Noise and coherence in optical image processing. II. Noise fluctuations

Fluctuations in image illuminance resulting from various sources of optical noise are studied as a function of the spatial coherence of the illumination. It is shown that noise fluctuations caused by the pupil plane can be reduced considerably by using incoherent, or even partially coherent, rather than coherent illumination. Conversely, noise caused by defects in the object plane is not affected by the degree of coherence, except for phase noise which is suppressed in incoherent light. Expressions for noise fluctuations are developed on the basis of a simplifying Gaussian assumption for the noise sources; the validity of this assumption is justified.