Describe the proper techniques for collecting fingerprint evidence from porous and non-porous surfaces.

Collecting fingerprint evidence requires different techniques depending on whether the surface is porous or non-porous. 'Porous surfaces', such as paper, cardboard, or unfinished wood, absorb fingerprints, making them more challenging to develop. The primary technique for porous surfaces is chemical processing. One common method is the use of iodine fuming, where the object is exposed to iodine vapors, which react with the oils in the fingerprint to create a temporary brown image. Another method is ninhydrin, which reacts with amino acids in the fingerprint to produce a purple-blue image. Ninhydrin is often used after iodine fuming because it develops fingerprints that iodine may not reveal. Another chemical is DFO (1,8-Diazafluoren-9-one), which is similar to ninhydrin but reacts more sensitively, producing a fluorescent image that can be photographed under laser or alternate light source illumination. Once a fingerprint is developed using chemical methods, it must be photographed to preserve it, as the chemical reactions can fade over time. 'Non-porous surfaces', such as glass, metal, or plastic, do not absorb fingerprints, making them easier to develop. The primary technique for non-porous surfaces is the use of fingerprint powder. The powder is applied to the surface using a brush, and it adheres to the oils in the fingerprint, making it visible. Different types of powders are available, including black powder, white powder, and fluorescent powder, each chosen based on the color and texture of the surface. After the powder is applied, the excess is gently brushed away, leaving the fingerprint visible. The fingerprint is then photographed. If the fingerprint needs to be lifted, it can be done using fingerprint lifting tape. The tape is applied to the powdered fingerprint, gently pressed down, and then carefully lifted, transferring the fingerprint to the tape. The tape is then placed on a backing card to preserve the fingerprint. Small Particle Reagent (SPR) is another technique used on non-porous surfaces, especially those that are wet or have been exposed to moisture. SPR contains fine particles that adhere to the fatty components of fingerprints. It is sprayed onto the surface, and the particles attach to the fingerprints, making them visible. For example, to collect fingerprints from a glass window, black fingerprint powder could be carefully brushed onto the surface. The excess powder would be brushed away, revealing any fingerprints present. The fingerprints would then be photographed and lifted using fingerprint lifting tape. To collect fingerprints from a piece of paper, the paper could be treated with ninhydrin. After the ninhydrin solution dries, any fingerprints present would develop into a purple-blue image, which would then be photographed.