A method to compute the correct estimates of the field performance of low light level imaging systems is described. The use of luminous equivalence ratio is suggested to correct for the errors caused due to the mismatch between the spectral distribution of the field source and the spectral distribution of the detector-calibrating source. Luminous equivalence ratios for some typical source–device combinations are computed. The results indicate that the extent of this correction becomes quite considerable for certain low light level imaging systems incorporating electrooptical devices having extended red response. A nomograph is presented to illustrate the use of this method for obtaining correct estimates of the field performance.
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