Correctly Prepare Photographs

Rule Out Matches Before You Measure Faces

by Joelle Steele

When comparing faces in photographs, images must first be properly prepared in order to rule out faces that resemble each other but that are not a match. This is especially true when comparing faces in biometric databases, and it’s a four-step process that requires a scanner and photo editing software such as Photoshop.

STEP ONE

Photos must be scanned at 300dpi to 600dpi so that the details of the faces will be visible, clear, and measurable down to the smallest detail. The photos should be enlarged to at least 15cm (6″) to 20cm (8″) in width. This high resolution is essential for making accurate measurements down to .08cm (1/32″). If entering the images into a database, the image size — not the resolution — can be reduced slightly prior to storage to reduce the file size.

STEP TWO

If the ears are visible, they are the very first things to compare. Ears are as unique as fingerprints in their size, shape, patterning, angle, and placement on the head. They do not move with changes in facial expressions. They are often at least slightly different on the same head. Ears can be examined from the side of the head and/or from a head-on face view. If any two lookalike people have even the slightest difference in ears, that difference will immediately rule out a match. Unfortunately, in many photographs, ears are not visible because they are obscured by hair, hats, and hoodies, or they are visible in one photo and not in the other. In those cases, it is necessary to continue to the next step.

STEP THREE

Align the pupils of the eyes in both photos using the pull-down horizontal gridline in your photo editing software. This is a far more practical and accurate means of alignment than the use of the Frankfurt Horizontal Plane.

STEP FOUR

Use your photo editing software to enlarge/reduce the irises of the eyes in both faces until they are exactly the same diameter. When the irises in both faces are the same size, the faces should now be the same size, and the facial features in both faces should align with each other.Sometimes a person has one iris that is larger than the other, and if that is the case, the same size discrepancy should exist in the other face.

SUMMARY

In general, there are many obstacles to comparing faces. Two different and unrelated people can look deceptively similar. Inadequate lenses and awkward camera angles can contribute to mis-identification. Standardized higher resolution images could greatly reduce problems of clarity, particularly in surveillance footage. Better care in taking mugshots head-on, as well as including clear profiles that require the subject to remove anything that is obstructing both of the ears before photographing them would prove very beneficial in ruling out false matches. The differences in ears are generally quite obvious, even to the untrained eye.

Including photos of the right and left ears on passports, driver’s licenses, and other forms of picture ID would likewise improve accuracy in identifying living people in person when comparing them to the faces and ears on their documents.

Accurately preparing images for comparison is the best way to authenticate identities. Anyone creating biometric software should keep this in mind, as should the people who intend to use that software.

This article last updated: 03/10/2014.