General Study Guide - Ontario Police College

- fluorescence or luminescence - is of a longer wavelength or lower energy than the light that
excited the molecule in the first place. Eg. Blue light of a forensic light source excites yellow
fluorescence of R6G .) If the excitation light is green, the observed fluorescence will be yellow or
red or even infrared, but not blue.
Most of the illuminating light is usually not absorbed but it is scattered or reflected from the surface
being examined i.e. the fluorescence is very weak. Filters which transmit the fluorescence but not
the illuminating light (excitation light) are therefore placed in front of the eye and camera to enable
the fluorescence to be seen and recorded.
At the crime scene, it is unknown beforehand what fluorescent items might be found. The most
useful combination of light source and viewing filter is blue light and an orange filter, but other
combinations can also be tried.
This colour wheel can be used for the
selection of a filter (i.e. goggles or
photographic lens barrier) to improve
contrast. Colours that are opposite on the
wheel are complimentary and will darken
the colour observed i.e. increase contrast
while adjacent colours on the wheel will
lighten the observed colour i.e. reduce the
contrast.
For example – Try orange goggles if
excitation light is in the blue range (400-
500nm).
Processing Technique - Fluorescence
Certain properties of perspiration, body oils and/or foreign substances contained in latent print
residue fluoresce when exposed to a laser or alternate light source. A filter is used to block the
incident light of the light source. No pre-treatment of the specimen is required; therefore, no
alteration of the specimen occurs.
Use on all surfaces
Non-destructive to specimen and subsequent examinations
Detects prints on surfaces not suitable for powders or chemicals
Detects prints not developed by other techniques