The concept of measurements of the instantaneous intensity of a monochromatic wave developed in the previous chapter assumes that any real photodetector provides a smoothing of the intensity of the incident wave. Even in the case of an ideally detecting device a shortest interval Δt exists, which is needed to initiate photoelectrons. For this reason any detectable quantity can not depend directly on the carrier frequency of the incident wave but only on lower frequencies. A quasi-monochromatic wave possesses properties of such a smoothed field. Moreover, since any detectable magnitude is proportional to the magnitude of light energy, the statistical properties of a quasi-monochromatic wave must be developed from measurements of the instantaneous intensity or from other quantities treated in terms of the intensity.
KeywordsProbability Density Function Thermal Radiation Black Body Radiation Statistical Ensemble Quadrature Component
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