Binocular Disparity in Aspherical Mirrors

An aspherical mirror is a convex spherical mirror whose radius of curvature decreases as the line of sight moves horizontally on the mirror from inboard to outboard. This differs from a regular spherical convex mirror which has the same radius of curvature everywhere on the mirror. Aspherical mirrors provide an increased field of view and larger image sizes than would be possible with a traditional spherical convex mirror. One potential concern with aspherical mirrors is binocular image disparity. Binocular image disparity in an aspherical mirror results from the situation where one eye sees an image on a portion of the mirror with a larger radius of curvature than the other eye sees. The difference in image sizes can cause discomfort to the person using the mirror and, if the difference is large enough, the person sees a double image. This paper describes a method for quantifying the binocular image disparity in aspherical mirrors. The method involves measuring images in the mirror in angular degrees with respect to the driver's eye. Binocular Image Disparity is defined as the percent change in angular image size seen by the driver's right eye versus left eye. Four types of targets were used to measure disparity: vertical linear and horizontal linear (one dimensional) targets, area (two dimensional) targets and volume (three dimensional) targets. Three eye to target distances were analyzed: 2000mm, 4000mm, and 6000mm. Four aspherical mirrors were analyzed. This method characterizes aspherical mirrors in terms of the maximum binocular image disparity and the maximum rate of change in binocular image disparity. Binocular image disparity was largely unaffected by target distance. The square area target was recommended over the other target types.