Thermal Microscopy in the Forensics Laboratory

Over the last few decades, forensic science has played an increasingly important role in tracking down criminals and narrowing-down the list of suspects that the police need to investigate. The ability to identify a suspect

from a DNA sample has helped identify many suspects over the years, but DNA samples are not always left at a crime scene and with no DNA database, trace forensic evidence is often what will lead the police to a suspect.

After any DNA samples are taken from evidence left at a crime scene, the search is continued with a hunt for any distinguishing hairs, fibres and particles that do not seem to belong at the scene.

Senior Forensic Scientist, David Sugiyama works in the trace evidence section of the Tulsa Police Department Forensic Laboratory where they analyse hairs, fibres, paint and particles of glass found on suspects, victims

and at crime scenes.

One of the distinguishing features of glass is its refractive index and Mr Sugiyama has found the Becke Line method for assessing a particles refractive index a fast and convenient way of telling if glass fragments found at a crime scene could be linked to a certain source. The key advantages of this technique are that it is non-destructive, relatively fast and inexpensive.

The Becke Line method involves placing a glass particle in a liquid of known refractive index and observing the direction in which the halo moves when the focal plane of the microscope is changed. The Becke Line moves towards material with the higher refractive index when the distance between the microscope objective and the sample is increased and towards the lower refractive index when the distance is decreased.