Identification from images: Theory and methods

The use of images for the identification of criminals is becoming more prevalent with the increased use of video surveillance systems. Any anatomical trait that is visible on an image could be used to identify an individual, as long as its usefulness as a biometric indicator is known and can be accurately measured. The mug shot, which was introduced in 1879 by Alphonse Bertillon was the first photograph used in forensic identification from images and since then the human face has been the focus for identification and recognition. However, the usefulness of the face or any other part of the body that could be measured from an image has not been thoroughly investigated. Population frequencies of various traits are known. However, many studies which investigate the frequencies of traits, use categorical scales of measurement. Categorical scales of measurement have been used to describe the human face and body for centuries, it is not a new technique. The advantages of using categorical scales to describe various anatomical features is, that it is inexpensive to study and does not require specialised technology. As long as an individual is well trained with sufficient knowledge of the human body, categorical scales are generally accepted as a means of describing human variation. The use of categories for description of the human body is currently accepted for research purposes and cases of skeletal identification. However, the use of categories is questioned when describing an individual from an image. A possible reason for this could be that in image analyses the traits are often too small to see, they are covered by clothing (such as those of the face by a balaclava) or they are subject to image distortion. Therefore, statements made by an expert witness in court proceedings regarding descriptions of anatomical features using categorical scales from images can often be questioned as it is primarily opinion based evidence.