An Alternate Light Source: ForensicFingerprint


 Alternate Light Source

Alternate Light Source or a forensic light source are used in crime scene 

investigation to identify, photograph and collection of many forms of physical 

evidence including latent fingerprints, hair and fibers, bruises, bite marks, 

shoe and foot prints, gunshot residues etc. due to their natural characteristic 

of light absorption or florescence effect Physiological fluids such as semen, 

saliva and urine are identified by using UV light. In any scene of crime the 

fast and accurate detection of physical evidence is very important.

The forensic scientists were used visible light sources for the examination of

 crime scenes. Some investigators were used ultraviolet (UV Black Light) at 

crime scene but white light was the mainstay. In 1970s the Royal Canadian 

Mounted Police (RCMP) added a new dimension. They used argon-ion lasers

 to forensic examination of physical evidence, but these expensive, bulky machines

 were not transported to the crime scenes, so their use was limited to the crime lab. 

In the late 1980s Portable lasers appear in the marketplace. They were capable of 

delivering a single wavelength of light. In the 1990s, the development of a high-intensity 

incandescent bulb revolutionized alternate light source and resulted in a wider range 

of wavelengths, from the ultraviolet range of the light spectrum to the near infrared.

 These machines were less than 20 pounds(9 kg) and they were providing an intense

 light beam that passed through various filters covering from visible green, blue, 

yellow and red light.

An alternate light source consists a laser or incandescent bulb, a filter or combination

 of filters that filters down the light into individual color bands (wavelengths) and 

enhance the evidence visualization by making evidence glow (fluorescence), evidence

 darkens (Absorption) by light interaction technique.


For different types of surfaces Different wavelengths (color bands) are required. Sometimes the background surface also glows under light source illumination. In this case, a wavelength of light is necessary that causes the processed print to glow (fluoresce) and not the background. The output power and the extent of wavelength tunability of the light source are proportional to the quality and quantity of evidence revealed. This is the ability of a forensic light source; UV lights cannot offer this selectivity due to its limited wavelength and low power.

Fluorescence is the key source of alternate light. One wavelength of light absorbs and the emission of light at a longer wavelength takes place. By using the screening filters emitted light can be detected and filters block out the other wavelengths of light.

Fluorescence -

Fluorescence is occur when light of a certain frequency and color strikes on an object, and light of a different color and frequency returns by the object. This fluorescence is very weak in intensity as compared to the light beam that created it, for visualizing this fluorescence, we must use filtration that blocks the visible light but passes the fluorescence. The UV light is invisible but it creates visible fluorescence from certain substances and objects. To view UV fluorescence no filtration is required. Clear glass or plastic lenses is recommended for eye protection.

For fingerprints detection, fluorescence is visible without the addition of any powders or dyes. EX- When light (blue-green) from a laser or incandescent source directed towards the organic materials in the fingerprint, it fluoresce yellow. It is difficult to lift prints from surfaces such as thin plastic bags, thin aluminum foil, rigid duct tape, concrete wall, heavily grained wood brick, printed glossy magazine pages, paper products, etc. by using traditional powder technique but by using Forensic Light Source techniques we have successfully seen and lift latent prints on these and many other types of textured surfaces. For processing of different types of surfaces different wavelengths are required and here the Forensic Light Source fulfilled the requirement having tunable or multiple wavelengths. We have faced problem when the background surface glow under light source illumination. To solve this, we need a wavelength of light that makes the print to glow and not the background. This means that different organic materials absorb light and they fluoresce at different wavelengths. So, fingerprint or a bite mark detected on materials is as diverse as on skin, paper, rubber, and cloth fabric. Two things are important first illuminating light is very critical to elicit the maximum fluorescence from the evidence, second the selection of the filter(s) is important.

Under room light Finger and palm prints are invisible on metal surfaces such as a gun or knife but prints can be seen when illuminated by green light and observed through orange filter. Apart from that there are fluorescent chemicals or powders used for latent fingerprint development. These powders have the ability that makes the latent residue to be fluorescing on a surface. For developing latent prints on porous surfaces such as paper and cardboard fluorescent liquids are used. After superglue fuming, fluorescent dyes are used to enhance developed latent prints

Body fluids - Body fluids saliva, urine stains, vaginal secretions and semen stains can be detected using alternate light sources. Some narcotics substances will also fluoresce. Under the light source illumination dried body fluids will glow. The body fluids will also fluoresce under UV black light but on many articles like clothing and sheets, these articles will also glow and hinder the evidence detection. It is advisable to use longer wavelengths to eliminate the background interference (background rejection). If your light source is powerful and you have more wavelengths then you will uncover the more evidence. Blood is not fluorescing under UV or visible blue light; it has luminesced by spraying some chemicals on it such as BlueStar and luminol.

Hair and Fibers

There are two light illumination methods to locate hair and fibers by using a Forensic Light Source. Lighting of a surface (floor or carpet) by applying oblique or parallel strong white light reveals small particles like hair and fibers. Under UV or Blue light hair and fibers will also glow.

Bruises/Bite Marks/Pattern Wounds

Forensic Light Sources may reveal bruise and patterned wound details that are invisible under normal white light illumination. Depending on the depth of the wound or bruise the wavelength of the instrument vary as different colors penetrate to different depths within the skin. Illumination is required for deep wounds to get enough skin penetration.

Gun Shot Residues/Explosive Residues

Gun Shot residues/Explosive residues are found in ammunition and explosives and are readily observed by using the appropriate illumination.  

Questioned Documents

Document forgery detected when we used alternate forensic light sources. Inks have different formulations and composition even within the same apparent, visible, color type and the ink in contact with the paper will produce detectable responses as the color of the light is tuned through the visible and infrared regions. IR sensitive film or a digital camera having IR sensitivity is required for visualization of evidence in IR lightening.

UV (ultraviolet) lights are used for examination of scene of crime to reveal evidence not visible to the eye under normal lighting. UV light reveals hidden blood, fibers and fingerprints, subcutaneous bruises on living and dead bodies. It’s also used on authenticating paintings, authenticating signatures, analyzing questioned documents at crime scenes and search for trace evidence on clothing etc. Blood on dark and patterned clothing is used to enhance through Infrared photography.

The installation of 1 and 3 watt Light Emitting Diodes (LEDs) in small, hand-held flashlights further advanced evidence recovery and their performance is remarkable. Precautions are always needed as Exposure to Alternate Light Source of radiation can result in serious, and sometimes irreversible, eye and skin injuries. Doesn’t look directly into the light, always wear protection (filters) on your eyes.

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