In examples , a relative skin reflectance of a captured image of a subject is determined. The determination selects from the captured image pixels of the subject’s face and pixels in the background and normalizes luminance values of the skin pixels using the background pixels . The relative skin reflectance value is determined for the captured image , based on the normalized luminance values of the skin pixels . Option ally the relative skin reflectance value is qualified , based on thresholds of skin reflectance values , as suitable for biometric use . Optionally , a non-qualifying captured image is flagged and , optionally , another image is acquired , or the non-conforming image is processed further to transform the image into a suitable image for biometric analysis.