ULTRAVIOLET and INFRARED Photography Summarized
A large part of the spectrum and its relationship to the world around us is
invisible because we are limited to seeing electromagnetic wavelengths which
extend only from those characteristic of violet-blues to those of the deep
reds. Photographic materials can extend our vision, especially when aided by
special light sources and various filters. The proper use of film, lights,
filters, exposure techniques and specific applications of the four basic
methods of photographing the invisible spectrum are discussed below.
Here is one example where I used infrared photography to yield useful
information to art historians about the "pentimenti" or underpaintings and
underdrawings in a painting by Velazquez called "The Forge of Vulcan" which is
located in the Prado Museum in Madrid, Spain.
The premise for photographing by REFLECTED ULTRAVIOLET or REFLECTED INFRARED
radiation is that we desire to "see" the interaction, by way of reflected
energy from our particular subject, of UV or IR rays as compared to the effect
on the same subject of visible light rays. The hope is that the subject may
appear differently by these rays than it does by "white" light or than it would
appear if seen through colored filters.
An analogy for the reason to apply these techniques is the lengths to which we
go to render a blue sky dark on a B&W print to contrast it against the white
clouds. This, of course, is accomplished by placing a yellow or red
(complementary colors to blue and cyan, the color of the sky) filter over the
camera lens. The filter does not allow the sky color wavelengths to pass on to
the film and thus the sky is rendered clear on the negative and dark on the
print. The point is that we can distinguish between two subjects of similar
tone, the sky and the clouds in this instance, but different color by filtering
out one of the colors with a filter of complementary color placed over the
camera lens. This principle applies also to the invisible areas of the spectrum
but we need films which can "see" in these areas in order for our own eyes to
see the differences if they exist.
Since UV and IR wavelengths are invisible to our eyes and thus can not be
assigned a "color" as such, B&W film is the most appropriate to use for both of
the above applications. Almost any B&W film can be used for reflected UV
photography although the slower emulsions seem to deliver somewhat better
negatives. To record the IR wavelengths Kodak High Speed Infrared film should
be used. It is also about the only IR emulsion readily available! Since the
film has no antihalation backing allowing light and IR to pipe into the
cassette it should be loaded into the camera in total darkness or under subdued
fluorescent illumination. Fluorescent tubes usually are very weak in IR output
diminishing the chance for fogging the film by IR rays passing into the
Associated with the lack of antihalation layer in Kodak High Speed Infrared
film, the fact that IR can pass through the film base largely unimpeded, this
sometimes causes artifacts to appear on the negatives. Pressure plates that
have a dimpled pattern embossed on them will cause a similar pattern of dots to
appear on the film and cutout areas in an otherwise smooth pressure plate may
also leave a matching impression but of lower density than the surrounding
area. Both of these can be prevented by covering the pressure plate with a
thin, smooth, IR absorbing material. Kodalith or other litho film, exposed and
processed to maximum density should work well for this purpose. It should be
attached with the emulsion side facing away from the pressure plate.
When attempting either UV or IR photography the subject must be lit with lights
which emit the wavelengths by which we wish to photograph.Electronic flash is a
good source of both UV and IR wavelengths. Some flashes have a UV absorbing
filter incorporated in the flash head and this may need to be removed (if
possible) for best results. However, even if not removed, there is usually
enough UV that "leaks" through so that at close range they still serve as
useful UV sources. Then, placing a UV transmitting filter (Wratten 18A) or IR
transmitting filter (Wratten 87 or 87C)over the camera lens will effectively
expose the film only to the wavelenghths of interest.
Under certain conditions it is possible to previsualize the approximate
appearance of a UV scene by installing into the camera a groundglass covered
with a fluorescing substance. The UV energy passing through the lens causes
this substance to fluoresce and the UV scene becomes visible in the finder!
This is also possible for IR but to a much less satisfactory extent by using screens
that exhibit a "reverse" fluorsescence effect (Stokes effect) whereby a
material illuminated with IR fluoreseces in the visible regin. There are also
evaporation based systems where the IR-formed image may be examined visually or
In order to continue to use a 35mm SLR camera for reflected IR photography even
when you use the visually opaque 87 filter for photography it is possible to
install the filter behind the camera's mirror mechanism and composition of the
IR picture becomes almost as easy as if you were using "light" for your
pictures. In some cameras with behind the mirror metering systems the filter
may need to be positioned just in front of the film between the film plane
guide rails. Metering off the film may be impossible.
While exposure is most appropriately determined in either case by making a set
of test exposures or by bracketing it may be possible to establish a UV or IR
"personalized speed index" by using one of the new ultrasensitive light meters
but metering through the UV or the IR filter and correlating the exposures that
seem to deliver negatives which you judge acceptable by your own standards with
those suggested by the meter. You should generally only attempt to establish
this personalized speed index metering in the reflected mode. Finally this does
not preclude bracketing but you may more consistently arrive at the proper
exposure with less waste.
The focal length of a given lens "changes" with wavelength and for this reason
its focus should be adjusted slightly when attempting to use it for other than
light wavelengths. There is a common misconception about the direction in which
this adjustment needs to be made.
In the case of IR the lens must almost invariably be moved somewhat further
from the film than visual focus demands. While simple lenses need to be moved
somewhat closer to the film in the case of UV rays, standard camera lenses need
most often to be moved also further from the film just as is the case with IR.
This is because most camera lenses are of achromatic color correction (unless
otherwise stated to be simple, very unlikely, or apochromatic in correction)
and bring two visible (except in some cases with lenses intended specifically
for UV photography) wavelengths to a common focal point. Since they "curve" the
chromatic focal plane about these two wavelengths the result is that both IR
and UV come to a focus farther from the lens than the visible rays. The use of
small apertures will in most cases diminish or eliminate the need to make a
focus adjustment. When using a fluorescing screen in UV photography the UV rays
are properly focused when the image on the screen is sharp.
I've already mentioned the term "fluorescence", and most of us are aware of the
term but may not be quite sure of what process is at work. When some subjects
are illuminated by certain wavelengths they reflect back not only the same
wavelenghts that they are illuminated by but they may transform some of these
incident wavelengths into usually longer ones. In a sense they change the color
of the light falling on them. When a subject behaves like this it is said to
FLUORESCE. Some subjects change short, ultraviolet energy into longer, visible,
wavelengths or colors. Others may change visible rays into yet longer, infrared
wavelengths. The former effect is, of course, visible while the latter is not.
This is the realm of FLUORESCENCE photography and to apply it you need to start
with a source that contains the wavelengths which the subject will transform
into longer wavelenghts. Usually over this source is placed an EXCITER filter
the function of which is to allow only those wavelenghts through which will
cause fluorescence. In UV work this is most commonly the Wratten 18A filter and
in the visible region of the spectrum the Corning filter #9788 can be used. To
photograph fluorescence excited by UV, which usually results in a visible
effect, color film can be used to good advantage with Kodachrome 64 being
paricularly suitable. Since not only the "new", visible, wavelenghts are
reflected from the subject but also some of the UV transmitted by the exciter
filter, a UV blocking filter, called a UV barrier filter, is placed over the
camera lens to allow the film to record only the fluorescence. For UV work this
barrier filter is the 2E or similar, pale yellow, filter. Exposure may possibly
be metered in the camera especially with the newer more sensitive in-camera
meters. No focus compensation is necessary.
Sometimes startling results can be obtained by using Infrared Fluorescence
(or as H.Lou Gibson calls it: IR luminescence) and one such example is
illustrated above where several documents were prepared by the parents of a
family and intended for their children to be read in the future. These were
then placed in a home-made time box and buried in the family lawn for 25 years.
The father wrote with a typewriter on one side and the mother with pen and ink
on the other. The typewritten message survived the intrusion of water into the
box, but the ink the mother used was hopelessly washed away so that her message
"into the future" was visually quite indiscernible. IR fluorescence made her
message reappear to the delight of the children (grown up by then).
To photograph IR fluorescence (or as H.Lou Gibson calls it: IR luminescence)
the B&W infrared film should be used. The exciter filter placed over the light
source must not allow any infrared rays through to the subject. The Corning
9788 is just such a filter and it can be obtained from the Corning Corporation.
It is a glass filter and since it will be used over the light source it can be
of low optical quality. Even so, it is almost twice expensive as the 18A filter
(which should be of optical quality) although for a much larger size. Six
inches square vs. three inches square for the 18A, at a price of about $75 for
the 9788 and $150 for the 18A. The barrier filter should be a deep red or
infrared filter such as the 87 or 87C. Camera focus should be adjusted as per
previous discussion. Metering is not possible because the effect ocurrs in the
IR and most meters are designed to meter light, not infrared. Even meters with
IR metering capability would probably fail to detect the presence of IR
fluorescence because of the very low amounts of IR produced in this fashion.
The four techniques briefly summarized above have widespread application in
document investigations and forensic photography, in surveilance and in
environmental studies. Also in medicine, mineralogy, philately, art history,
Generally one sets out on a voyage of discovery when faced with a new subject.
One tries each method in turn, hoping that one of them will yield better
information about the subject than that which the unaided eye can perceive.
Each new subject becomes a most exciting photographic adventure.
TO PHOTOGRAPH BY REFLECTED ULTRAVIOLET
The sample is illuminated by "white" light and an 18A filter is placed over the
camera lens, tightly fitted. If the subject does not fluoresce then you can
place the filter over the light but you need to work in a dark room.
This technique is useful for the photographic enhancement of rashes and other
skin disorders and also for detecting alterations in documents, etc.
Suggested light source: Electronic Flash (best if flash does not have UV
Filter on camera : 18A
Appropriate Film : Most any B&W film, especially slower emulsions
Lens : Should be able to transmit UV. Most camera
lenses are suitable for long wave UV associated
with use of 18A filter over lens.
Under special conditions, such as when a short wave UV source is available,
photography is done in the dark and the 18A filter is not used. Subjects that
fluoresce will be recorded as a mixture between reflected UV and visible
fluorescence. When photographing with short wave UV (around 254nm or less)
energy, typically provided by special sources, many standard camera lenses are
unsuitable because the glasses they are made of absorb such wavelengths. Quartz
or mirror lenses may need to be used although pinholes and some plastic lenses
are an alternative. Under very short wave UV conditions the gelatin of the
emulsion itself may absorb the incident rediation. In this case special
emulsions with silver halides deposited on the surface are used.
Exposure : Generally determined by trial.
Focus : Camera focus must be adjusted for critical use.
TO PHOTOGRAPH FLUORESCENCE EXCITED BY UV RADIATION
In its most common practice the sample must be irradiated with UV rays and due
to fluorescence it will "GLOW" in the visible region of the spectrum. The
sample will appear in various colors as a result of the effect the sample has
on the incident energy. Because samples also reflect some UV as well as
fluorescing, the unwanted UV is removed with a "barrier" filter opaque to UV.
The filter which limits the incident energy to a desired spectral region is
known as the "exciter" filter. This technique is particularly useful for the
identification of minerals and for "fingerprinting" documents such as stamps
and currency. It is also used in forensic work with fluorescing powders in
Light source suggested : Electronic Flash or UV emitting lamps
operating in a darkened environment.
Exciter filter suggested: with lamps it's built in so none needed
with electronic flash use 18A over flash.
Barrier filter suggested: Wratten 2A or 2E
Appropriate film : Color reversal daylight film seems best.
Effect is in color so B&W film less useful.
Exposure : Can often be determined with built in meter but light level
is much lower than it might appear visually.
Focus : Visual focusing on groundglass possible.
TO PHOTOGRAPH BY REFLECTED INFRARED
The sample can be illuminated by "white" light and an 87 or 87C 9or similar)
filter is placed over the camera lens, tightly fitted. In a darkened room the
filter may be placed over the light source. Sometimes a filter is not required.
For example, a flatiron may be used as a source of IR radiation and if
operating in a dark room no filters are needed.
Some photographers use a #25 filter over the lens of SLR cameras in particular
so that they can focus on the image in the groundglass. Alternatively, the
visually opaque IR filter may be inserted behind the camera's mirror.
This technique is useful for the recording of subcutaneous (below skin) veins,
for the detection of alterations or forgeries of documents, for
Light source suggested : Electronic Flash, floodlights or daylight.
Filters suggested : 87 or 87C over camera lens.
Film : Kodak High Speed Infrared
Exposure : Determined by trial, use manufacturer suggested data,
or using CdS Luna Pro and metering through 87C filter
set ASA guide to a speed of 2400.
Focus : For critical results it must be adjusted.
TO PHOTOGRAPH FLUORESCENCE EXCITED BY LIGHT IN THE INFRARED (LUMINESCENCE)
As in fluorescence excited by UV certain samples transform shorter wavelengths
into longer ones which in this case are in the invisible IR region. This
effect is sometimes called "luminescence" and the sample is usually irradiated
with BLUE-GREEN light without any IR present in the beam. A barrier filter
which allows only the newly created IR wavelengths to pass into the camera is
placed over the lens. Best to work in a dark, IR free, environment.
This technique is particularly useful for the study of inks, hardwoods and
forgery detection in forensic photography.
Light source suggested : Tungsten or Electronic Flash.
Exciter filter suggested: Corning 9788 over light source plus Corning
3966 heat absorbing filter placed between
source and the 9788 if using tungsten source.
Certain blue-green lasers also may be used
without any filters.
Barrier filter suggested: Wratten 87, 87C or 88A
Film : Kodak High Speed Infrared
Exposure : Determined generally by trial. Expect it to
be extremely great.
Focus : Must be adjusted for critical results.
Note: when IR reflected light photography is done with COLOR INFRARED film,
then "white" light plus IR must fall on the sample, thus a Wratten #12 is used
over the camera lens to remove from the incident radiation the blue light to
which the three layers of the IR COLOR film are all sensitive.