Forensic Image Comparison Services

Forensic Image Comparison is the process of comparing two images including known and unknown subjects within digital imagery. Does the unknown subject within an image match the known subject? Primeau Forensics image comparison and identification services of unknown subjects/objects for the trier of fact in legal proceedings. In addition, we offer our opinion along with an in-depth analysis based on similarities and differences in many cases.

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The Scientific Method: ACEV-R






How does Forensic Image Comparison Work?

Forensic image comparison is the process of comparing an unknown and known subject when at least one variable has been captured in imagery. An assessment is made between features of the captured imagery regarding identification (as opposed to a demonstrative exhibit). For example, photographic comparisons include:


  1. A facial comparison between an unknown subject depicted in a surveillance image with an identified suspect.
  2. The comparison of objects, such as a vehicle, depicted in surveillance images with those recovered in an investigation.

-SWGDE Guidelines for Forensic Image Analysis PG 6

image comparison services

Quality of Imagery for Image Comparision

The first step in the investigation is assessing image quality. In order to accurately interpret the content of an image, it is imperative to recognize the conditions and limitations. These limitations most commonly occur during the time of capture, editing, or processing. For example, imaging conditions that can impact the appearance of objects or subjects include:

  • Resolution
  • Frame rate
  • Bit rate
  • Perspective changes
  • Compression artifacts
  • Optical defects
  • Sensor defects
  • Lighting conditions
  • Motion or focal blur

Image Comparison Criteria

We compare images using practices approved by the scientific community. Criteria for these practices include class characteristics and individual characteristics.

Class Characteristics

Identifiable features that narrow the likelihood that a known and unknown object or subject belongs to the same group. Consistent class characteristics between questioned objects and subjects can only suggest similarities, however. They cannot, by themselves, infer a positive identification.

individual Characteristics

Observable characteristics that distinguish objects within a class from one another. Individual characteristics result from random natural processes, manufacturing processes, intentional alteration, and wear-and-tear. Moreover, the ability to identify a specific object or subject requires similarity between individual characteristics. Analysts determine the sufficiency of these characteristics based on quality, quantity, and persistence of imagery details.

Firstly, a thorough examination of class and individual characteristics between the known and unknown allows an analyst to form an opinion. Secondly, if the objects or subjects in question are the same, the analyst will eliminate all others of the same class. 

Standards set by SWGDE and LEVA define comparison criteria.

Comparison of Images from Video Evidence


The digital image comparison investigation is not bound by images extracted from surveillance equipment, but images of sufficient quality. These images are created by many types of digital camera systems and image formats. With this in mind, images must be of sufficient quality to analyze and extract characteristics for the photo forensics analysis.

The most prolific source of evidence for police is from video images. However, video is not an accurate medium for identification on its own. This is because digital compression removes and adds information that wasn’t originally included. An image forensic expert MUST be able to interpret the data in order to provide an accurate comparison investigation.

Generating an Opinion

Photographic comparison incorporates analysis of imagery, comparison of individual features, evaluation of the comparison’s significance, and comparison verification. 


Primeau Forensics utilizes a scale commonly known as the “likelihood scale” or “scale of conclusions.” The scale describes the strength of our opinions coupled with the comparison investigation. Though this confidence scale originated in the United Kingdom, it is now used internationally. See the scale in the figure to the right.

image comparison

Expert Witness Testimony

As an expert witness, we must present our observations to the trier of fact in a simple, unbiased method. Reports are written from a scientific foundation to ensure that both testimony and reports uphold integrity. 


We do not generate opinions about the likelihood of objects or subjects based on subjective reasoning, rather scientific analysis. 


Furthermore, we will involve experts from other industries should it lend a hand in strengthening our investigation. This will, in turn, strengthen our reports and testimony.


If you have imagery that you question or need help understanding, please contact us for a pro bono consultation.

Primeau Forensics

Primeau Forensics