From Fundamentals of Digital Imaging
8.2 Radiometry and Photometry
Since we are interested in measuring the quantities that are of importance to the eye, we need to determine the transformation from external radiometric quantities to quantities that are meaningful physiologically. The first step is to characterize the radiation source, f ( ?). Qualitatively, the function is described in terms of intensity or brightness. However, the term brightness indicates a human response. For this work, the units of f ( ?) need to be stated more precisely. There is a need to distinguish between the quantities sensed and measured by an instrument and those related more directly to human perception. The major distinction between radiometry and photometry is that radiometry measures absolute quantities, while photometry measures or computes quantities as they relate to the human observer. Furthermore, there is a need to define precisely what is the quality of interest and what is actually being measured. More in-depth discussions on radiometry and photometry are found in texts such as [19, 20, 160, 168, 199, 313].
From a physical point of view, the radiant power emitted by a source is measured in watts. Likewise the power received by a detector is measured in watts. Since the power is a function of wavelength, the unit should be watt/nm. The total power integrated over all wavelengths is called radiant flux or radiant power and is measured in watts. We denote radiant flux per nm by f r( ?). Because radiant flux does not indicate...
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