Visual performance and comfort of computer user depends on the quality of computer display. A host of visual symptoms has been associated with poorer display quality, such as eyestrain, eyesore and dry eyes. The lower pixel density of earlier displays is the likely source of adverse user experiences, as it forces users to adopt a close viewing distance and results in heightened accommodative and convergence stress.
The critical criterion in evaluating the pixel density is number of pixel per degree, or PPD. It is defined as the number of pixels occupying a certain extent of visual angle in relation to the user’s vision. When the PPD surpasses the psychophysical resolution of human vision, optimal visual functions for screen viewing is expected. Higher PPD also provides additional benefits to viewers, such as permitting smaller screens and richer display content. However, since displays with high pixel density are very expensive, there needs to be a compromise between increasing pixel density and holding down the cost of computer display.
PPD is directly affected by two factors: pixel density and viewing distance. Indirectly, several factors determine PPD by altering viewing distance. These include screen size, display form factor (phone, tablet, laptop, or desktop screen), viewing angle, and luminance/illumination conditions.
The present analysis defines the meaning of the threshold PPD, and explains how it can be quantified. Its relationships to screen size, pixel density, and display form factor are also illustrated. Calculation of PPD for presently available/popular displays with various form factors is conducted to evaluate their PPD relative to a theoretical threshold, 1 pixel/arcmin. A series of experiments are proposed to investigate the threshold PPD, the relationship among the factors affecting threshold PPD. Their ultimate goal is to provide specifications for the screen size and PPD for different display platforms.
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