VISUAL ACUITY AND HYPERACUITY

Airy disk. Point-spread function in the image of a diffraction-limited optical instrument with a circular pupil. Diffraction limit. Minimum dissipation of spatial information in the imaging of an optical system, due to the aperture restriction in the propagation of electromagnetic energy. Fovea. Region in the center of the retina where receptor elements are most closely packed and resolution highest. Hyperacuity. Performance in task where thresholds are substantially lower than the grain of the receiving layer. Light. Visually evaluated radiant energy. In this chapter radiant and luminous energy terms are used interchangeably. Optical-transfer function. Modulation in the transmitted images of spatial sinusoids, as a function of their spatial frequency; it is complex, that is, has amplitude and phase terms. Psychophysics. Procedure for studying an observer’s performance by relating the variables of physical stimuli to measurements of associated responses. Point-spread function. Spatial distribution of energy in the image of a point object. Snellen letters. Alphanumeric characters of defi ned size and shape used in standard clinical testing of visual acuity. Spatial frequency. Number of cycles of a sinusoidal grating target per unit distance. Commonly cycles/degree visual angle. Superresolution. Ability to garner knowledge of spatial details in an optical image based on previous available information, either by extrapolation or averaging. Vernier acuity. Performance limit in the alignment of two abutting line segment; it is the prime example of hyperacuity. Visual acuity. Performance limit in distinguishing spatial details in visual object. Visual angle. Angle subtended by an object at the center of the eye’s entrance pupil; it is a measure of distance in the retinal image. 4

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