Impression Evidence

There are many forms of impression evidence that can be encountered at the laboratory.  These include fingerprints, palm prints and footprints which are discussed in the latent print section.  Additional types of impression evidence can include footwear impressions, tire impressions, bite marks, glove prints, lip and ear impressions and fabric impressions.  The most common type of other impressions encountered at the laboratory is footwear.

Footwear identification involves comparing the soles of a suspect’s shoes to impressions or prints left at a crime scene.  An impression is usually a three-dimensional impression that could be found in mud or a soft material; a print is an image left on a solid surface by substances such as dust, powder or liquid.

At the crime scene the print or impression must first be photographed with a scale to show all the observable details of the impression.  Photographs should be taken directly over the impression or print.  The use of side lighting can help to highlight many details that will aid in future examination.  The next step is for the crime scene technician to attempt to remove the original impression or print for submission to the laboratory for comparison.

If the evidence is a print in dust or dirt on a floor tile, glass, paper or drywall this can be easy to remove.  However if the print is on a surface that cannot be submitted to the laboratory, the crime scene technician may be able to preserve the print in a manner similar to lifting a fingerprint.  A large enough piece of lifting material is laid over the print and a fingerprint roller is used to eliminate any air pockets.  The lifting material is then removed lifting the print from the surface.  Another technique is to use an electrostatic lifting device to recover the print.

If the evidence is an impression in soft earth the technician will obtain a dental stone cast after photography.  If the impression is in snow there are specialized products available to help protect the impression during the casting process.

If the footwear print is in blood there are numerous chemical developers that can be used to enhance the print.  Examples of these chemicals include amido black, leucocrystal violet and diaminobenzidine.  They react with the blood to form a colored complex that can be more easily visualized.  (It is important to note that research has shown that a number of blood enhancement chemicals have been shown not to have a negative effect on DNA profiling of blood samples.)

Once the crime scene evidence has been photographed, removed or processed, it is submitted to the crime laboratory.  In addition, the Investigator has to obtain the suspects footwear for comparison. 

Footwear Evidence

At the laboratory the analyst will produce known prints of the suspect footwear then compare these images to the photographs, impression casts or prints retrieved from the crime scene.

The evidentiary value of the evidence will be determined by examination.  Some questioned items can be excluded by size, shape or design.  However identification cannot be made on the basis of class characteristics alone.  Identification is only possible when individual characteristics are present.  These would be random characteristics that have been imparted to the sole after production.  When these characteristics are present in sufficient quantity and quality it can be possible to identify a shoe sole as the source of print or impression left at a crime scene.