|Films & Filters
“... and everything looks worse
in black and white.”
- Paul Simon, Kodachrome
On the most basic terms, the
three layers in film can be thought of as three black and white emulsions
containing metal halide grains layered onto a flexible base. The layers
consist of a blue sensitive layer (yellow dye), a green sensitive layer
(magenta dye) and a red sensitive layer (cyan dye). The amount of dye
released by the light passing though each of the layers is inversely
proportional to the amount of light of a particular wavelength (color)
is received from the scene, i.e. the more blue there is the less yellow
dye is released by the blue sensitive layer and since these layers are
on top of each other, there will be more magenta and cyan in the layers
underneath. The more green there is the less magenta dye, and consequently
more yellow and cyan. The three dye layers, when added together, produce
the colors we see blue, green and red. These are called "color reversal
Below are some diagrams that show the
basic cross section structure of film and the spectral sensitivities
of the layers. The top layer is sensitive to the blue wavelengths,
the yellow dye layer blocks the passage of blue light after it "exposes"
the metal halide grains in the blue layer, so only the green and red
light passes through, after the green wavelengths pass through the
green sensitive layer they are blocked so that only red light is left.
The second layer is sensitive to green
and blue light and the third is sensitive to red and blue light. Since
all of the layers are sensitive to blue light a blue blocking filter
layer (yellow) is added under the first layer which effectively stops
blue wavelength photons from penetrating past the blue sensitive layer;
it blocks blue light from getting though to the green sensitive layer
(which produces magenta color dye) and the red sensitive layer (which
produces cyan colored dye). Under these layers is a base and an antihalation
backing which stops light from reflected off of the back of the film
and going back up though the layers and provides some structural rigidity
to the film.
1. Simple Cross Section
2. Color Absorption
3. Color Film Layers
1. Simple Cross Section: shows the
emulsion with metal halide grains, the base (which constitutes the
majority of the thickness of film) and the antihalation backing.
2. Color Absorption: film cross section
with the blue blocking layer and different color dye layers labeled,
and the corresponding absorption curves according to the wavelengths
for each of the dye layers.
3. Color Film Layers: color film cross
section showing all of the layers and a description of each. Some
terminology used to qualitatively describe color; Hue - the property
of a color that distinguishes it from another color, i.e. red, blue
or green, Chroma/Intensity - saturation, the purity or vividness of
a color, Value - distinguishes the relative presence or absence of
Reversal, RGB sources and CMY negatives.
7. Film Sensitivity
4. Exposure: simple diagram showing
the exposure of film, the resulting colors and the negative image
that is recorded on the exposed film. 5. Developing: diagram of the developing process with
the removal of the colors, and the resulting developed positive color
6. Color Reversal, RGB sources and
CMY negatives: diagram illustrates the separate wavelengths that are
received by each of the film layers (RGB), the resulting negative
images for each (CMY), and the positive image that is produced.
7. Film Sensitivity: shows the result
of films with different sensitively to red light, note the intensity
of the barn in each image.
color reversal concept may sound confusing, and it is, but with a little
practice, or brute-force memorization, you can learn it. Just remember
that the colors (wavelengths) of light mix differently then the watercolor
paints you used in grade school.
First described by James Clark Maxwell
in the mid 1800s, the color additive theory describes how be perceive
color and how colors are created. Essentially white light is a combination
of many different colors, a continuum of wavelengths organized into
"bands" which we label with names (blue, green, red etc). When equal
parts of each of the three major bands are combined you get white
light. White light is the sum of red, green and blue.
Red, green and blue are the "primary"
colors of white light. All three colors will result in white, the
absence of all three will produce black.
When two primary colors of light are
added together, you get a color that is brighter than either of its
These are the "additive" combinations:
Red + Green = Yellow
Red+ Blue = Magenta
Blue + Green = Cyan
By using unequal amounts of red, green
and blue light you can create new colors. Using red, green and blue,
the entire spectrum of visible light can be created.
A TV monitor uses additive color. Three
beams of electrons corresponding to red, blue and green are projected
onto a fluorescent screen. The pixels of the screen are made of triads
which are sensitive to the three colors, based on the proportion of
red, blue or green light striking the triad the pixel can appear in
any single color. [more
So far light has been referred to with
terms like "color", "wavelength" and "photon". These terms will be
reviewed in greater detail in another module but a basic explanation
is required to understand the usage of these terms here. Basically
the physics explanation of what light is involves two ideas; the first
is that light has wave like properties (refraction of colors
in a prism) and particle like properties (the heating of a surface
in the sun). The wave explanation allows for the splitting-up of the
electromagnetic spectrum into colors (UV, Blue, Green, Red, Infrared,
etc.) and the particle (photon) explanation allows for the explanation
of the energy imparted to a surface by light of different wavelengths
(UV photons have the most energy, shortest wavelength, and infrared
have the least, longest wavelengths, etc).
Molecular scattering takes place in
the atmosphere between the camera and the ground and has the tendency
to reduce clarity. UV and blue wavelength photons that have been scattered
by molecules in the atmosphere, called "diffuse skylight", enter the
camera lens along with the photons reflected from scene and blur the
image. Filters are used to reduce the film's exposure to these shorter
blue and UV wavelengths. IR films (longer wavelengths) are less sensitive
to atmospheric scattering, and are often used in conjunction with
filters to increase clarity.
It is necessary to know how to read
spectral response graphs. Along the x axis is wavelength in micrometers
(um) or nanometers (nm) usually for visible wavelengths, and along
the y axis is the percent reflectance (or absorption). Having such
graphs allows for the separation of characteristic materials within
an image based on the "brightness" or percent reflectance; if you
know very little about an area, but have near infrared wavelength
imagery, it is possible for you to "classify" certain features based
only on their reflectance, and it is possible to discriminate healthy
vegetation, tree types, soil/rock types etc. based solely on how bright
they appeared in the image using the spectral response curves for
these materials as an interpretation aid.
Below is a diagram showing the spectral
response (percent reflectance) of two film types, natural color and
color infrared (CIR). Notice that with the CIR film the shorter wavelengths
are blocked (yellow hashed lines). This is done using filters which
serve to enhance clarity by blocking diffuse skylight; the glass of
the filter itself appears yellow in color (not blue). Reflectance
as a function of wavelength is an important fundamental concept, the
following graphics show the different reflectance characteristics
for common generalized surfaces.
8. Natural Color and CIR
9. Generalized Reflectance
10 a. Panchromatic
vs. Color Infrared (CIR)
10 b. Spectral Signatures
of Conifer and Hardwood Trees (Pan vs. CIR)
8. Natural Color and CIR Film: spectral
response curves and the corresponding image, note the effect of the
filter on the infrared photography which blocks UV and blue wavelength
light improving image clarity.
9. Generalized Reflectance Curves:
graphic shows the idealized reflectance characteristics for common
surfaces, note the "green blip" and the "red dip" in the visible wavelengths
for vegetation, also note the strength of the reflectance in the IR
wavelengths. In order for a surface to be "separable" the reflectance
must be significantly different, so the further separated the curves
are, for a particular wavelength, the more separable the features
10a. Panchromatic vs. Color Infrared
(CIR): this pair of grayscale images illustrates the difference between
CIR and black and white (panchromatic) photography, note the differences
in the trees and row crops.
Spectral Signatures of Conifer and Hardwood Trees (Pan vs. CIR): This
generalized plot of the reflectance of Hardwood and Conifer trees
for the different film types and the corresponding wavelength ranges
shows how spectral seperability depends on film type.
filters it is possible to produce a wide array of images with different
degrees of brightness, saturation and tone/color variation. Filters
can selectively block different wavelengths, this enables color infrared
and natural color photography to capture specific reflectance characteristics
of vegetation or other features of interest. With a carefully planned
and executed aerial photography mission it is possible to determine
the exact specifications that are optimal for your purpose. Basic knowledge
of film and filter combinations will allow you to make educated choices.
Professional aerial survey firms will be able to determine the exact
combination of film and filter for you. The diagrams below describe
the properties of some common filters and the sensitivities of the dye
layers for generalized natural color and CIR film. Notice the extended
sensitivity of the CIR film, and the broader range of shorter wavelengths
that are blocked by the filter.
11. Common Filters and Wavelengths
12. Color Film and UV (Haze)
13. Color Infrared (CIR) Film,
Yellow (Blue) Blocking Filter
11. Common Filters and Wavelengths:
the number designations are used to label the filtering of certain
wavelengths, these are standardized filter ratings.
12. Color Film and UV (Haze) Filter:
diagrams show the filtered wavelengths and the sensitivity of each
of the layers according to wavelength for natural color film.
13. Color Infrared (CIR) Film, Yellow
(Blue) Blocking Filter: diagrams
show the filtered wavelengths and the sensitivity of each of the layers
according to wavelength for color infrared film.
image medium type provides the most detailed image?
The Developing Process
Diapositives, also called "color positive
transparencies", are the highest resolution photographic medium available,
they are used for fine scale precision photogrammetric projects requiring
exacting precision and interpretability. A semi-opaque glass table
with a backlight are necessary, as well as high magnification viewing
equipment. Diapositives are expensive to have produced (more than
$100 each) and for most routine interpretation or mapping purposes
they are not necessary.
film is an art. Although machines process conventional film effectively,
sometimes they don't work. Aerial photography is developed by hand,
experienced dark room personnel are extremely valuable people because
they have learned from the experience of developing hundreds, maybe
thousands, of air photos what the ideal balance of tones and colors
for a particular area should look like. Achieving the "best" positive
image from an aerial photograph requires a good eye for color and knowledge
of how to correct for certain factors like lighting geometry changes
along a flightline, overcast conditions, or color hues in vegetation
or soil that show-up in aerial photography as a result of the lighting
geometry. It is for this reason that when a flight is contracted from
an aerial photography firm that you, the client, have the right to inspect,
and in rare cases reject, the final products. Aside from choosing the
optimal film, filter and lens combination, the process of actually developing
the positive images is very important.
Type of Conventional Aerial Imagery
Care in the developing of the photography
is important for interpretation purposes. In some cases CIR photography
may be "over developed", the red hues may be exaggerated giving the
interpreter a false impression of vegetation health. This problem
is especially acute with the interpretation of wetlands. Using aerial
photography to map natural landscapes requires field experience, or
on the ground images in some cases, to develop a sense or "signature"
of what the features of interest look like. Time of year, especially
in the case of wetlands, is a very important consideration. The ideal
time to have a site flown may be different for different types of
vegetation. Leaf-on or leaf-off photography, as well as when certain
species come in to bloom, may be required. For this reason locals
and experts who are familiar with the study site should be consulted
prior to planning an aerial photography mission.
Aerial photography is typically
Color Infrared Film (CIR)
- black & white (panchromatic, grayscale)
- natural color (red, blue and green)
- color infrared (false color, black
Color infrared film is often
called "false-color" film. Objects that are normally red appear green,
green objects (except vegetation) appear blue, and "infrared" objects,
which normally are not seen at all, appear red. The quality of the film/camera,
time of year, climatic conditions and how the film is developed influence
how landscapes appear in CIR photos. The primary use of color infrared
film is for studies involving vegetation such as wetlands mapping or
ecosystem monitoring. Healthy green vegetation is a very strong reflector
of infrared radiation and usually appears bright red on color infrared
photographs (depending on how the film is developed). Color Infrared
(CIR) uses special film, lenses and filters to capture reflected near
infrared energy (sometimes called Near IR), NOT emitted thermal infrared
(heat). A common misconception about "Infrared Photography" is that
it is associated with heat - this is wrong. Color Infrared (CIR) photographs
capture sunlight that has been REFLECTED from the surface, NOT emitted
(that's Thermal IR).
The graph below shows the wavelength
ranges and describes an important concept for remote sensing - atmospheric
absorption and reflection. The maximum reflectance from earth's surface
is in the UV and Visible wavelengths, notice that the variability
in the reflectance in the non-visible wavelengths beyond red is highly
variable. CIR film responds to the chlorophyl in plants, green and
red reflectance, as well as plant moisture content due to the structural
changes in the leaf it controls (leaf thickness). Filters are used
to enhance the film's sensitivity to infrared reflectance.
Wavelength Ranges Labeled and
Visible and IR Wavelengths,
Below are three images of a power plant.
Wavelengths in the natural color visible (400-700 nm), photographic
IR (0.7-1.3 um) and the thermal infrared (3-14 um) parts of the electromagnetic
Thermal (not a photograph!)
why is my hat red?
CIR vs Natural Color Photos
Lines Per Millimeter
Resolution for aerial photographs
is determined by a number of factors, some of which you can control
and some you can't. Those factors that cannot be controlled are the
weather, the air quality (dust and smoke) and in some instances the
visibility of the target(s). The factors that can be controlled are
the time of year, the geometry (though altitude, lens, film format)
and stereo overlap. Another consideration is the film type and filters
because they can have a strong influence on the resulting image quality.
The resolution of a camera system is measured
using a standardized scheme that is based on "lines per millimeter".
The quality of a lens is often measured with this quantity. Basically
the "test pattern" (see below) is photographed from a fixed altitude,
and the number corresponding to the smallest number of line pairs visible
(per millimeter) is recorded.
Film manufacturers sometimes use "MTF-curves"
(Modulation Transfer Function) on their films. The "lines per millimeter
resolution" is also used to describe lens capabilities, but for most
traditional ground-based photography MTF-curves are more useful than
the line pairs per millimeter because lines per millimeter can be
measured with different light situations. For aerial photography however
resolving power is the most important consideration. Lines per millimeter
will remain a standard upon which resolution is based for a long time
wavelengths are the shortest of the normal photographic range of the
electromagnetic spectrum, they can be acquired with standard black and
white film as well as specialized film and pure lenses and filters especially
manufactured for UV light. Filters for UV photography are glass only,
film based emulsion filters do not work. Ultraviolet photography is
used by crime scene analysts due to the different absorption and reflectance
properties of some substances or interest. Bruises and other markings
that may have been removed, or are not apparent on the skin surface,
can be photographed in the ultraviolet because these wavelengths penetrate
the skin slightly.
Nature photographers use UV photography
as well. Insects and flowering plants have been studied using UV photography.
The coevolution of insects and plants has led to some very interesting
Ultraviolet aerial photography is also
used to census animals in snow environments. The fur of what appear
to be white animals, like the baby seals, is dark black in UV wavelengths
(because their fur absorbs those wavelengths), so they contrast very
sharply against a snow background (which reflects UV). UV aerial photography
is also useful for identifying oil on the surface of the water. This
is because UV wavelengths are sensitive to the smoothness of the water
surface; oil on the water surface tends to smooth small capillary
waves caused by wind, the smoother surface causes a contrast in reflectance
with the surrounding water.
Below are some diagrams and images
that pertain to UV photography. UV filters are used with natural color
and CIR photography to screen out UV wavelengths so they do not blur
the images. UV wavelengths reveal certain surface features better
than other wavelengths, as is the case with salt flats.
Common UV Filters
UV Imagery of salt (left most
image) in Angstron units (Å), 1 Å = 10^10
Left: A) Normal River Water, B)
Diesel, C) Gas, D) Oil
Boat wake through diesel spill
Left: normal beside UV photo,
showing locations of white baby seals.
normal photo of a white animal hide sled, and a UV photo.