Forensic Image Comparison

Forensic image comparison is the process of comparing objects or people when at least one variable is captured in imagery, and making an assessment of the correspondence between features of the captured imagery regarding identification or elimination.

So what does that mean?

Here are a couple examples of forensic image comparison:

  • A facial comparison between an unknown subject depicted in a surveillance image with an identified suspect. In other words, is the suspect the person in the video committing the crime?
  • The comparison of objects, such as vehicles, depicted in surveillance images with those recovered in an investigation. The suspect drives a Nissan Altima with a dent on the lower left bumper, is that the same car in the surveillance video that captured the accident?

Conditions for Comparison

In order to accurately interpret the content of an image under examination, it is critical that the expert evaluates and recognizes the conditions and limitations that occurred during image capture, processing or editing. Every condition can impact the appearance of subjects or objects depicted. Important conditions to understand include, but are not limited to:

  • Resolution
  • Frame Rate
  • Bit Rate
  • Perspective Changes
  • Compression Artifacts
  • Optical Defects
  • Sensor Defects
  • Lighting Conditions
  • Motion or Focal Blur

Exemplar VS Evidence

Evidence: Images or videos that contain objects or people that are considered UNKNOWN

Exemplar: Images or videos that contain objects or people that are considered KNOWN

Comparison Criteria

Images must be compared using the accepted best practices approved by the scientific community. These best practices include the acceptable criteria for comparison. The criteria are divided into two categories, class characteristics and individual characteristics.

  1. Class Characteristics: Class Characteristics are the identifiable features that assist in narrowing the statistical probability that a questioned object or person belongs to the same group as a known object or person that share the same features. Consistent class characteristics between questioned objects and individuals can only suggest similarities. They cannot, by themselves, be used to infer a positive identification.
  2. Individual Characteristics: Individual characteristics are the observable characteristics which differentiate objects within a class from one another. Individual characteristics arise from such events as random natural processes, manufacturing processes, intentional alteration, and wear-and-tear. The ability to identify a specific person or object requires a correspondence of individual characteristics. The analyst will determine the sufficiency of characteristics based on their expertise, through careful consideration of the subject matter, as well as the quality, quantity, and persistence of details in the imagery. No arbitrary number of characteristics is required.

A thorough examination of class characteristics and individual characteristics compared and contrasted between questioned and known objects/persons may allow an experienced and properly qualified analyst to form an opinion. If it is safe to do so, the objects are the same, eliminating all others of the same class.

The image comparison criteria are defined by the standards presented by SWGDE (Scientific Working Group on Digital Evidence) as well as LEVA (Law Enforcement and Emergency Services Video Association)