POWER SPECTRAL MEASUREMENTS BY THE DIFFRACTION OF COHERENT LIGHT
RIVERSIDE RESEARCH INST NEW YORK
Pagination or Media Count:
The theory and technique of estimating power spectral densities by Fraunhofer diffraction are investigated analytically and experimentally. A technique for estimating the spectral density is developed by which the estimate is independent of changing light levels in the diffraction system. It is shown that the estimated spectrum is biased and in the case of a rectangular aperture, the error can be written in terms of the Hilbert transforms of the spectrum. The variability of the estimate is computed and the ratio of the variance to square mean is shown to approach unity for large apertures, indicating the need for smoothing by additional filtering. Reasonable bounds on the bandwidth of the scanning filter are established theoretically. The experimental results are found consistent with the theoretical evaluations. Techniques for reducing the zero-order light level are investigated. The spectral densities of several common photographic films are experimentally determined and compared with models of grain-noise and total film-noise.
- Lasers and Masers