Written by Arthur H. Borchers
forensic investigation require accurate documentation. The actual scene of any
incident cannot be frozen in time and brought before a trier of fact sometime
in the future. The best way to preserve a scene is through detailed
admissible photographs be true and accurate representations of the original
scene. Many photographers understand their cameras are recording only the
visible light their eyes can see without understanding the complexities of how the
light and their cameras interact.
Sir Isaac Newton
showed what we take to be white light when passed through a prism comes out as
rainbow: red, orange, yellow, green, blue, indigo, and violet. Now it is known that light bends when moving from air to glass (or other medium) and
exits in its component colors. Visible light and its inherent colors are part
of the electromagnetic spectrum and expressed as wavelengths (Figure 1)
between 380-400 and 700 nanometers (nm)—or, in slightly more understandable
terms, 0.0004 to 0.0007 millimeters.
At both ends of
the visible light range are infrared (IR), or “below red,” and ultraviolet
(UV), or “beyond violet.” The infra/below and ultra/beyond prefixes derive from
the light’s frequency value, not the wavelength. UV-IR areas of the spectrum
are not generally perceptible by the human eye. Some snakes can sense IR or
heat sources, even in total darkness. Birds and insects can see in the UV range
to better visualize flowers and food sources. Both IR and UV have forensic
applications, but capturing images utilizing their illumination requires
special care and equipment.
Light striking an
object can be transmitted, reflected, absorbed, or converted (Robinson, 2010). Humans
perceive the color of an object because the object either absorbs or reflects
visible light: an apple absorbs all light except red, which is reflected back
to our eye; grass reflects green light; and black objects reflect no light.
Glass transmits light. Tinted glass transmits only specific wavelengths and
eliminates all others. Filters which exploit the transmission property will be discussed
a bit later.
Forensic UV has
been in use for years because UV light will cause organic and other chemical
compounds to fluoresce. Fluorescence is a reaction to UV light where a
substance absorbs the UV photons, atoms in the substance are excited, and then they
emit a photon of a longer wavelength—typically in the visible spectrum.
Commercially, this can be seen in the use of “whiteners” in laundry detergent
and fluorescent highlighters and markers that appear to “glow.” The color of
the fluorescence is related to the chemical structure of the substance being
UV light is also broken
into three general bands: UV-A, UV-B, and UV-C. UV-A, also called longwave or
near UV, has a wavelength of about 320 to 380-400 nm. UV-B (mediumwave UV)
falls between 280 to 320 nm. UV-C (shortwave UV) ranges between 200 to 280 nm. Most
of the UV light that reaches the Earth from the sun is UV-A from 315 to 380 nm because
all lower wavelength light is blocked by our atmosphere. UV-C is not naturally
occurring on Earth and is dangerous to humans. UV-C is used as a commercial
disinfectant in airplanes, hospitals, some office environments, and water-treatment
facilities. The UV-C light destroys the DNA material within bacteria, viruses,
and protozoa. For forensic purposes, use of UV-C lamps should only occur on a
crime scene after any DNA search and recovery.
Below 300 nm,
normal optical glass and its coatings absorb UV light. So, when using this
light for imaging purposes, specialized lens materials like quartz or calcium
fluorite are required. Lenses with these materials are quite expensive, often costing
several thousands of dollars. Modern camera lenses are designed to reflect UV
away from the sensor. The lower-wavelength UV photons are higher-energy than
both visible and IR light, making its use dangerous to the eye and exposed
skin. UV-protective glasses and high-SPF sunscreen protection are required.
Forensic UV light
sources typically operate in the UV-A range and some visible light may also be
emitted. A yellow filter is often used on the camera to enhance the fluorescent
image by eliminating the bluish light.
IR light is
divided into three common bands: IR-A, IR-B, and IR-C. IR-A, also called Near
IR, has a wavelength range between 700 and 1400 nm. IR-B extends from 1400 to
3000 nm. IR-C is between 3000 nm and 1 mm. For photographic purposes, IR-A is
the only range used. The IR laser in most laser scanners operates in the IR-B
range, at 1550 nm.
In the film era,
UV and IR images could only be captured with specifically formulated media. UV
was captured only on black-and-white film. IR film was extremely sensitive and
had to be loaded into the camera in complete darkness. The filter used, Kodak
Wratten 18A, is no longer manufactured. The replacement filters, the U-360 and
UG-11, are flawed with a near-IR light leak. For black-and-white film, that was
not a significant issue as the film was not very red-sensitive. Using those
filters on a digital sensor did not produce the expected results, primarily due
to a lack of understanding about how digital sensors work.
In simple terms, a
digital sensor detects focused light falling on its surface. A sensor has
millions of photodiodes—or photosites—arrayed
across its surface. A patterned color filter covers the sensor and makes each photosite sensitive
to only one wavelength of light: red, green, or blue (RGB). The most-used
pattern is called a Bayer Filter (Figure 3).
If the light falling onto a photosite with a red filter has a red component, the light is converted to an electrical charge. The information from the red, blue, and two green photosites are resolved to form the color information for an individual photo element, or pixel, in the final image. Sensors are typically sensitive to light wavelengths from about 350–1050 nm. A special filter called a hot-mirror is mounted over the image sensor. The hot-mirror prevents image capture of fringe UV and IR light, making the camera capture light ranges typical of film cameras. Figure 4 depicts an absorbing hot-mirror filter. There are reflecting versions of hot-mirrors that appear more mirror-like.
Modern DSLR lenses
often do not work well in UV-IR applications, as their lens elements are
designed with either coatings or glass that filters or reflects UV light away
from the sensor. When they do work, one must use long exposures and higher ISO
settings, requiring the use of a tripod. Due to the different wavelengths of UV
and IR light, they focus at a different point than visible light. On film
camera lenses, an IR dot was often included on the lens barrel (Figure 5).
The procedure to focus an IR photo was to manually focus your composition,
install any filters required, note the distance on the lens barrel, then turn
the focus ring so that distance aligned with the IR dot, and then release the
shutter (Figure 6). Older lenses from film cameras often lack the extra
protective coatings, which makes them especially useful in UV-IR applications.
Focus for UV images is accomplished by turning the focus ring to the left by a
similar amount as in IR (Figure 7).
To avoid the UV
lens-coating issues, a useful alternative is to search out a film camera lens. In
a lab setting, an alternative is to use an old enlarger lens. Enlarger lenses
are designed to project a focused center-to-corner image across a sheet of
photo paper, called a flat-field design. Using an enlarger lens on a camera
will provide better final coverage in-document or focus-stacked, layered images.
Enlarger lenses lack the ability to focus, so a helicoid extension tube or rail-mounted
bellows is required. (See Figures 8 & 9). The use of an extension
tube or bellows also gives the added benefit of being used in macrophotography
On the UV end of
the spectrum, melanin in the body left as the result of a bite mark or deep
bruise will absorb UV while the surrounding tissue reflects it for a
significant amount of time—potentially several weeks—after the original bruise
fades. Fingerprints and body fluids will absorb UV-C and stand out from a
reflective background. New versus old paint on a car or wall can be easily
identified. Inks all have different reactive qualities for questioned-document
examination in both UV and IR. Writing on burned documents can often be
discerned under IR light. Tattoos on decomposed or burned bodies may be
enhanced to assist in identification. To avoid any confusion with color issues,
UV-IR images are best viewed in grayscale.
forensic reflected UV imaging systems, called RUVIS, include both quartz lenses
and an image intensifier that make such units prohibitively expensive for most
auxiliary lighting, long exposures, and a tripod are often required. The normal
color image on the left in Figure 11 is a black shirt with gunshot
residue from four shots that was captured at 1/30, f/8, at ISO 800. The image
on the right was captured with a 780 nm IR filter, 30 sec., f/11, at ISO 3200.
The bullet holes and varying amounts of gunshot residue due to varying distance
of each shot can be seen.
can be modified by removing the hot-mirror from the front of the sensor.
Commercial camera conversions will remove the hot-mirror and install either a
clear or IR-only filter in its place. Replacing the hot-mirror with a clear
filter is called a full-spectrum conversion, leaving the camera with broader
than original UV-IR coverage and sometimes allow handheld operation. A better
practice is to always use a tripod to ensure a steady image. While it is
possible to remove the hot-mirror yourself, having a skilled vendor perform the
modification will provide a guarantee the operation is done competently, and
the camera will focus correctly.
A camera with full-spectrum
conversion can be equipped with a wide range of broad or narrow range filters,
depending on your application or desired effect. The filter selection includes
a hot-mirror (Figure 3), so that near normal appearance images can be
In the DSLR age,
filters are not often used. Certain film filters (FLD) were used to correct for
fluorescent lights while using daylight balanced film. When using black-and-white
film, color filters were used to enhance or subdue tones for artistic purposes.
In a forensic setting, filters were rarely used. Now, DSLR cameras can be
adjusted internally for custom white balance, increased color saturation, and
other artistic settings. Neutral-density filters may be used to extend exposure
time for creative reasons to include highlighting a laser path or object
motion. In UV-IR photography, filters are used to custom tailor the specific
wavelength of light striking the camera sensor. There are dozens of filters to
Filters for UV-only
applications often have a slight IR, near 700 nm. This can be very detrimental
to UV photos, as IR overpowers the less-prevalent UV light. Filters for UV
applications are substantially more expensive, often costing several hundred
dollars. The Venus U filter shown below (Figure 12) is a UG-11 filter
with a dielectric coating on the front and back surfaces for UV light only
Filters often have
transmission graphs available. The diabatic logarithmic scale shown in Figure
13 demonstrates the optical density and wavelength suppression. In this
instance, a U-340 filter is shown with and without a pairing with an S-8612
filter. The critical line in this graph is 1E-03, which shows that the U-340
alone has an IR leak above the 1E-03 line at 650–800nm. The U-340 used with the
S-8612 filter does not extend above the 1E-03 line and therefore would be an
acceptable pairing for UV-only use. IR filters are more reasonably priced and
the numbers in their name often represent the light cut-off lower limit of IR
transmission, e.g., 715, 780, 830, 850, and 1000 nm (Figure 14).
Lighting for UV-IR
applications is an important consideration. IR is often considered in the
context of heat. Outdoor scenes are helped by the sun. However, anyone with
forensic experience knows most serious events happen at night. LED and
fluorescent lamps will not work for IR. Photographic flood lights or quartz
halogen work lights will provide broad spectrum light suitable for UV-IR
purposes. Some LED lamps are available in the 365-nm range, but the majority
are commercial “black lights.” UV-B/C lamps are “germicidal” bulbs with peak
output in the 254-nm range.
A standard photo
flash can be modified for UV-IR photography by removing the plastic filters in
front of the xenon flash tube. Once the original filters are removed, application-specific
filters can be mounted over the bare flash tube to limit output to required wavelengths,
as well as give some protection to and from the exposed parts. Extreme caution
should be taken when opening a flash, as the internal capacitor can retain an injurious
electrical charge. If you are not comfortable with such a modification, the
camera modification vendors can often do the work.
UV-IR is not a
normal, everyday photo assignment. Understanding the complex nature of UV-IR
spectrum is demanding. The moderately expensive gear requires justification
that an administrator might not understand without a demonstration of its
value. The fact that UV-C can damage potential DNA recovery may cause further
hesitancy. However, the potential benefits are significant. The skill requires
dedicated practice and documentation to have value and be accepted in a court
setting. Your commitment as a forensic photographer is key. Merely having the
equipment is just a first step. Frequent practice will make you ready when the
important job comes in.
About the AuthorArthur Borchers is an adjunct instructor for
the Homeland Security Training Institute at the Suburban Law Enforcement
Academy / College of DuPage and a forensic consultant with Larsen Forensics
& Associates, both in Glen Ellyn, Illinois. Borchers has advanced training
and experience in photography, photogrammetry, firearms, shooting, crime scene,
and traffic-crash reconstruction after retiring from the Oak Park (Illinois) Police
Department. He is also a contributing author to Sanford Weiss’ forthcoming
book, Forensic Photography for the Preservation of Evidence from CRC
Robinson, E. M. Crime
Scene Photography, 2nd Ed. (2010) Cambridge: Academic Press.