The discussions of physical FOMs above did not attempt to relate the measured values to the visual performance of the user. However, in some cases, it was appropriate to provide limited comments on the impact of the FOMs on user visual performance. In the following sections, system performance as a function of user visual performance is explored in greater depth. The eye has its own transfer function which must be considered when the display image is viewed. Previously, the FOMs for displays were categorized into four domains: Spatial, spectral, luminance, and temporal (Table 5.2). These image domains parallel analogous human visual performance domains. The spatial domain includes those display parameters associated with angular view (subtense) of the user and coincide with user’s visual acuity and spatial sensitivity. The spectral domain consists of those parameters associated with the user’s visual sensitivity to color (wavelength). The luminance domain encompasses those display parameters identified with the overall sensitivity of the user to illumination levels. The temporal domain addresses display parameters associated with the observer’s sensitivity to changing levels of light intensity.
The human eye has an extraordinary visual capability. It can perceive light within the spectral region of 0.38 μm (violet) to 0.78 μm (red). It consists of a central region, containing cone detectors, which provides detail and color perception (decreasing with decreasing cone density away from the center, fovea); and a peripheral region, containing rod detectors, which provides black and white perception and motion detection. The maximum sensitivity of the cones is about 555 nm and is 507 nm for the rods. The eye has 10 decades of dynamic sensitivity, which usually are divided into three ranges: Photopic (day), mesopic (twilight), and scotopic (night) (Bohm and Schranner, 1990). Adaptation to these varying levels is achieved through the changing pupil diameter from 2.5 to 8.3 mm. The temporal integration time of the eye is about 200 msec. Its resolution capability (for sine waves) is better than 1.72 cy/mr. However, these characteristics vary with age and viewing conditions.
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