Alternate Light Sources
Written by Keith Breeding   

The basic theory
behind alternate light sources

BECAUSE THEY ARE USED routinely in the laboratory and in the field, alternate light sources (ALS) help discover evidence that would otherwise remain invisible. ALS systems provide a cost-effective, versatile set of tools to the forensics investigator. High-end ALS systems are based on powerful 500-watt xenon arc lamps. These light sources have proven valuable tools in forensic work. While most in law enforcement may be familiar with the term ALS, it is useful to review some of the basic scientific principles and the theories of light that drive forensic applications. It is also useful to explore how ALS can be applied in different situations.

Light Theory

What exactly is light? In the simplest terms, light is energy. Two theories are typically used to describe light’s properties: wave theory and particle theory. When light interacts with large items, we typically explain that interaction using wave theory. When light interacts with objects on a molecular level, we have to shift to particle theory. When discussing forensic light sources, it can be useful to switch back and forth between the two theories in order to explain a point.

Wave theory refers to light as having properties similar to the waves in the ocean. As light energy moves through space it propagates like a wave. Each specific color of light has a wavelength, which is a term that refers to the distance between wave peaks. For example: A specific color of blue might have a wavelength of 450 nanometers (nm). One nanometer equals one billionth of a meter. All colors can be referred to by their wavelength.



Figure 1—Visible light in the electromagnetic spectrum.

The electromagnetic spectrum is often illustrated on a linear scale—as in Figure 1—with the longest wavelengths to the right and the shortest to the left. Visible light (ranging from violet to red) makes up only a small part of the full spectrum. Ultraviolet (UV) light —with shorter wavelengths—lies to the left of violet, while infrared (IR) light—with longer wavelengths—lies to the right of red. Other kinds of energy continue off the left side of the scale, such as x-rays and cosmic rays. Micro-waves and radio waves continue off the right side of the scale. Your eyes, however, are only sensitive to the range of visible light— approximately 400nm to 700nm—so we will confine our interest there.

Particle theory is based on the presupposition that light is composed of small particles called photons. A photon is a pure energy particle that travels at the speed of light. A bright light emits many photons while a dim light emits few. Photons are absorbed or reflected when they come in contact with matter.

Another point to consider is that the human eye does not see all colors equally well. The eye is most tuned to see the color green and is progressively less sensitive the farther the color lies from green on the scale. This effect is readily observed on a rainbow, which appears bright in the middle and dim on the edges.

The term white light refers to the light emitted by the sun, composed of equal amounts of energy at all visible wavelengths. If a color is absent, then the light will not appear white; instead, it takes on some color that is the combination of the colors present. Mono-chromatic light is light composed of a single wavelength. Lasers produce monochromatic light. Lamp-based ALS systems use filters with different bandpasses to control color. The term bandpass refers to the amount of light on either side of a center wavelength. For example: 650nm ± 25nm is a 50nm bandpass.

The use of filters makes an ALS extremely versatile. An ALS based on a 500W xenon arc lamp puts out equal amounts of light at all colors. By using high-end, narrow bandpass filters, any center wavelength can be created using a single instrument.

Absorption

All substances tend to transmit, reflect, or absorb light of different colors. Consider a piece of paper. When a white light shines on white paper, the paper will look white. This is because the paper is reflecting all colors back to the eye. Remember, white light is composed of equal amounts of all the colors. If we shine white light on blue paper, however, the paper looks blue. This is because there is dye in the paper that absorbs all the colors and only reflects back the blue light. This principle of absorption can be exploited to help us find evidence. The idea is to enhance the difference between the color of the evidence and the color of the background surface.


Figure 2—Comparison of colored lights shined on colored surfaces.

To understand how this works, consider a white surface with three colored dots of blue, yellow, and red (Figure 2). If we shine white light on the surface, the paper looks white. The blue dot, however absorbs all colors except blue. The yellow dot absorbs all colors except yellow, and the red dot absorbs all colors except red. In each case, the colored dots only reflect back a single color.

If we shine a blue light on the same surface, the white surface will reflect the blue. It can only reflect blue, however, because there are no other colors present to reflect. The blue dot still reflects the blue light. But the yellow and red dots absorb the blue light. Remember: Yellow only reflects yellow and red only reflects red. Since they cannot reflect blue, the yellow and red dots turn black. If a yellow light is applied, the surface will turn yellow and the blue and red dots will turn black. The process continues for a red light.

In a practical application, we discover that blood absorbs light very readily at 415nm (which is the wavelength for blue). This means that if we shine a 415nm light on blood, it will absorb the light and turn darker than it appeared without the 415nm light.



Figures 3a and 3b—Demonstrating the principle of absorption.

There are two ways to accomplish this. In a dark room, we can put a 415nm filter in front of a white light source so we only apply 415nm light. Since there is no other light in the room, there is nothing to interfere with the process. If we are in a lighted room, we simply move the same kind of filter in front of a camera—or we wear goggles that filter out all but the 415nm light. Figure 3a and Figure 3b show the results of enhancing a faint bloody fingerprint using the principle of absorption.

Diffused Reflection


Figure 4—The angle of incidence and the angle of reflection are equal.

One principle of light is that it always propagates in a straight line. When light comes in contact with a surface, it will either be reflected, pass straight through the surface, or be absorbed. When light is reflected, the angle of reflection will be equal to the angle of incidence (Figure 4).


Figure 5—Reflection from a flat surface and from a rough surface.

Reflected light always obeys this principle, but it looks very different when reflected from a rough surface than from a smooth surface. Figure 5 illustrates the difference. On a smooth surface, light stays together and continues on its original path. When applied to a rough surface, however, it scatters in many directions. The scattering is not random, as it still follows the laws of physics, but it may appear to be rather random if the surface is especially rough. When light reflects off a smooth surface, we refer to this as specular reflection. When light reflects off a rough surface, we refer to it as diffused reflection.

Let’s go back to the example of the bloody fingerprint. If we are fortunate enough that the print is on a smooth surface, we start by holding the light source at a 45-degree angle. We then slowly change the angle until we catch the diffused reflection and enhance our ability to see the print. If we do this right, we will see light reflected off the print and the substrate will appear dark. This makes for a light print on a dark background. Remember, light shining on the smooth surface is not reflected to our eyes or to the camera. We are only picking up the light from the rough surface of the print.




Figures 6a and 6b—Before using diffused reflection and after.

Figure 6a and Figure 6b show the results of enhancing bloody fingerprints on a chair using diffused reflection. Remember that in this case, the fingerprint ridges appear white, whereas with the absorption method the prints appeared dark.

Photoluminescence

Some body fluids will absorb light of a particular color and then emit light of a different color. This is referred to as photoluminescence. Whenever this occurs, the wavelength of the emitted light is always longer than the wavelength of the absorbed light. Here is an example: A substance can absorb blue light and emit red light—but a substance will never absorb red light and emit blue light. This is sometimes called red-shift, since red is the longest wavelength of visible light.

To use this natural effect to enhance evidence, we can use our white-light ALS, but it is necessary to use very tight filter control. We only want to apply to the evidence light that can be absorbed, and we only want to see the light that the evidence emits. Some ALS systems use inexpensive filters. While this makes for a less-expensive system, it is usually ineffective since the absorbed light may wash out the emitted light.

When using a high-end ALS, problems of overlap are not an issue and the investigator can clearly see evidence.



Figure 7a and 7b —Viewing a fingerprint with ALS but without goggles (left); the same fingerprint viewed with ALS and goggles (right).

Using photoluminescence is easily achieved with a properly tuned ALS and camera barrier-filter or goggles. The correct wavelength of light is applied to the substance with the ALS and the goggles allow the user to only view the “glowing” evidence. It can be quite striking the first time a new investigator puts on a pair of colored goggles and sees evidence that was not visible without them (Figure 7a and Figure 7b).

Conclusion

It has been illustrated that principles of light can be used by investigators to find and enhance evidence. Certain ALS systems can be valuable tools in that they offer high performance, affordability, and versatility.
There are many different ways to use ALS systems. The three methods shown here are the most common and can be used to find evidence. Other techniques are also available and may be explored in future articles.

About the Author

Keith Breeding is business manager with Rofin Forensics USA. He has a background of 15 years in scientific-lighting technology, as well as a background in engineering and business development for forensic, medical, and industrial applications. He can be reached by e-mail at: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

 


ORIGINALLY PUBLISHED:
"The Basic Theory Behind Alternate Light Sources", written by Keith Breeding
January-February 2008 (Volume 6, Number 1)
Evidence Technology Magazine
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