Modus Operandi
Written by David A. Thornton   

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Alternate Light Sources

A CRIME-SCENE SEARCH is incomplete without an examination using an alternate light source (ALS). Using the phenomenon of fluorescence, otherwise overlooked biological and trace evidence will often leap from the background.

Generally, an ALS is a device that transmits light through a filter—or a series of filters—to pass a specific wavelength through a delivery device that shines the light on evidence. The ability to project a specific wavelength separates the ALS from the patrol officer’s flashlight. The officer’s white flashlight beam is polychromatic, where all of the colors in the visible spectrum mix to create a white shaft of light. An alternate light source, on the other hand, is considered to be monochromatic because all of the transmitted light has the same wavelength and frequency.

The three key properties of light are 1) intensity, or the measure of energy over the period of the wave; 2) wavelength, or the distance between the peak of one wave and the next; and 3) frequency, or the number of waves over a period of time. All three properties should be taken into account when considering an ALS because of the effect that the transmitted light will have on matter.

Light has been studied for hundreds of years and theories regarding the nature of light continuously evolve. Newton, Planck, Einstein, and Hawk-ing have all attempted to explain the properties of light. One researcher, Johann Wilhelm Ritter, discovered ultraviolet (UV) rays when experimenting with the rate at which various colors darkened paper that was saturated with silver nitrate. Light, as a part of the electromagnetic spectrum, has a dual nature and can display properties of a particle and a wave at the same time. It has packets of energy called photons riding on the wavelengths that determine the colors of visible light. Different wavelengths and amounts of energy have different effects on matter.

Different wavelengths and frequencies deliver different amounts of energy to the atoms in a substance, causing it to glow. Increasing the frequency of light will result in a proportional decrease in the wavelength and a proportional increase in the energy in the photons. Biological fluids will fluoresce between 400 and 450 nanometers, the UV portion of the electromagnetic spectrum; other trace evidence may fluoresce in the 570 to 680 nanometer range. The energy contained in each photon is transferred to electrons in matter. Here is how that happens:

A cloud of electrons buzzes around an atom’s nucleus. The more energy an electron has, the farther away it is from the nucleus. As it loses energy in the form of light, it sinks toward the atom’s center to its ground state (the electron’s preferred distance from the nucleus). When electrons are continually excited by an ALS, they are not allowed to return to their ground state in order to be charged up again. As a result, the intensity of the fluorescence actually fades. This process is called photo bleaching.

The energy in photons not only causes the fluorescence but can also damage biological evidence. The fluorescent black light, although not a true ultraviolet light, produces light near the UV-A range and creates a fluorescent effect. The shorter UV-A region is within the true ultra-violet region of the spectrum but it is the shorter wavelengths of the UV-B that damages DNA and can create sunburns on the operator’s skin if exposed for too long.

When an ALS is shined on evidence and the light is absorbed and then re-emitted, it is called photo-luminescence. This can be fluorescence, a temporary glowing that lasts as long as the light excites the material; or it can be phosphorescence, a glow that persists after the excitation stops.

Maximizing the fluorescence of the evidence while minimizing the background fluorescence can be challenging. When looking for a particular kind of evidence at specified wavelengths, crime-scene investigators know that the fluorescence will be at a longer wavelength. Therefore, they use barrier filters to block out the unwanted light wavelengths to visually intensify the fluorescence.

After light has been absorbed and then remitted by the electrons, it is always at a longer wavelength known as Stoke’s Shift. Electrons will be excited at one wavelength but will tend to release energy at another. For instance, Rhodamine is excited at 555 nm (the green region) but emits light at 627 nm (the orange or red area). Knowing the specific wavelength of the re-emission helps CSIs to select the best barrier filter.

Using an ALS during a crime-scene search will yield more evidence at a crime scene. Gaining the maximum benefit from the light depends on the operator’s understanding and use of frequency, wavelength, and intensity to best advantage.

About the Author

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it is a crime-scene investigation training consultant with Thornton Consulting & Investigation in Thornton, Colorado. He has 17 years of law-enforcement experience, is a professional educator and law-enforcement trainer, and is a Certified Crime Scene Analyst with the Inter-national Association for Identification. He is currently overseas training Afghan police officers in basic policing skills, and their commanders in training management.

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