Role of depth in eye distance measurements: comparison of single and stereo-photogrammetry.

Photogrammetry has been used as an alternative to direct measurements to obtain facial distances for a variety of anthropometric applications. Taking measurements from photographs is less intrusive to subjects and reduces screening time, but measurements from single frontal photographs neglect depth information and may be inadequate for screening purposes. This study examined the role of depth in measurements of palpebral fissure length, interpupillary distance, inner canthal distance, and outer canthal distance using single- and stereo-photogrammetry; an operator selected landmarks on single and stereo digital photographs displayed on a computer monitor. Depth was not found to make a significant contribution to eye distances in an idealized system where the real-world coordinates of points on the eye were known from three-dimensional calibration of stereo photographs. However, the differences found between measurements taken from single frontal photographs and those from stereo-photogrammetry indicated that measurements from single photographs are prone to errors due to misalignment of the camera, the face, and the calibration instrument during image acquisition; if single photographs are to be used, the placement of these components should be carefully monitored.