref. 4).
10
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3. THEORY
3.1 Applicable Phenomena
The appearance of a thin oil film on water is associated
with that part of the incident light that is reflected
from the water-air interface or from the oil -air and oil-
water interfaces. Where the oil film displays colors,
these colors are the result of interference between re-
flections from the two oil interfaces.
3.2 Reflectivity
Reflectivity is defined as that fraction of light (inten-
sity) incident upon a surface that is reflected. Re-
flectivity is a function of the angle of incidence (from
the normal) and the refractive indices of the media.
The Fresnel equation applies to reflection from the sur-
face of a transparent medium in air:
R = u
tan(i+r)
In this equation, the relationship between i and r is ex-
pressed by Snell's Law:
sin r = y-j
In these equations:
R = light reflected
light incident
i = angle of incidence (see Fig. 3)
r = angle of refraction (see Fig. 3)
y. = refractive index of air, taken as 1.0
y = refractive index of denser transparent
medium
11
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FROM SOURCE
TO EYE
OIL FILM
WATER
Figure 3. Reflection and refraction of light by a thin film (from Ref. 7)
12
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We calculated the values of reflectivity from equation (1),
noting that angle of incidence is measured from the normal
and using the standard value of y for water of 1.33 and an
average value for oil of 1.5.
R(water)
0.021
0.060
0.21
film should be distin-
guishable from a water surface because of its higher
reflectivity if it is observed at a near vertical
viewing angle.
(b) As viewing angle (angle of incidence) becomes less
vertical, reflectivity of oil compared to water de-
creases until water has a higher reflectivity than
oil. This latter case represents glare.
(c) Except for very high angles of incidence (approaching
a horizontal viewing angle) only a small fraction of
incident light is reflected. The rest is transmitted
into the water or oil medium.
i
30
60
75
0
0
0
The tabulated values
(a) Even
without col
R(oil)
0.041
0.090
0.16
imply that
or, an oil
3.3 Interference
Normal illumination, or "white" light, contains all wave-
lengths in the visible spectrum, and each wavelength rep-
resents a color to the eye. Light has wave-like properties
similar to an AC voltage or current. Two light rays of the
same wavelength can reinforce each other, cancel each other,
or have some intermediate additive effect, depending upon
whether they are in phase, 180° (h wavelength) out of phase,
or with an intermediate phase relationship. Optically, ef-
fects from these phase relationships are termed interference.
Rainbow effects (iridescence) from an oil film on water are
due to interference; a given film thickness will produce dif-
ferent phase relationships among the various colors contained
in white light. Whether a given wavelength or color is rein-
forced or cancelled out depends upon its relationship to the
film thickness.
13
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Any two rays of light, of the same wavelength, X,
emanating from the same point of a broad, uniform
source will be in phase. Suppose ray 1 is reflected
from the top surface, and ray 2 is reflected from the
bottom surface of an oil film on water. Additionally,
since the oil film has a higher refractive index than
that of water, ray 2 will undergo a phase shift upon
reflection of 180°, or % wavelength. Figure 3 (Ref. 7)
schematically shows the longer optical path of light
ray 2 compared with that of ray 1. The difference in
optical path length, AL, is
AL = 2t (y2- sin2i)% (3)
where
t = film thickness
y = refractive index of oil
forming the film
i = angle of light incidence
(the same as viewing angle)
measured from the normal to
the surface.
For any given wavelength of light, the interference effects
depend upon the number of wavelengths in AL, taking into
account the 180 phase change (% wavelength) that occurs
at the oil-water interface.
If AL contains a whole number of wave-
lengths, maximum destructive interference
will occur; that is ray 1 and ray 2 are
out of phase, and will cancel each other.
Mathematically, this condition is,
AL = xX (x = 1,2,3,4, etc) (4)
If AL contains an odd number of half
wavelengths, maximum constructive
interference will occur; that is,
ray 1 and ray 2 are in phase, and
they reinforce each other. Mathe-
matically, this condition is,
AL = x| (x = 1,3,5,7, etc) (5)
14
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Intermediate conditions will give
partial interference.
Transposing equations 4 and 5 allows calculation of the
wavelengths that give maximum effects.
For destructive interference:
X = ^ (x • 1,2,3,4, etc.) (4a)
/\
For constructive interference:
X = ^- (x = 1,3,5,7, etc.) (5a)
J\
Since normal illumination contains all wavelengths, a given
film thickness will selectively reinforce some wavelengths
and cancel others, changing the color composition of the total
light reaching the eye and giving the effect of visual color.
For illustration, assume an oil film, refractive index = 1.5,
viewed at an angle of 45°. In Table II are the calculated
wavelengths within the visible at which interference will
occur for different film thicknesses. Wavelengths between
408 and 816 nm are considered to be in the visible, or close
enough to influence the visual effect (see Fig. 1).
The approximate resulting color, listed in Table II, is ob-
tained through the following type of reasoning; 300 nm film
is used as the example:
Constructive Interference: at 544 nm (yellow-green)
Destructive Interference: at 408 nm (violet)
and at 816 nm (red)
Net results: Original white light plus
green, and minus red and
violet results in a net
green light.
The color sequence deduced as a function of thickness is, in
fact, the sequence found experimentally. Examination of the
photographs confirms this.
Thus far we have established that films thinner than 150 nm
should display no color since wavelengths subject to inter-
ference are in the ultraviolet, outside the visible. Between
75 and 150 nm, there is some slight influence in the violet/
blue end of the visible. As a speculation, this may contri-
bute to the "silvery sheen" effect, which we interpret as a
film that shows no distinguishable color but which does
15
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TABLE II
THEORETICALLY IMPLIED COLOR VS. WAVELENGTH
Constructive
Destructive Resulting
Film
Thickness, nm
75
150
200
250
300
AL
nm
204
408
545
677
816
Interference
at nm
408
816
(none)
451
544
Interference
at nm
(none)
408
545
677
816,408
Color
Approx.
Slight bl
warm tone
purple
blue/blue
green
uish
green
16
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have a pearl-like or metallic luster. We also see that
films in the 200-300 nm range should display a variety
of colors of high purity since in this range there is
only one, or at most two wavelengths that show strong
constructive interference.
As film thickness increases, a greater and greater
number of wavelengths exhibit strong interference.
Table III is a list of wavelengths subject to con-
structive and destructive interference versus film
thickness. With an increasing number of wavelengths
(colors) that are intensified or depressed by a film,
the net color becomes a blend of colors distributed
through the visible spectrum, so that the apparent
color becomes less and less pure. In the extreme,
such as with a 3000 nm film, so many colors are in-
volved that the result approaches the "whiteness"
of a complete spectrum, and the effects become a
weakening alteration of light grey and dark grey
bands instead of a repeating sequence of pure colors.
We can now summarize a theoretically deduced trend of
appearance versus film thickness;
Thickness less than 150 nm:
no color apparent
Thickness of 150 nm:
warm tone apparent
Thickness of 200 to 900 nm:
variety of colors, including
purple, magenta blue, green,
yellow, showing considerable
purity, with a net rainbow-
like effect
Thickness greater than 900 nm:
colors showing progressively less
purity, degrading toward grey as
thickness increases.
17
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TABLE III
WAVELENGTHS SUBJECT TO INTERFERENCE VS.
THICKNESS
Film Wavelengths for
Thickness Constructive Interference
(nm) (nm)
Wavelengths for
Destructive Interference
(nm)
300
600
900
1200
1500
1800
2100
2400
2700
3000
544
652,465
699,544,445
725,593,502,435
742,528,544,480,429
753,653,516,616,466,426
762,672,601,544,497,457,
423
768,687,622,568,522,484,
450,421
733,699,639,588,544,506,
474,445,420
777,710,653,604,563,526,
495,466,441,418
408,816
816,544,408
816,612,490,408
816,652,544,466,408
816,680,583,510,453,408
816,699,612,544,490,445,
408
816,714,634,571,519,476,
439,408
816,725,653,593,544,502,
466,435,408
816,734,668,612,565,525,
490,459,432,408
816,741,679,627,582,543,
509,479,453,429,408
18
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3.4 Fluorescence Not Significant
Fluorescence is the absorption of light at a wavelength
appropriate to the structure of the absorbing substance
with prompt re-emission at a longer wavelength. Some
components of petroleum, notably aromatic and other ring
species, exhibit fluorescence in the UV and short visible.
As a generality, absorbtion is in the 200 to 400 nm range,
with re-emission in roughly the 350 to 450 nm range.
Compared with the interference effects, the contribution of
fluorescence to the visual appearance is negligible be-
cause of the relatively small fraction of oil components
affected, the low intensity of re-emitted light, and the
low sensitivity of the eye in this spectral region.
It is possible that fluorescence may contribute to (but
not dominate) the luster of the "silvery sheen" effect.
Fluorescence may also contribute to photographic re-
cording of a thin film out-of-doors because the photo-
graphic emulsion has higher sensitivity to this spectral
region than has the human eye.
19
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4. EXPERIMENTAL
4.1. General
4.1.1. Preparing and Photographing Laboratory Oil Films
The two main aspects of laboratory experimental procedure
were physical and photographic. Physical procedure in-
cluded the equipment and techniques used to prepare the
oil films in small trays in the laboratory; photographic
procedure included equipment and lighting for producing
the photographic record. Although both procedures are
basically simple, some difficulties were encountered in
the physical procedure. A sufficiently detailed account
is given so that others who may wish to perform similar
or related experiments will be able to proceed with the
fewest possible problems.
4.1.2. Other Observations
Besides photographing oil films in the laboratory, a
variety of observations were made in the laboratory
and out-of-doors, but were not usually photographed.
These were done to guide and interpret the laboratory
procedures, to establish the correspondence of labora-
tory and non-laboratory effects, and to generalize
and order the results. These observations included:
(a) An extensive series of small-pan observations
in the laboratory to verify the similarity of
effects produced by different oils on dis-
tilled and sea water.
(b) Daily observation of a weathered oil-on-water
film in a 4 x 6 foot tray, out of doors, for
several weeks. The effect of viewing angle
and outdoor illumination -- sky and cloud con-
ditions, intensity of light, sun angle -- were
observed.
(c) A number of out-of-doors observations of small-
pan oil films. These films were identified!
with those formed in the laboratory. These es-
tablished the identity of results as seen and
photographed with laboratory lighting and as
21
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seen under ideal outdoor lighting. For example,
the greater brilliance and intensity of irides-
cence illuminated by white clouds (ideal) was
clearly evident when compared with illumination
by clear blue sky.
(d) Occasional visits to nearby oil spills. Some
photographs of these observations are included
to illustrate certain points.
4.2 Physical Procedure
The basic setup was simple. For each film formed, a tray
approximately 9 x 12 inches was lined with clear plastic,
a liter of the desired water was added and allowed to calm,
then a measured volume of oil was expelled from a syringe
onto the water surface.
4.2.1 Plastic Liner
The thin, clear plastic film liner served two purposes.
First, it could be thrown away after one use, and a fresh
liner could be used for the next oil film. This precluded
the need for scrupulous cleaning of the pan between succes-
sive films. Perhaps more important, it provided a container
with reproducible surface properties and avoided possible
effects of variation in tray cleaning. The liner was care-
fully handled to avoid contact with the hands or other source
of contamination. To smooth the sheet into the pan, fresh
Kim-Wipes (lintless paper towels) were used.
Second, the clear liner made it possible to vary the back-
ground brightness when white, grey or black paper was in-
serted between the liner and the tray bottom (see Section
4.2.2.6, Background).
Some plastic sheet materials strongly inhibited the spread-
ing of oil. A&P Clear Freezer Wrap was satisfactory from
this standpoint. One roll of polyethylene gave good results.
Other rolls (different sources) inhibited spreading.
4.2.2 Measurement of Film Thickness
If a known volume of oil spreads uniformly to cover a known
area, then the film thickness can be computed.
. volume
t ~
22
-------
If volume is expressed in microli
if area is expressed in square me
the oil film thickness in nanomet
micron) is:
t(nm) =
iters (1 yl = 10" cm ) and
meters (1 m2 = 104 cm2), then
nanometers (1 nm = 10-9m = 1Q-3
(7)
m
4.2.2.1 Water Surface Area
Area was deduced by measuring the depth of a known volume of
water (1 +_ .01 liters) in the tray. Corrections were applied
to compensate for the sloping sides and rounded bottom edges
of the pan. A check was made by measuring pan dimensions at
the water line, correcting for rounded corners.
The two determinations checked within 5%. Allowing for the
fact that the plastic liner did not completely follow the
tray contour, we estimated the total uncertainty in area at
10%. The surface area so determined was:
A = 0.068 +_ 0.0034 m2
For convenience in computing nominal film thickness, a nominal
area was used:
A = 1/15 m2 = 0.067 m2
4.2.2.2 Oil Volume
A Hamilton microliter syringe (25 yl capacity, 1/2 yl gradua-
tions) was used to meter oil volumes up to 20 yl. For larger
volumes, up to 200 yl (0.2 ml), a normal 1 ml syringe, gradu-
ated in 0.01 ml, was used.
If the syringe was in continuous use with the same oil, it was
not cleaned between successive uses. At the end of continuous
use, or when changing oil, the syringe was flushed several
times with carbon tetrachloride, after which the plunger was
removed and rinsed with acetone. The barrel was attached to
a vacuum line, acetone was drawn through by vacuum to rinse,
and the barrel was then dried with air aspirated by the vacu-
um. A clean syringe was first flushed five or six times with
the oil to be used.
The estimated overall accuracy of
0.5 yl for the 25 yl syringe, and
delivered oil volume ;s +_
+ 10 yl for the 1 ml syrinqe
23
-------
This takes into consideration inaccuracies in scale reading,
dribbling, and small droplets that occasionally remained at-
tached to the delivery end of the needle.
4.2.2.3 Accuracy of Film Thickness
In nearly all cases, the oil spread over the entire water
surface, and thus the nominal film thickness was 15 nm per
microliter of oil. Uncertainties in oil film thickness
owing to the oil volume measurement are:
Nominal Oil Volume Nominal Oil Thickness, Uncertainty,
nanometers nanometers
1-20 15 - 300 + 7.5
50 - 200 750 - 3000 + 15.7
Additional uncertainty owed to surface area is: +_ 5%.
Resulting total uncertainty is as follows:
Nominal Oil Thickness Total Absolute Error Percent of
nanometers
15
38
300
750
300
nanometers
+ 8.3
+. 9.4
+. 22.5
+. 187.5
+ 300.0
Nominal
+ 55
+ 20.5
± 7-5
+ 15.7
+ 10.0
4.2.2.4 Oils Used
The need to pick up and deliver oil through the fine needle
of a microliter syringe limited the choice to low viscosity
types. Those used were:
Mediterranean Crudes (dark brown)
Light Arabian
Agha Jari (Iranian)
Domestic Crude (Dark brown)
South Louisiana
24
-------
Distillate (Clear with light yellow
to red tint)
No. 2 Fuel Oil
Samples were withdrawn from full or nearly full drums
and placed in polyethylene bottles. Small ullage in
the nearly full drums implied they had never been or
were infrequently opened. Thus, there had been negli-
gible loss of volatile components, and the samples
were representative of the parent stock.
4.2.2.5 Water Used
Distilled water (with and without background dye: see
Section 4.2.2.6) and visually clean, coastal sea water
(salinity 26 ppt) were used as substrates for several
hundred oil films. Since the spreading of the test
oils was apparently identical on all these waters,
no further characterization of water was made.
Because of this identity in behavior, distilled water,
with or without dye, was used for the films photographed
for this report. The properties of distilled water are
less variable then those of tap, river, estuarian, or sea
water. The advantage is clear should there be future
need to compare results obtained at different laboratories
or at different times.
4.2.2.6 Background Brightness
Background brightness was varied to represent a range of
real life bottom and water conditions. In shallow, clear
water over a light sand bottom, most of the light incident
on the water surface penetrates to the bottom and is re-
flected back through the water surface with little absorp-
tion. At the other extreme, the water is so deep, or the
bottom so dark, that almost all the incident light is ab-
sorbed below the water surface and the water looks almost
black. The former gives a high background brightness; the
latter, a low background brightness.
In a shallow pan with clear water, the high brightness
situation was simulated by white paper (or white tray bot-
tom) under the plastic liner. Black paper under the liner
gave a medium high, rather than a low, background brightness
owing to reflection from the surface of the plastic liner
itself.
25
-------
Low background brightness was obtained by dyeing the
water. Sheaffer Permanent Jet Black ink (2% ml per liter)
gave an intense dark color that completely obscured a
white tray bottom. The ink did not inhibit oil spreading
on distilled water; it did inhibit spreading on sea water.
On undyed water, however, representing true field condi-
tions, oil spreading on distilled water and sea water are
equivalent (Section 5.2). For experimental reproducibility,
distilled water is preferable (Section 4.2.2.5). Thus
ink-dyed distilled water was used to simulate a low back-
ground brightness.
Other dyes tried were unsatisfactory. Liquid food coloring
gave excellent opacity and did not inhibit spreading on
either distilled or sea water, but gave unnatural colors.
RIT fabric dye gave excellent opacity and good color, but
strongly inhibited spreading on both distilled and sea water.
4.2.2.7 Surface State
Oil placed on the calm water surface spread to form a
static film of nearly constant thickness and of nearly
uniform color. To simulate the more realistic condi-
tions of an active water surface, two things were done.
(a) By blowing onto the film in the middle of the
tray, a hole was formed in the film, exposing
a circular patch of bare water. Around the
bare water, the oil film tapered in thickness
from zero at the edge to the full thickness
away from the edge.
(b) The tray, containing the water and oil film,
was agitated by a laboratory shaker table.
Horizontal orbital motion of the shaker table
generated waves about %-inch to 1-inch trough-
to-crest.
Besides providing glittering reflections from the irregular
surface, the waves produced variations in film thickness.
The film is compressed at the crest and stretched thin in
the trough, similar to the effects of sound propagation in
a fluid where the medium is compressed at the node and rari-
fied at the antinode.
26
-------
The sequence of operations was:
(a) Observe static film as laid on calm water
(b) Observe film with hole blown in it
(c) After calming, start shaker table and
observe film
(d) Observe film after stopping shaker table and
while allowing surface to calm.
4.3 Photographic Procedure
No special photographic equipment was needed except
supplementary devices for critical focusing at object:
image ratios of about 6:1. The lighting was arranged
to give optimum conditions -- a bright white uniform
source with an area large enough so that its image,
reflected by the water surface into the lens, would
substantially fill the negative area. This simulated
illumination by a uniform white sky.
A schematic, Figure 4, shows the general arrangement used.
Details are in the following sections.
4.3.1 Camera and Objective
A 35 mm Leica Ilib body was mounted on the rear plate of
a Leica sliding focusing attachment. A 9-cm Leica Elmar
f:4 objective with lens shade was mounted on the front
plate. The sliding focusing attachment provided ground
glass viewing and critical focusing with a 10X magnifier.
The attachment added 11.8 mm to the normal lens flange to
focal plane distance, acting as an extension tube or bel-
lows for closer-than-normal focusing. The 9-cm focal
length, compared to the normal 5-cm objective, increased the
object-lens distance to about 60 cm (24 in.) for more con-
venient working at the reduction ratio of 6 or 7 to 1.
4.3.2 Lighting
A No. 2 photoflood lamp (color temperature 3400 K) or a
500 watt 3200K lamp (color temperature 3200 K) in a 12-inch
aluminum reflector with diffusing screen provided good il-
lumination. For maximum light and to fill the negative
area with its image, the diffuser was close to the water
27
-------
Figure 4. Arrangement for photography oil film in tray.
28
-------
surface, about 12 inches. The "earth from space" effect
in the photographs clearly illustrates the lamp placement;
the circular light source covers the height of the photo-
graph, but does not quite cover the full width of the
format.
Illumination at the water surface varied from about 4000
lumen/ft^at the center to about 3000 at the edges. In-
tensity at the center was about half that of a normal sun-
lit outdoor scene; variation over the field represented
a one-half stop exposure variation.
4.3.3.1 Film and Exposure
For the laboratory photographs, High Speed Ektachrome
(ASA 125 with 3200K lamp) was used at first; later on,
Kodachrome II Professional, Type A (ASA 40 with 3400K
lamp) was adopted. The higher contrast and greater ap-
parent saturation gave transparencies that seemed closer
to the visual impression. Most of the out-of-door photo-
graphs were on High Speed Ektachrome, daylight type.
Exposure for the tray photographs, as indicated by an inci-
dent light exposure meter, was 1/100 sec. at a marked aper-
ture of F:6.3. Effective aperture (correction for close-up
focusing) was f:8.
4.3.3.2 Processing
The exposed film was given to a local dealer for normal
processing by Kodak. There were no special arrangements.
Results were satisfactory. Slight variations in overall
tone were found (slight green cast vs. slight blue cast)
among rolls processed at different times, but these dif-
ferences were insignificant for the purpose at hand.
29
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5. RESULTS
5.1 General
This section presents the results obtained from laboratory
observations and includes photographs of oil films formed
in small trays. Additional photographs of oil films taken
in the field are included to show the correspondence be-
tween effects in the laboratory and in the field.
5.2 Spreading on Different Waters
The degree of spreading between films of Arabian and No. 2
Fuel Oil on sea water and distilled water, in 1/15 m2 trays,
was compared (Table IV).
Although in the thinnest films there are differences in
spreading tendency between different oils, the spread of
each oil is nearly identical for sea water and distilled
water. This fact allowed the use of dyed distilled water
as a controlled substrate for experimental purposes.
Photographs of water-to-water comparisons are not included
in this report.
5.3 Photographs of Laboratory Oil Films
In the color photographs, the oil films range from 15 to
3000 nm (nominal) thick (Figs. 5 thru 21). Films of Light
Arabian represent the entire range. Films of South Louisi-
ana represent the range 15 to 300 nm (nominal). Agha Jari
and No. 2 Fuel Oil represent two thicknesses each: 15 and
150 nm. This series used dyed water for a dark background
and shows nearly pure optical effects from the surface film
alone, undegraded by extraneous light from glare or back-
ground.
The four photographs in Figure 22 show the effects of back-
ground brightness on visibility.
Figures 5 through 21 each have three or four different photo-
graphs of the same oil film (one film per figure), with each
photograph representing a different surface state. The arrange-
ment of photographs is a consistent vertical sequence from top
to bottom:
Film as formed
Hole blown in film
Shaker table in operation
At rest after shaker table
31
-------
TABLE IV
COMPARISON OF OIL SPREAD ON SEA WATER AND DISTILLED WATER
Oil/Water
Fraction of Surface Covered
oil , yl*
Light Arabian
Sea Water, undyed
Distilled water, undyed
Distilled water, dyed
1
10
0.8
0.8
0.8
1.0
0.9
1.0
1.0
1.0
1.0
No. 2 Fuel
Sea water
Distilled
Oil
undyed
water, dyed
0.5
0.5
0.8
*Film thickness = ylx!5 for coverage - 1.0
0.9
1.0
32
-------
If one of the spaces within the sequence is blank, there
is no photograph of that particular surface state.
5.3.1 Effect of Film Thickness
Figures 5 through 12 record the changing appearance of
Light Arabian oil films as thickness increases from 15 nm
to 3000 nm. The main points are:
(a) Films less than about 150 nm show no color, but
are brighter when compared with bare water because
of the higher reflectivity of the oil film. There
is no color for these thicknesses because inter-
ference occurs only at wavelengths shorter than
those in the visible spectrum.
(b) At about 150 nm, a bronze tint is seen in the
calm film, as laid. Surface activity introduces
an additional purple tint owing to local thick-
ening.
(c) From 300 to 750 nm, strong color and brilliant
rainbow coloring (iridescence) prevails, the
latter when surface activity causes thickness
variations.
(d) At 1500 nm, brilliance is lost from the rainbow,
and at 3000 nm, considerable dulling is evident.
As film thickness increases beyond this, irides-
cence will disappear completely except where there
is pronounced local thinning, and the film will
assume the color of the bulk oil.
5.3.2 Effect of Oil Type
Figures 13 through 17 similarly show the appearance of
South Louisiana films from 15 nm to 300 nm. Once nearly
complete coverage is obtained at 38 nm (2.5 ul), the ap-
pearance of these films is nearly identical with their
Arabian counterparts. As a further comparison, 150 nm
films of Agha Jari and No. 2 Fuel Oil (Figs. 19 and
21) are similar in appearance to the Arabian and South
Louisiana.
Differences are found in the thinnest films, formed by
1 yl of oil. Arabian and Agha Jari spread uniformly to
give thin films, about 15 nm thick, with reflectivity
somewhat higher than that of the water.
33
-------
The South Louisiana and No. 2 Fuel Oil (1 yl) (Figures
13 and 18) give incomplete films with a Swiss cheese ap-
pearance of decidedly higher reflectivity than do the
other two oils. Note, however, that the greater reflec-
tivity is associated with a lower surface coverage and,
thus, a film thickness greather than that obtained with full
coverage. In fact, these films of low coverage do indeed
appear similar in brightness to the 38 or 75 nm films of
Arabian.
Thus, even when spreading characteristics of an oil do
vary, owing to type, composition, or other factors, the
appearance of the film, in terms of brightness or color,
is appropriate to the actual local thickness, in accord
with theory.
5.3.3 Effect of Surface State
5.3.3.1 Blown Holes (Films with tapered edges)
The main purpose in exposing the bare water was to demon-
strate that even very thin films, 15 and 38 nm, displayed
a reflectivity greater than water. In fact, comparing
these photographs with those of the fully spread films
(Arabian and Agha Jari, Figs. 6 and 20), it is evident that
the detection, or visibility, of thin colorless films de-
pends on the ability to make such a comparision. The eye
is unable to make quantitative estimates of brightness with-
out comparing an unknown to a reference or standard. This
is a significant aspect of visibility and detectability.
With films thick enough to show iridescence, 300 nm and
thicker, the tapered edges show the sequence of the inter-
ference colors as thickness increases. This sequence is
bronze, purple, blue and green; as the film becomes thicker,
the sequence repeats (Figures 10, 11, and 12.).
From the discussion of theory and the application of equa-
tion 3 (Section 3), one would expect this sequence to re-
peat for approximately each 300 nm increment of thickness.
Counting sets of bands in the 300 to 3000 nm films shows
this to be true, allowing for an occasional extra band due
to ripples. In the 1500 and 3000 nm films, one can also
see that, with an increase in thickness and in the number
of wavelengths subject to interference (Table III) some
colors drop out of the sequence and the intensity or
purity of color degrades.
34
-------
Figure 12b is an enlargement showing the 3000 nm film in
greater deail. The changing appearance with each 300 nm
increment, although sometimes subtle, can be seen. The
major changes reflect the groupings in Table I; the first
two sets of bands are bright, the next three degrade to
brick-red and turquoise, and ultimately color is lost and
the bands are light-dark. This one photograph in itself
summarizes the thickness-appearance relation.
5.3.3.2 Waves (Shaker Table)
Where a thin, colorless film covers the entire field of
view, (Fig. 5 and 6) waves or chop on the water produce
a glitter of reflected light spots that make it hard to
identify these films. A difference in reflectivity be-
tween a film and the adjacent bare water can be obscured
by the brightness differences in the glitter pattern.
The relative reluctance of South Louisiana and No. 2 Fuel
Oil to spread tends to be preserved under wave conditions.
Because of the incomplete coverage, bare water can be seen
and the colorless film can be discerned. The lower spreading
tendency affects the films up to 150 nm, and the local thick-
ening gives the appearance of a nominally thicker film.
The wave condition also illustrates well the progressive
dulling of iridescence as film thickness increases beyond
1000 nm.
5.3.3.3 Calm After Waves
There frequently occurs, in actuality, the condition of calm
after waves; during the lifetime of a film it can be exposed
to changing weather and sea conditions. Three effects of
wave actions were noted in the laboratory when the surface
came to rest after wave making on the shaker table.
(a) The tendency of wave action to work a compressed deposit
into a larger, thinner film is illustrated in Figs. 13 and
18, where the initial "Swiss cheese" films of South Louisiana
and No. 2 Fuel Oil have been worked into a more continuous
film. This "working out" was observed in experiments on the
open sea (Ref. 8).
(b) A weathering effect is quite noticeable with the 150 nm
films. Loss from the Arabian film, Fig. 8, has been enough to
decrease the apparent film thickness to about 75 nm as evidenced
by the change from the original yellow color to a colorless
35
-------
reflecting film. The South Louisiana film, Fig. 16
does not show this effect markedly. This may be be-
cause of a difference in duration of exposure to wave
conditions, or it may represent a true difference owed
to differences in solubility and volatility of light
components. The Agha Jari film, Fig. 21, shows an
intermediate effect. The No. 2 Fuel Oil film shows a
notably different phenomenon. The loss of components
from the film, and perhaps their partial solution in the
water, changed the interfacial properties so as to in-
hibit spreading. Examination of the original color
slide shows that the deposit had become an array of
small discrete flecks of oil separated by apparently
open water that gives the film a grainy appearance.
In the paragraph above, material loss from the film was
attributed to volatilization and solubility. A third
possibility is that some of the oil became stuck to the
tray liner during agitation; this was, in fact, observed
with thicker films. The behavior of No. 2 Fuel Oil, how-
ever, substantiates that even though material loss via
edge-sticking may have occurred, it was not the sole
mechanism. The change in spreading properties requires
a selective loss of constituents; this indicates that
volatility and solubility are involved, since these pro-
cesses are selective with respect to molecular weight and
molecular structure.
(c) A third result of wave action noticed especially with
the thicker film was the creation of apparently permanent
or long-lasting variations in film thickness that give rise
to color variation and streaking in the quiescent film.
5.3.3.4 Background Brightness
Figures 22a, 22b, and 22c show 15 nm, 105 nm, and 300 nm
films of Arabian oil. In each case, one half the tray con-
tributes a high background brightness and the other half,
a medium to medium-high background brightness. These films
were gently disturbed to develop local variations in thick-
ness. For comparison, Figure 22d shows a 300 nm film over
a dark background (dyed water), at rest, after wave-making
on the shaker table.
The increased visibility and brilliance owed to the low
background brightness is evident. Also evident in the
trays having both bright and medium backgrounds is the
apparent discontinuity at the background boundary, even
though the oil film is continuous over both backgrounds.
36
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Because only a fraction of the incident light is subject
to interference, the inherent visual effect is readily
diluted by extraneous light. If the incident light is
nearly normal to the surface, most of the light will
penetrate. Simple measurements with a photographic ex-
posure meter indicated that about 2% to 5% of the incident
light is reflected by a bare water surface, and about 5%
to 10% by a surface with an oil film. This is as predicted
by theory. Of the 90% that penetrates, some will be ab-
sorbed in the water column, some absorbed by the bottom, and
some reflected by the bottom. After reflection, and after
further attenuation, this light returns through the oil film
to the observer. The amount of returned light depends on
the absorbtivity of the water and the bottom and on sus-
pended particles that can reflect light directly. This
returned light, added to the interference effects, dilutes
the color effects and reduces the interference effects, the
contrast, and the relative brightness between oil-covered
areas and bare water areas.
In its extreme, a bright background can overwhelm the opti-
cal effects of an oil film as shown in the photographs.
The effect is similar to projecting a slide or movie in a
darkened room and then turning on the house lights.
5.4 Field Observations
Our own and the published results of other observers substan-
tiate the effects observed in the laboratory and add further
information that expands upon the laboratory results.
5.4.1 The Appearance of Oil Films
Comparison of relationships between film thickness and visual
appearance shows excellent agreement among our laboratory
photographs, the API data (Table I) and the General Re-
search Corporation data (Figure 2).
Figures 23, 24 and 25 are of oil films formed by a spill of
No. 2 Diesel Fuel on fresh water, the Raritan River. Figure
23, a view from a bridge across the river, shows a colorless
oil film along the right bank; Figure 24, a thicker irides-
cent film; and Figure 25, a film with a circle of bare water
created by tossing a stone. The effect is similar to blow-
ing a hole in the laboratory film, and demonstrates unequi-
vocally the presence of the film.
Figure 26 shows a film of No. 2 Fuel Oil from a spill on es-
tuarian water, the Arthur Kill. The low background brightness
37
-------
and overcast sky produced clear color effects similar to
those in the laboratory.
5.4.2 Viewing Angle and Viewpoint
Figure 27a, colorless films of an unknown oil on estuarian
water shows how a high viewing angle (bottom of photograph)
gives better visibility. As distance from the viewer be-
comes greater, and thus the viewing angle departs more from
the vertical, the oil film becomes less visible and finally
is obscured by the increased glare from the surface. A
similar effect is apparent in Figure 27b, a brightly iri-
descent film of No. 2 Fuel Oil (which can be seen better in
Figure 26, taken at a high viewing angle).
Figure 28 shows the results of a 50 gallon per minute sea
water discharge (Raritan Bay) containing 100 ppm South Loui-
siana Crude. In Figure 28a, taken from the vessel and show-
ing the discharge, it is hard, if not impossible, to see the
resulting oil film because of the poor viewing angle. On
the other hand, Figure 28b, taken from a helicopter at 100 ft.
altitude, clearly reveals a film described by the observer
as "sheen, non-metallic, no color".
Figure 29 illustrates a reversed situation. The discharge
here is 25 gallons per minute and contains 10 ppm South Loui-
siana. The oil discharge rate was too low to yield a large
continuous film, and formed instead the discontinuous streaks
and patches of thin colorless film. From the vessel, these
were visible; from the helicopter, however, there was no vis-
ible result of the discharge. In this case, a close viewpoint
permitted the detail to be seen, whereas at a distant view-
point, this detail could not visually or photographically be
resolved.
5.4.3 Illumination, Background Brightness and Surface States
These factors are considered together because it is sometimes
hard to separate them in a real situation.
References 5 and 6 emphasize that an overcast sky gives better
visibility than a blue sky with sun, and our experience is
certainly in accord. Interestingly, with an overcast sky,
illumination need not be intense. Figure 30 is a photograph
of an iridescent patch of oil on a blacktop driveway on a
rainy day. Light intensity from the overcast sky was 450
lumens/ft^, about 1/60 of the intensity on a sunny day. The
primary factors are the overcast sky, giving a broad uniform
light source, and the dark background.
38
-------
In sunlight, best results are obtained when the area
observed is shaded from direct sunlight and illuminated
only by light from the sky. Figure 31 is a photograph of
a colorless film viewed from our vessel, whose shadow can
be observed in the foreground. In the shadow, the oil film
is readily discerned. In the sunlit upper right portion,
the film is not detectable owing to an adverse combination
of illumination, background brightness, and surface state.
If sunlight illumination cannot be avoided, the sun should
generally be at the observers back for best visibility, or
at least not more than 90 degrees to either right or left.
See also Reference 6.
39
-------
Calm, as formed
Hole blown
Waves (shaker table)
Figure 5. Light Arabian Crude (1 yl). Thickness is 15 nm.
41
-------
Calm, as formed
Hole blown
Waves (shaker table)
Figure 6. Light Arabian Crude (2.5 yl). Thickness is 38 nm.
43
-------
Calm, as formed
Hole blown
Waves (shaker table)
Figure 7. Light Arabian Crude (5 yl). Thickness is 75 nm.
45
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 8. Light Arabian Crude (10 yl). Thickness is 150 nm.
47
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 9. Light Arabian Crude (20 yl). Thickness is 300 nm.
49
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 10. Light Arabian Crude (50 yl). Thickness is 750 nm.
51
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 11. Light Arabian Crude (100 yl). Thickness is 1500 nm.
53
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 12a. Light Arabian Crude (200 yl). Thickness is 3000 nm.
55
-------
Figure 12b. Light Arabian Crude.
Enlarged view of 3000 nm film (Figure 12a)
57
-------
Calm, as formed
Waves (shaker table)
Calm after waves
Figure 13. South Louisiana Crude (1 yl),
Thickness would be 15 nm for full coverage,
59
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 14. South Louisiana Crude (2.5 yl). Thickness is 38 nm.
61
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 15. South Louisiana Crude (5 yl). Thickness is 75 nm.
63
-------
Calm, as formed
'
Hole blown
Waves (shaker table)
Calm after waves
Figure 16. South Louisiana Crude (10 yl). Thickness is 150 nm.
65
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 17. South Louisiana Crude (20 yl). Thickness is 300 nm.
67
-------
Calm, as formed
Waves (shaker table)
Calm after waves
Figure 18. No. 2 Fuel Oil (1 yl).
Thickness would be 15 nm with full coverage,
69
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 19. No. 2 Fuel Oil (10 pi). Thickness is 150 nm.
71
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 20. Agha Jari Crude (1 yl). Thickness is 15 nm.
73
-------
Calm, as formed
Hole blown
Waves (shaker table)
Calm after waves
Figure 21. Agha Jari Crude (10 yl). Thickness is 150 nm.
75
-------
a. 1 yl 15 nm (if full coverage)
b. 7 yl 105 nm (if full coverage)
c. 22 yl 330 nm
d. 20 yl 300 nm
Figure 22. Effect of Background Brightness of Visibility.
(a),(b),(c) Bright and Medium Background; (d) Dark Background.
77
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;*•
Figure 23.
No. 2 Diesel Fuel on Raritan River,
Figure 24.
No. 2 Diesel Fuel on Raritan River
(iridescence).
Figure 25.
No. 2 Diesel Fuel ,
with bare water exposed.
Figure 26.
No. 2 Fuel Oil on
Arthur Kill (iridescence)
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a. Unknown oil on Arthur Kill
b. No. 2 Fuel Oil on Arthur Kill
Figure 27.
Diminishing visibility as viewing angle departs from vertical
81
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Figure 28.
Visibility of 50 gpm, 100 ppm
oil-water discharge, from the
surface (a) and from the air (b).
Figure 29.
Visibility of 25 gpm, 10 ppm
oil-water discharge, from the surface
(a) and from the air (b).
83
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Figure 30.
Undimini shed visibility of driveway oil film in
low intensity light.
Figure 31.
Reduced visibility in direct sunlight compared
with visibility in shadow.
85
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6. DISCUSSION
6.1 Inherent Optical Properties and Visibility
This investigation provides direct answers to the questions
that prompted it:
(a) Thin oil films have defnite optical character-
istics that produce inherent visual effects
with an orderly relationship between film
thickness and appearance. This relationship
is independent of oil type. The most evident
aspect of appearance is color, with orderly
variations in the number of colors present
and in their purity or intensity.
(b) Visibility is not inherent or constant, but is
the product of the inherent visible effects and
of the various factors external to the film that
can degrade or obscure the inherent effects.
These factors include sky conditions, sun posi-
tion, state of the water surface, character of
underlying water and bottom, viewing angle, and
distance between observer and film. Light in-
tensity does not appear as a strong factor.
6.2 Results are General, not Unique
The laboratory photographs are an internally consistent set
of data that shows the inherent appearance of oil films and
variations in visibility that result from viewing conditions.
We must now establish that these results and the conclusions
based upon them are completely general. Correspondence be-
tween laboratory results and observations in actual situations
is just one aspect of the generality sought.
One way to establish generality is by comparing enough obser-
vations to show that the same effects are found in a wide
range of circumstances.
The appearance-thickness relationship demonstrated in our
laboratory photographs corresponds closely if not identically
with those of the American Petroleum Institute and the General
Research Corporation. Taken as a whole, these three sets of
data include laboratory and field observations, a minimum
of five and probably six or more types of oil, a minimum of
three different observers, a minimum of two different water
types, and an unknown number of different viewing conditions.
The net results, with regard to the thickness relationship,
87
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is that inherent appearance is a function of thickness
and is not a function of oil type or of water type. A
further implication is that our laboratory photographs
show the same visual effect as do oil films in the field.
This is further supported by our photographs of real
fi1ms.
The effect of ambient or viewing conditions upon visi-
bility also shows agreement between our laboratory re-
sults and field observations. Photographs in this report
of real situations bear out the trends established in the
laboratory, especially with respect to background (optical
character of water and bottom) and surface condition. Com-
ments of the Coast Guard and other observers upon sky con-
dition and sun position are the same as ours. During the
course of studying oily water discharges (to be reported
separately), extensive observations were made from the
air. These confirmed the trend indicated in the labora-
tory that water chop and high background brightness (be-
cause of light return from the particles in turbid water)
lessen visibility, especially for colorless films.
When comparing a range of observations, we find agreement
in all respects and an absence of contradiction and con-
clude that our photographic data and the conclusions of-
fered can be applied to all instances of thin oil films
on water.
A second approach to assess generality is to compare re-
sults with theory. Two optical phenomena are involved:
the relative reflectivites of oil and water surfaces, and
the interference between light reflected from the top and
the bottom oil film surfaces.
Reflectivities of 2% to 5% for water and 4% to 10% for oil
are obtained both from theoretical calculation and measure-
ment. Only that small fraction of the incident light
reflected by the oil film is involved in creating the
optical effects of brightness and color that we see. With
only 10% of the light being "processed" by the film before
reaching the eye, it follows that, if even a small fraction
of the "unprocessed" light is returned to the eye, the ef-
fects resulting from the film will be diluted and weakened.
The observed effects of background brightness and of view-
ing angle (glare) upon visibility, even to the point of
making a film non-visible, are in complete accord with this
physical picture.
88
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The photographs of laboratory films were made under ideal
conditions; glare was eliminated by the higher viewing angle,
and background minimized by dying the water. Because of this,
they show with high purity the inherent visual effects pro-
duced by the oil film itself. Comparing the photographs with
the effects predicted from increasing the thickness (Equation
3), one sees complete correspondence: the onset of color
is at the correct thickness; the sequence in which colors
appear is correct; the reduction in the number of colors
with increasing thickness is appropriate to the increasing
number of wavelengths in interference; the progressive
dulling colors, with their eventual disappearance into
greyness, is as expected.
Also apparent from Equation 3 is the fact that the visual
effect of an oil film is, for practical purposes, inde-
pendent of the type of oil. Thickness and refractive index
are the only film properties that enter the equation. The
refractive indexes of most components of oils are between
1.4 to 1.5 and the limits are 1.3 to 1.7. The maximum
variance from the average value 1.5 is 4^13%, which is neg-
ligible compared with the 200:1 ratio of thickness values
investigated. Once again, the photographs bear this out.
From the theoretical standpoint, we see that:
(a) The two principles of reflectivity and inter-
ference adequately explain the quantitative
as well as qualitative appearance and visi-
bility of oil films less than 3000 nm thick.
(b) No other phenomena need be invoked to explain
these findings.
In summary, the data from this laboratory and from other sources
agree: our data are adequately explained by the above physical
principles; and these principles are fundamental and general.
Therefore, the effects reported here should apply, and do apply
without restriction, to all thin oil films on water.
6.3 Field Application of Results
Many of the details developed can be useful in field observa-
tions and may be especially helpful to inexperienced observers.
Some aspects are discussed below.
89
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The thickness-appearance relationship can be used with
complete confidence to estimate the thickness of an oil
film. This, together with an estimate of area, yields an
estimate of the amount of oil in the film. For this
purpose, the relationship listed in Table I is conven-
ient. Table I is simple enough to commit to memory. We
suggest that thickness in nanometers be used, since the
surface coverage in milligrams per square meter is nu-
merically the same as thickness in nanometers.
If observation conditions do not permit a fine judgment
of color purity or of film brightness, a simple scale of
"no color - bright color - dull color" still provides
useful information on the approximate thickness.
Conversely, if careful observation is possible, one can
make up a table, based upon our photographs, that is more
detailed than Table I (see Section 6.4).
The series of laboratory photographs has proved useful in
our laboratory as a direct illustration of actual appear-
ance that avoids the imprecision of verbal color descrip-
tion. They have been useful in indoctrinating new obser-
vers and in refining the perception of more experienced
people. They have been used in training courses given at
this laboratory for field personnel of Federal and State
organizations.
Appreciation of the factors affecting visibility should
help the observer evaluate what he sees. Although some
of the factors are not under his control (sky conditions
and water background), to some extent he may be able to
select the most favorable viewing position and angle or to
otherwise influence the total set of conditions under
which he must operate.
6.4 A Modified Thickness-Appearance Table
By using the guidance provided by theory and the effects
shown in the photographs, especially Figure 12b, one can
make more refined estimates of film thickness. In Table V,
a set of colors is referred to as being characteristic of
a given film thickness; remember that the range of colors
represents a small range of thicknesses, more or less
ending at the nominal value cited.
90
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Appearance
TABLE V
SCHEMATIC BASIS FOR THICKNESS-APPEARANCE RELATIONSHIP
Thickness Range Description
(nanometers)
Colorless Films
Up to 150
Onset of Color
Approx. 150
Pure Rainbow Colors 150 to 900
Dull, Impure Colors 900 to 1500
Films reflect more light than does water, and look
brighter. May need adjacent bare water for comparison.
Apparent brightness increases with thickness. At about
75 nm and thicker, a pearly or metallic luster is
usually apparent.
First color seen is a warm tone, more bronze than
yellow.
As film thickens, deep violet or purple appears,
these colors begin the first set of rainbow bands.
The set of bands around 300 nm are in the sequence:
bronze, purple, blue, green, in order of increasing
thickness. These colors are pure and intense.
The set of bands around 600 nm are slightly less
intense than at 300 nm, and have a modified color se-
quence: yellow, magenta (reddish violet), blue, green.
They are quite pure.
Main characteristic is reduction in number and
purity of colors. Colors at 900 nm are a rich terra
cotta (brick red) and turquoise (rather bright blue-
green). At 1200 nm and 1500 nm these colors are pro-
gressively duller or less pure looking. These sets of
bands may also contain a trace of white or pale
yellow.
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TABLE V (Continued)
Appearance
Thickness Range
(nanometers)
Description
Light and Dark Bands
with Little Color
1500 to 3000
Any color present is merely a tint in the light and
dark alternating bands.
At 1800 nm, the contrast between light and dark
bands is strong, but weakens as thickness increases.
At 3000 nm, it is apparent that interference effects
are weak, and they will quickly disappear as thickness
increases.
vo
ro
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7. GLOSSARY
cm centimeters
gm gram
lumen/ft^ intensity of illumination, same as foot-
candle
m meter
_3
mg milligram, 10 gm
_3
ml milliliter, 10 liter
-9 -3 °
nm 10 meters, 10 micron, 10 Angstrom units
00 -1-8
A Angstrom unit, 10 nm or 10 cm
X wavelength in nm
-4
y refractive index; also micron or 10 cm
yl microliter, 10 liter or 10~ ml
93
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8. REFERENCES
1. Federal regulation, "Part 610 - Discharge of Oil" issued
under Section ll(b) (3) of the Federal Water Pollution
Control Act, as amended (84 Stat. 92' 33 U.S.C. 1161),
and printed in the Federal Register, Vol. 55, No. 177,
September 11, 1970.
2. Bikerman, J.J., Surface Chemistry, Academic Press (New
York) 1958 (2nd Ed.); or other similar sources.
3. Manual on Disposal of Refinery Hastes: Vol. I Haste
Hater Containing Oil, pg. 10, American Petroleum In-
stitute, 1963 (7th Ed.).
4. Allen, A.A., and Schlueter, R.S., Estimates of Surface
Pollution Resulting from Submarine Oil Seeps at Plat-
form A and Coal Oil Point, Technical Memorandum 1230,
November 1969, General Research Corporation, Santa
Barbara, California.
5. Wobber, F.J., Imaging Techniques for Oil Pollution Sur-
vey Purposes, Photographic Applications in Science,
Technology and Medicine, Vol. 6, No. 4 (July 1971).
6. Orthlieb, F.L., LTJG, USCGR, Photographic Detection of
Ship-Generated Oil Slicks. Final Report, February 1971,
Coast Guard Office of Research and Development, Wash-
ington, D.C.
7. Short!ey, G. and Williams, D., Elements of Physics,
Prentice-Hall (Englewood Cliffs, New Jersey), 1955
(2nd Ed.); or other college physics textbooks.
8. "Pollution in Navigable Waters", Hearings before the
Committee on Rivers and Harbors, House of Representa-
tives, 71st Congress, 2nd Session on H.R. 10625, A Bill
to Amend the Oil Pollution Act of 1924. Part I.
May 2, 3, and 26, 1930, p. 41-49, Washington, U.S.
Government Printing Office, 1930.
95
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
w
THE APPEARANCE AND VISIBILITY OF THIN
OIL FILMS ON WATER,
Hornstein, B.
Environmental Protection Agency
Edison Water Quality Laboratory
National Environmental Research Center
Edison, New Jersey 08817
In-House Report
Environmental Protection Agency report
number EPA-R2-72-039,, August 1972.
Oil films of controlled thickness up to 3000 nanometers, upon water
surfaces in the laboratory, confirm an inherent and orderly thickness-
appearance relationship which is independent of oil type and water type.
These laboratory studies also investigated the effects of viewing condi-
tions upon the ease of visibility of these thin films.
Out-of-doors observations were made; these and the observations
reported by other sources were found to correspond with the laboratory
results. The visibility of a thin oil film depends not only upon its
thickness-dependent inherent appearance, but also upon conditions ex-
ternal to the film. These include nature of illumination and sky con-
ditions, sun angle, color and depth of water, color of bottom, and
viewing angle.
Color photographs are included for illustration of the points
discussed.
*0il-water interfaces, *Thin films, *0il, *Color, Theoretical analysis,
Laboratory tests, On-site investigations, Water pollution, Oil pollution,
Water pollution effects.
Visibility, Appearance, Iridescent films, Optical interference,
Reflectivity.
05A
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
US DEPARTMENT OF THE INTERIOR
WASHINGTON, D C. 2O24O
Bernard Hornstein
Edison Water Quality Laboratory, NERC
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