HUMAN CONTRAST SENSITIVITY IN COHERENT MAXWELLIAN VIEW - EFFECT OF COHERENT NOISE AND COMPARISON WITH SPECKLE

Lasers have been used in vision for measuring the neural contrast sens itivity function (CSF) by forming interference fringes on the retina. We distinguish among three kinds of illumination with lasers: incohere nt (without noise), Maxwellian or coherent (with coherent noise), and diffuse coherent (with speckle). The three have different characterist ics and different CSF's. A coherent imaging system is designed to meas ure the CSF with fully coherent illumination. This is the CSF of the w hole visual system, although it is measured with gratings imaged on th e retina. It therefore differs from the neural CSF's measured by other authors with partially coherent illumination. However, the neural CSF 's are also obtained in this study with and without noise. The effects of coherent noise and speckle on both the visual system and neural se nsitivities are studied and compared. Coherent noise differs from spec kle in the following ways: (1) It behaves as a high-pass filter, reduc ing sensitivity in the low-spatial-frequency range, whereas speckle is a low-pass filter; (2) quantitatively, coherent noise reduces neural sensitivity by a factor k(m)' with a maximum value between 4 and 6, wh ereas speckle reduces neural sensitivity by a factor k(s) with a maxim um value of similar to 25 (1.4 log units) for a 3-mm pupil and up to 3 5 (1.55 log units) for a 1-mm pupil; (3) the masking effect of the coh erent noise is affected by changes in luminance but not by changes in pupil diameter; however, the pupil size is the main parameter affectin g the masking effect of the speckle. (C) 1997 Optical Society of Ameri ca.