Infrared photography

The invention: The first application of color to infrared photography,
which performs tasks not possible for ordinary photography.
The person behind the invention:
Sir William Herschel (1738-1822), a pioneering English
astronomer
Invisible Light
Photography developed rapidly in the nineteenth century when it
became possible to record the colors and shades of visible light on
sensitive materials. Visible light is a form of radiation that consists of
electromagnetic waves, which also make up other forms of radiation
such as X rays and radio waves. Visible light occupies the range of
wavelengths from about 400 nanometers (1 nanometer is 1 billionth
of a meter) to about 700 nanometers in the electromagnetic spectrum.
Infrared radiation occupies the range fromabout 700 nanometers
to about 1,350 nanometers in the electromagnetic spectrum. Infrared
rays cannot be seen by the human eye, but they behave in the
same way that rays of visible light behave; they can be reflected, diffracted
(broken), and refracted (bent).
Sir William Herschel, a British astronomer, discovered infrared
rays in 1800 by calculating the temperature of the heat that they produced.
The term “infrared,” which was probably first used in 1800,
was used to indicate rays that had wavelengths that were longer than
those on the red end (the high end) of the spectrum of visible light but
shorter than those of the microwaves, which appear higher on the
electromagnetic spectrum. Infrared film is therefore sensitive to the
infrared radiation that the human eye cannot see or record. Dyes that
were sensitive to infrared radiation were discovered early in the
twentieth century, but they were not widely used until the 1930’s. Because
these dyes produced only black-and-white images, their usefulness
to artists and researchers was limited. After 1930, however, a
tidal wave of infrared photographic applications appeared.The Development of Color-Sensitive Infrared Film
In the early 1940’s, military intelligence used infrared viewers for
night operations and for gathering information about the enemy. One
device that was commonly used for such purposes was called a
“snooper scope.” Aerial photography with black-and-white infrared
film was used to locate enemy hiding places and equipment. The images
that were produced, however, often lacked clear definition.
The development in 1942 of the first color-sensitive infrared film,
Ektachrome Aero Film, became possible when researchers at the
Eastman Kodak Company’s laboratories solved some complex chemical
and physical problems that had hampered the development of
color infrared film up to that point. Regular color film is sensitive to
all visible colors of the spectrum; infrared color film is sensitive to
violet, blue, and red light as well as to infrared radiation. Typical
color film has three layers of emulsion, which are sensitized to blue,
green, and red. Infrared color film, however, has its three emulsion
layers sensitized to green, red, and infrared. Infrared wavelengths
are recorded as reds of varying densities, depending on the intensity
of the infrared radiation. The more infrared radiation there is,
the darker the color of the red that is recorded.
In infrared photography, a filter is placed over the camera lens to
block the unwanted rays of visible light. The filter blocks visible and
ultraviolet rays but allows infrared radiation to pass. All three layers
of infrared film are sensitive to blue, so a yellow filter is used. All
blue radiation is absorbed by this filter.
In regular photography, color film consists of three basic layers:
the top layer is sensitive to blue light, the middle layer is sensitive to
green, and the third layer is sensitive to red. Exposing the film to
light causes a latent image to be formed in the silver halide crystals
that make up each of the three layers. In infrared photography, color
film consists of a top layer that is sensitive to infrared radiation, a
middle layer sensitive to green, and a bottom layer sensitive to red.
“Reversal processing” produces blue in the infrared-sensitive layer,
yellow in the green-sensitive layer, and magenta in the red-sensitive
layer. The blue, yellow, and magenta layers of the film produce the
“false colors” that accentuate the various levels of infrared radiation
shown as red in a color transparency, slide, or print.relationship to the color of light to which the layer is sensitive. If the
relationship is not complementary, the resulting colors will be false.
This means that objects whose colors appear to be similar to the
human eye will not necessarily be recorded as similar colors on infrared
film. A red rose with healthy green leaves will appear on infrared
color film as being yellow with red leaves, because the chlorophyll
contained in the plant leaf reflects infrared radiation and
causes the green leaves to be recorded as red. Infrared radiation
from about 700 nanometers to about 900 nanometers on the electromagnetic
spectrum can be recorded by infrared color film. Above
900 nanometers, infrared radiation exists as heat patterns that must
be recorded by nonphotographic means.
Impact
Infrared photography has proved to be valuable in many of the
sciences and the arts. It has been used to create artistic images that
are often unexpected visual explosions of everyday views. Because
infrared radiation penetrates haze easily, infrared films are often
used in mapping areas or determining vegetation types. Many
cloud-covered tropical areas would be impossible to map without
infrared photography. False-color infrared film can differentiate between
healthy and unhealthy plants, so it is widely used to study insect
and disease problems in plants. Medical research uses infrared
photography to trace blood flow, detect and monitor tumor growth,
and to study many other physiological functions that are invisible
to the human eye.
Some forms of cancer can be detected by infrared analysis before
any other tests are able to perceive them. Infrared film is used in
criminology to photograph illegal activities in the dark and to study
evidence at crime scenes. Powder burns around a bullet hole, which
are often invisible to the eye, show clearly on infrared film. In addition,
forgeries in documents and works of art can often be seen
clearly when photographed on infrared film. Archaeologists have
used infrared film to locate ancient sites that are invisible in daylight.
Wildlife biologists also document the behavior of animals at
night with infrared equipment.