What is 'substrate control' in CSI?

What is 'substrate control' in CSI?

Dr. Richard Saferstein: 'Substrate control' is an interesting concept because one of the things that we're most concerned about in Forensics is contamination. That is, what we collect at the crime scene and what we find in the crime lab, is that ultimately something that we can relate to the perpetrator, to the victim? Or, is it something that came about as a result in inadvertent contamination let's say by a police officer who happens to be investigating that crime scene or a bystander? So in certain situations, a 'substrate control' is taken of areas that we think are uncontaminated, that are near important areas of the crime scene that contain important items of physical evidence. And we examine these 'substrate controls' to make sure that they are free of contaminants. So when we do examine the evidence that we've collected and the other 'substrate control' and we have information that we can extract from that piece of evidence, we know that that information is free of contamination, free of contaminants.