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Chapter 8:
Image Contrast

8.1 Radiometry

Radiometry generally applies to the distribution of quanta over space, angle, and time. Specifically, radiometry applies to energy, whereas photometry applies to luminous energy, measured in Talbots. In the current discussion, radiometry applies to both. The terminology is defined in Table 8.1.

Within the field of optics, flux is generally accepted as flow. However, in physics, flux indicates the strength of a field per area. Sensors for commercial applications are frequently defined by luminance in lux, whereas sensors for scientific applications are normally specified in photons or electrons. Quantum efficiency indicates the conversion of photons to electrons.

8.2 Expression of Contrast

The contrast of an object is defined as

C = (ΦOP − ΦBP)/(ΦBP + ΦD),

where ΦOP is the flux of the object pixel, ΦBP is the flux of the background pixel, and ΦD is the flux of the detector. The range of contrast is from −1 to +∞. A negative contrast indicates a dark object within a bright background. A positive contrast indicates an object that is brighter than the background. Quite often both objects are rather dark.

The flux of the detector has numerous components, as described in Chapter 15. The two most common are dark current and read noise. They are often stated as electron currents.

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