How does alternate light source work

The object then emits a monochromatic light of a different colour and wavelength. Due to loss of some energy, the fluorescence emitted has less intensity than the light initially provided.

This emitted light can be detected by the help of certain filters because to visualise this less intensity radiation, it is essential to block the light at all other wavelengths.

Thereby, these filters aid in making the fluorescence light visible by passing only the desired fluorescence. Alternate Light Source finds high usage in crime scene investigation because it has the ability to locate the materials which can be made to fluoresce, if given the corresponding radiation. Irrespective of the nature of the material on which the evidence is present, the organic materials like biological evidences like body fluids, human remains, teeth, bones, etc.

For instance, for the naked eye, it is next to impossible to detect a fingerprint. But with the assistance of ALS, a blue-green light from a suitable light source can be allowed to fall on the suspected area. The organic material present in the fingerprint will eventually start to give a yellow fluorescence. Thus, a fingerprint can be easily detected without the use of any chemical powder or dye. Different organic materials will have the tendency to interact with different wavelengths and thus other biological samples can also be detected and analysed in the similar manner.

As different materials will emit different colour of light, it is now the skill of the investigator to correctly interpret the emitted radiations. Other parameters to be taken care of are:.

Applications of ALS are numerous. They help in narrowing down the area for sample collection. A number of physical evidence types are known to fluoresce when exposed to ALS. These include:. Body fluids can be detected using alternate light sources. Body fluids that can be detected include; saliva, blood, semen, urine and vaginal secretions.

The near-ultraviolet light radiations are used for identification of semen stains. Blood can be made to glow by treating it with certain chemicals like Luminol. Hair and fibre can be located with the help of ALS by using oblique light on a surface. This will help in revealing small particles of hair and fibre, which can then be collected. In some cases, if the hair or fibre has been dyed there is a chance that it will illuminate under UV light or blue light.

Analysis of Weapons like gun or knife can be done using ALS. They are made up of metals and when examined under normal visible light they do not yield any information as such. But when viewed under green light and observed via an orange filter, it is possible to locate fingerprints and palmprints present on the evidence. Analysis of Trace evidence like gunshot residue, paint chips, crystals, etc.

Due to their small size and quantity, it is very difficult to locate and then efficiently collect them. The gunshot residue pattern can be analysed just by observing the pattern, eliminating the need to touch it. By using ALS at different wavelengths, these particles uniquely interactwith the radiation and emit peculiar patterns. Thereby, allowing each evidence to be detected uniquely. Under these conditions, the biological material had markedly higher fluorescence than the inert material.

This study had demonstrated that it was possible to not only detect but also to differentiate the bone and tooth fragments when mixed with other debris.

On the surface this seems logical however, the goggles used with almost all forensic light sources are plastic and not indexed. They are not uniform. They are uniform enough for our eyes to see the evidence well enough but the camera is not as forgiving as our eyes. The camera filters provided by your forensic light source manufacturer are indexed and reproducible. That means someone using a Mini-CrimeScope illuminating at nm photographing something using the Orange 21 filter is going to see the same thing anyone else who is using that same combination.

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 nm which is the wavelength for blue. This means that if we shine a nm light on blood, it will absorb the light and turn darker than it appeared without the nm 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 nm filter in front of a white light source so we only apply nm 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 nm light.

Figure 3a and Figure 3b show the results of enhancing a faint bloody fingerprint using the principle of absorption. 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.

The benefit here is that the ridges fluoresce brightly and when photographed, a multi-colored or confused background can be minimized to the point where it does not show up in photographs. Fluorescent liquids are used to develop latent prints on porous surfaces such as paper and cardboard.

Fluorescent dyes are used to enhance latent prints developed using superglue fuming. The use of alternate light sources provides a highly practical and efficient means of locating physical evidence at crime scenes.

The invention and perfection of Light Emitting Diodes LEDs has further advanced evidence recovery since even 1 and 3 watt LEDs can be installed in small, hand-held flashlights, and their performance is remarkable. Don Penven has more than 35 years direct and indirect experience in law enforcement. He currently serves as a technical support representative and technical writer for Sirchie Finger Print Labs.

He maintains the Blog: www. Printer Friendly Page.