There are several different types of residues found both in ammunition and explosives, which fluoresce. By using a tunable Forensic Light Source, observations can be made on items without touching the surface, to see if fluorescent residues are present.
In addition, the scanning of any observers clothing after a shooting may help in identifying the shooter due to the ejecta that comes from the barrel of the gun when it is fired. The same ejecta residue would also appear on the hands of the shooter linking them to the fired gun.
Typically the material that will fluoresce is residue that will be ejected out of the barrel, as well as material ejected from the breach.
GSR particles fluorescing on the hand that fired the weapon; SP540nm illumination, orange camera filter. Click for High Resolution.
While there is some debate as to what is the component is that fluoresces, it is secondary for the purposes of detection. The individual particles will fluoresce using illumination between 450nm and 500nm and the use of an orange goggle or camera filter.
GSR on cloth; SP540nm illumination, Orange camera filter
As always, the detection is background dependent as the particles are very small. Detection and documentation requires near ideal conditions.
While the particles are seen typically through fluorescence it is also possible to see the particles in the Infrared. Many times the infrared light improves background rejection which improves visibility of the particles and aids in collection. While many forensic light sources illuminate in Infrared wavelengths, the difficulty lies in the visualization as Infrared effects cannot be seen with the naked eye. Visualization requires the use of Infrared sensitive film or an Infrared sensitive digital camera, such as the Universal Imaging System.
While the particles typically fluoresce in the visible wavelengths, they typically absorb in the Infrared. If they did fluoresce in the Infrared, they would appear as white particles on the Black and White film or Black and White digital camera.
Cloth (under plastic) fired at from a distance of 1 foot; visualized under White light.
Cloth (under plastic) fired at from a distance of 1 foot; visualized under 475nm with a 550nm camera filter. Click for full resolution.
Cloth (under plastic) fired at from a distance of 1 foot; visualized under IR630nm light with a 715nm longpass IR camera filter. Click for full resolution.