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Fig. 9-23. Solar Radiometric Input. Radiometric input (radiance in Wrrr2 nr1 [wavelength]) of the Sun at sea level as a function of the wavelength.

The integration of the spectral radiometric input over the spectral bandwidth gives the power density in the spectral band of interest To first-order we can assume that lambertian (ideal) reflection with a constant reflection coefficient occurs at the target scene (this approximation holds for small spectral bandwidth). Hie area of the ground pixel resulting in back-radiated power determines the power density per solid angle. The atmosphere attenuates this radiation by a constant transmission factor (again invoking an approximation for small spectral bandwidth). Hie effective aperture at orbital altitude collects a very small fraction of this radiation resulting in the power at the entrance of the optics. The signal power is attenuated further by transmission through the optics, ultimately resulting in a lower power level at the detector pixel. During the integration period a certain amount of energy (power times integration period) is accumulated in each pixel. This energy is divided by the energy of one photon (which is wavelength dependent) resulting in the number of available photons per pixel. The quantum efficiency of the detector transforms this number of photons into the number of available electrons. These electrons comprise a charge packet and correspond to the output signal of the detector.

To fully characterize the radiometric performance of an instrument, we must also determine the signal-to-noise ratio and dynamic range. The signal-to-noise ratio

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