Post Number: 349
|Posted on Monday, February 09, 2009 - 07:18 pm: ||
Test prints at a crime scene usually refer to fingerprint dusting, in situations where comprehensive crime processing is not warranted (i.e., spending days completely fuming and chemically processing all surfaces, often causing considerable destruction to anything examined).
When there is doubt about whether a particular dusting powder technique will work well on a surface, test prints "may" be placed on an innocuous area (such as the backside of a desk, where it is not suspected that a perpetrator may have touched) to determine if the process chosen (such as a contrasting color fingerprint powder with a camel hair brush) develops the test prints without "painting" the surface.
In reality, most crime scene technicians are able to determine at a glance which powder and brush/applicator will work well on most surfaces and there is seldom a need for test prints, unless the surface seems to have an unusual coating of polish, cooking grease, or other contaminate.
Of course, since Crime Scene Technicians do not have ESP and cannot know whether a suspect had natural friction ridge skin secretions on their fingers and palms (versus contaminates from hair oil, food, etc.), there is never a guarantee that actual latent prints from a perpetrator (if present) will develop as nicely as test prints from the technician (which are often made by rubbing a finger against the side of his/her greasy nose to ensure fine adherence of the fingerprint powder).
In a forensic laboratory, test prints on a non-evidentiary surface are normally processed for each examination as a quality control step to ensure that the reagent or other procedure is performing as expected. Without doing a test print, a forensic scientist cannot know if the absence of friction ridge detail is due to no one touching the surface versus a reagent (or a procedure) that is performing poorly.