Forensic Light Source (FLS)
Is the use of specific wavelengths of light, to enhance the contrast between the evidence we look for, and the surface, or substrate, it may be present on, during a forensic investigation.
The visible spectrum for the human eye is usually demonstrated through the colors of a rainbow: red, orange, yellow, green, blue, indigo, and violet. The wavelengths these colors represent are between 700nm (red) and 400nm (violet).
A very simplified way to explain the use of FLS is that we use specific shorter wavelengths of light (U.V., blue, green) and look for the resulting longer wavelengths which are emitted by the interaction between the wavelength we use, and the substance we are looking for.
Now at times, this is a substance we think may be present (e.g. seminal fluid) or it may be a solution we added, as we know it will adhere to what we are looking for (e.g. indanedione zinc chloride use to find fingerprints).
White (regular) light exam
Addition of orange filter
Addition of 532nm (green) light
Seminal fluid stain visualized with FLS
In physics there is a theory referred to as Stokes' Law which states the fluorescence emission (light being emitted) is at a longer wavelength than the incident light.
This means that when a short wavelength of light (used for searching) is absorbed and re-emitted, it loses energy and becomes a longer wavelength of light. This change from a higher energy (lower wavelength) light to a lower energy (higher wavelength) of light is referred to as Stokes' Shift.
Knowing what the change will be for a specific wavelength of light when it is absorbed and re-emitted by a specific substance (like the one we are looking for), we can anticipate what particular wavelength, or colour, of light to use as we look for that specific substance.
Once we know what color to look for, we use filters to narrow down the range of what we can see. They literally filter out other colors that may interfere with or prevent us from seeing what we are specifically looking for.