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Chapter 5:
Perceptual Quality and Utility Measures
The literature on display quality and utility deals with both physical and perceptual measures. The previous chapter defined and described physical measures; this chapter describes common perceptual measures. Later chapters will show the results of studies using these measures in order to demonstrate the effects of display parameter differences. Perceptual measures are those that involve the HVS and thought processes (known as the cognitive system). These measures are used to assess the quality or utility of a display. Whereas physical quality measures are made at the output of the display, perceptual quality measures include the human visual and cognitive systems of the image chain (Fig. 1.1). It is sometimes the case that the HVS is the limiting factor in the display chain. For example, under normal viewing conditions, the observer may not be able to resolve all of the low-contrast detail presented on the face of the display. The purpose of a display is to present information for analysis and decision making. The quality of the information presented must be sufficient to enable accurate analysis and decision making in a timely manner. The value of the information presented defines utility. At some point on a quality continuum, there may be no improvement in utility. The relationship between quality and utility is non-monotonic as shown in Fig. 5.1. Measurement of display utility involves both the visual and cognitive systems. Perceptual measures of quality and utility range from subjective ratings of display quality (e.g., poor, fair, good) to measures of task performance using the display (e.g., detection of lesions on lung x rays). At an intermediate level, observers may make estimates of their ability to perform defined tasks on a display. The National Imagery Interpretability Rating Scale (or NIIRS, commonly pronounced “nears”) is an example of a task performance estimate scale. Although physical and perceptual measures are related, they do not always relate in a linear fashion. The HVS has finite limits - we cannot see infinitely small detail. Thus, improving resolution beyond our ability to see the image detail can reduce the usefulness of a display.
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