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Chapter 1:
System Considerations
1.1 Radiometry 1.1.1 Blackbody radiation Blackbody radiation is the emission of radiant energy which takes place from a blackbody at a fixed temperature. A blackbody is an ideal body which absorbs all incident radiation and reflects none. Its radiating and absorbing efficiency, called its emissivity factor, is unity. A graybody is an object with an emissivity factor less than unity. Although an ideal radiator, a blackbody should not be considered a meaningless abstraction. On a cosmological level, cosmic background radiation which came into being shortly after the creation of the universe has been observed to fit a blackbody curve with a high degree of precision. In the context of electro-optical system design, the ideal assumption of a blackbody is extremely useful because it represents a limiting case or may be an approximation to a real set of conditions, as for example in the calculation of stray radiation from an internal baffle. Blackbodies are used as calibrated sources in simulation and spectrometer applications. 1.1.2 Planck's equation Blackbody radiation has a spectral energy distribution as a function of temperature which is described by Planck's equation (see Fig. 1.1): L(λ)=2hc2λ51exp(hc∕λkT)−1Wm−3sr−1, where L is spectral radiance h is Planck's constant, 6.6262 × 10−34 Joule (J) c is the velocity of light, 2.9979 × 108 m∕s λ is the wavelength in meters k is the Boltzmann's constant, 1.3806 × 10−23 J∕K T is absolute temperature in degrees K.
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