Alternate Light Sources and How CSIs Use Them An easy way to remember what an alternate light source (ALS) is would be to describe it as any device that emits any color except white light. As you may know, light travels in waves—just like radio and TV signals. Light waves are part of the electromagnetic spectrum. Think of electromagnetic( EM) waves as being just like the waves you see along the beach, except that EM waves have a much shorter wavelength. An ocean wave is measured in feet or meters from the starting point of one wave to the starting point of the next wave.
When looking at a chart depicting the electromagnetic spectrum we see that the longest wavelengths are apparent in electrical power and telephone signals. At the other end of spectrum (very short waves) we find gamma rays and just beyond them are the x-rays. Moving toward the longer wavelengths, the next area is ultra violet light (UV)—the shortest light beams—to the longer wavelengths of Infrared (IR). In between UV and IR are the visible light frequencies of violet, blue, green, yellow, orange and red. When all of these visible light bands combine, the result we see is white light. OK, this ends the physics lesson.
The most important thing to remember is that the different visible light colors can be used to cause a phenomenon known as fluorescence. In other words, certain substances fluoresce or emit a new light, when exposed to certain colors of alternate light sources.
The very first ALS used by law enforcement was a longwave ultraviolet light. UV light, as it is often referred to, or “Woods Light,” or “Black Light.” During the mid-1980s law enforcement became interested in ALS.
First it was the use of lasers in the crime lab. It was discovered that some untreated latent fingerprints on certain objects would fluoresce under laser light. As the laser began finding its way into many crime labs, the price began to come down. Well, leave it to some resourceful forensic scientists to discover that if certain colors of laser light would produce fluorescence, so why not use lower powered, less cumbersome and lower cost lights. Research found that ALS in the blue range (410nm-470nm) was the most effective in producing fluorescence in many substances.
A number of advantages are apparent when alternate light sources are used to locate and identify physical evidence. A principal advantage is that the light reflected from multicolored backgrounds is minimized. This is a big plus when photographing the evidence.
The most common uses for ALS at crime scenes are locating physiological fluids like saliva, urine and semen. UV light works best on these substances but blue light often produces good results. Bone fragments and teeth are visible using both UV and blue light. And certain narcotics like crack cocaine will fluoresce under blue light. Cyanoacrylate (superglue) fuming for latent prints on non-porous surfaces is a favored method used in crime labs and crime scenes; but one problem is that the developed prints are white in color and on light backgrounds they are virtually invisible.
Search For Bone Fragments
To compensate for this problem, the developed prints may be dusted with conventional or fluorescent latent powders in order to provide photographic contrast. Liquid dyes may also be used such as Ardrox, Basic Yellow and Rhodamine 6G. Thus treated, the cyanoacrylate prints are then exposed to ALS permitting visual inspection of the fluorescence and photographic recovery. Most people are aware that UV (black) light causes certain materials to “glow in the dark,” and forensic examiners use UV light at crime scenes and in the laboratory to look for items that fluoresce.
Contrary to what you may see on certain TV shows—blood does not fluoresce. But the script often has crime scene investigators flashing around a blue light and the supposed blood emits a blue-white color. In reality, blood absorbs most light frequencies—like UV and Infrared (IR) (and everything in between), so on most backgrounds—particularly light-colored backgrounds—droplets of blood will appear as black spots. Blood, however, will luminesce, or glow in the dark, when treated with a chemical like Luminol. But that’s for another article.
The most frequently used alternate light sources are in the blue range. Just like radio and TV signals, these wavelengths are measured in meters. A radio signal at a frequency of 144 megahertz (MHz) has a wavelength of about 2 meters (6.56 ft.). The most popular blue light frequency for forensic work is around 455 nanometers (nm)—a nanometer being one billionth of a meter. But there’s a catch—although alternate light sources do cause fluorescence in certain objects, the fluorescence is so weak that the visible light overwhelms this fluorescence. But all isn’t lost. Investigators can block out the visible light by looking through a barrier filter, which blocks out the source beam and allows the weak fluorescence to be seen. When the light source is either violet or blue an orange barrier filter is used. Thus the orange filter blocks the visible light from the light source and passes the weaker fluorescence. Yellow, orange or red light ALS sources require use of a red barrier filter.
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If you would like more information on alternate light sources, use the link below to download the free instruction manual of an ALS kit that offers seven different light frequencies.