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The previous chapter described the operation of displays and discussed the effect of hardware controls on image appearance. This chapter describes how physical quality measures are used to characterize the performance of a display. Chapter 5 will describe perceptual measures of quality and utility. Previously quality was defined as the degree to which a displayed image accurately portrays the original “scene” (e.g., a lung or a section of terrain) and accuracy was defined in terms of spatial and spectral response. Quality can thus be defined in terms of a mathematical relationship between the image and scene and is a function of both the image capture device and the display. Quality can be measured in both the physical and perceptual domains. The true scene/image relationship is defined in the physical domain, while perceptual judgements can be used to estimate the relationship. For example, contrast can be defined in terms of luminance differences or in terms of perceived brightness differences. Utility, on the other hand, is the value of the displayed image to an observer. Value is typically measured in terms of the ability to perform a specified task. Whereas quality can be measured in both the physical and perceptual domains, utility is measured in the perceptual and cognitive (eye and brain) domains.
Many measures have been devised to characterize and report on the quality of displays. This chapter describes the measures used in later chapters to select and optimize displays, although not all of the measures described here will be used in this book. They are discussed because readers may encounter them in other display literature. In the following chapters, we use a subset of these measures to define desired display performance.
Physical measures of display quality can be categorized in terms of resolution, contrast, noise, and artifacts and distortions. Resolution relates to the ability to see fine detail in an image. Contrast refers to tonal or color differences within the image. Noise refers to unwanted signal variation, either spatially or over time. Artifacts are unwanted image variations generally resulting from processing or signal transmission. Distortions are defined in terms of departure from straightness and linearity. Physical measures can also be categorized in terms of the measurement domain. Measurement domains include spatial, luminance, spectral (color), and temporal or time-related.
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