Chapter 7 - Ultraviolet, Infrared, and Fluorescence

Abstract

The light that is the foundation of photography is related to the electromagnetic spectrum. This spectrum depicts light as a wave of radiation, which has a peak height (amplitude), a distance between peaks (wavelength) and a number of complete waves passing a point each second (frequency). When light strikes various surfaces, it can interact with those surfaces differently, and crime scene photographers can use these differences to visualize otherwise “invisible” evidence. Ultraviolet (UV) light can be used to search for faint blood stains, tracks, or trails. UV light can also cause nonblood body fluids, some fibers, and some fingerprints to fluoresce. The evidence can then be photographed while fluorescing and collected. Infrared (IR) light can also react differently to different substances and substrates, making it possible to visualize various types of evidence not noticed in the visible light range. Inks with different chemical makeup can frequently be distinguished, and gunshot residues on dark fabrics can more easily be seen. Printing or writing on charred documents can sometimes be visualized by the use of IR lighting techniques. Some inks can be made to fluoresce in the IR range when stimulated with blue–green light. Many types of evidence can be made to fluoresce in the visible light range of the electromagnetic spectrum, enabling the evidence to be seen, photographed, and then collected.