Ultraviolet Light Analysis

Ultraviolet (UV) light technologies are used for multiple purposes in forensic investigations, including authenticating paintings and other fine art, authenticating signatures, analyzing questioned documents, illuminating latent fingerprints at crime scenes and trace evidence on clothing, analyzing ink stains, and revealing residual stains of body fluids.

Ultraviolet light analysis and other optical examination techniques are recommended by the Federal Bureau of Investigation guidelines as the first choice to examine biologically contaminated evidence. This is because ultraviolet analysis is not destructive. It allows precise images and preliminary identification of the evidence before other analytical methods, such as luminol or washing solutions, are applied.

Body fluids such as saliva, semen, vaginal fluids, urine, and perspiration give off fluorescent light when illuminated by a source of ultraviolet light, which is a very efficient method for detecting such stains in a crime scene or in objects collected from the scene, such as clothing, towels, bed sheets, or decorative items. Even dried stains become fluorescent under UV light.

Forensic technicians also use UV light technologies, such as ultraviolet monochromators or optical spectroscopy, to detect the presence of illegal or controlled substances or their residues in unidentified samples, or to determine how many types of ink or pens were used in a forged document.

Ultraviolet reflectance spectrography generates images from ultraviolet radiation in a technique known as RUVIS. This technique allows the detection of latent fingerprints on nonporous surfaces without dusting or chemical treatment as well as of those previously treated with superglue fumes (cyanoacrylate vapor). RUVIS produces clear detailed images that can be either photographed or filmed, depending on the equipment in use. The RUVIS technique basically consists of the generation of UV light by an external source, which is focused on the surface containing latent prints. The UV light is diffusely reflected from the fingerprint residues on the nonporous surface toward an optical filter and passes through an objective lens into an image intensifier that converts it into visible light, thus producing the fingerprint image. Latent fingerprints are those that are invisible to the naked eye and must be detected by optical devices before being photographed. For forensic purposes, all technical data involving the picture is also recorded, such as the type of camera, lens, film, shutter speed, camera position, angle, and distance from the object. These records ensure the reproducibility of the image by independent analysts, thus preventing accusations of image manipulation to force a print match.