What is a Forensic Light Source?

What is a forensic light source? That blue light you always see the CSIs shining on the crime scenes while wearing big orange goggles is not some science version of a psychedelic party. What investigators are doing in that situation is making use of a forensic light source to see hidden objects or objects in a different way than under normal light. Normal white light is actually a combination of all the different colors of the rainbow, and the forensic light source is merely separating out a certain color (typically blue light), so that only one wavelength of light is represented. For example, blue light has a wavelength of around 450 nm, and red light has a wavelength of around 700 nm in the electromagnetic spectrum.

When light is shined on a surface, the surface can either absorb, reflect, or transmit the light. In special situations, the light that is shined on an object is absorbed by that material and then re-emitted at a different wavelength (called a Stoke’s shift), which is known as fluorescence. Typically when an object fluoresces, the light is re-emitted at a lower energy state, and therefore, at a longer wavelength. In the case of blue light as the incident light, the re-emitted fluorescence usually occurs in the orange spectrum – that’s why investigators wear orange goggles! In order to see the faint fluorescence, investigators must block out all the blue light from entering their eyes since that would drown out the faint fluorescence, so orange goggles will only allow orange light to reach their eyes, and hence, the fluorescence is observed.

Many body fluids, organic materials and fibers will fluoresce under the forensic light source, and other materials such as blood, gunshot residue, and some inks will absorb blue light to appear dark under the light source. These substances are often not observable to the naked eye, and especially if the surface was wiped down or cleaned. The forensic light source is always a good starting point in a crime scene search because it is portable, quick, cheap (once the unit has been paid off), and most of all, non-destructive, meaning that it will not damage or harm any of the evidence.