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Use of the Alternate Light Source for Glass Side Determination

The most common glass found in structure windows is “water floated glass”. During the processing the individual sheets of glass take on absorption characteristics on one side and reflective characteristics on the other side in reference to Ultra-violet radiation in the high band end. The glass is not installed in structure according to which side is UV absorbing and which side reflects. But each sheet of glass is the same on any given side throughout the sheet. Often a broken pane of glass at a crime scene is contaminated from outside elements and a careful examination of the glass can reveal which side was facing outward when the window was broken. In a situation where the glass was either clean or contaminated on both sides this method cannot be used.

The low strength less expensive UV light sources seems to work better than the higher intensity light sources. So if you are on a budget this is a good thing. Simply shining the UV light on the target glass pane will result in an image that either appears to have depth like looking into a pool of clear water or a reflection effect will occur like shining a light on a mirror. What ever occurs on one side of the glass will be the opposite for the reverse side of the glass pane.

When a window is broken out of a frame in a structure the investigator should attempt to find a fixed fragment still in the frame. Examination of the fixed fragment will determine if the outside of the broken window pane reflects or absorbs the UV light. Following this test the investigator will now know how to lay the fragments in relationship to outside or inside facing of the glass pane in question. This will allow the investigator to apply standards for determining direction of force and also make reassembling the window fragments much easier and quicker.

Figures 1 and 2 are the same fragment of glass exposed on the respective opposite sides. It demonstrates what will be seen by the investigator when employing this technique. Please note, no filters are necessary or required.

I learned this technique from Mr. Larry Stringham, CSI Supervisor, Cape Coral Police Department, Cape Coral, Florida.

If you have any questions you may contact me:

David A. Lounsbury
Coordinator, Criminal Forensic Studies Program
Florida Gulf Coast University
10501 FGCU Boulevard South
Fort Myers, Florida 33965-6565


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