Perceptual assessment of laser-speckle contrast

A procedure is described for perceptually assessing the effects of laser speckle on target discrimination in the context of developing a laser-display measurement standard. The perceptual approach avoids the difficulties inherent in assessing speckle contrast photometrically. Size thresholds for triangular test stimuli were obtained using an orientation discrimination task. Data were obtained for both a laser display and an LCD monitor using both a method of constant stimuli and a QUEST procedure. The identification threshold was 29% higher for the laser display. Given that the measured spatial resolutions of the displays were similar, the difference in threshold was attributed to the effects of laser speckle. Laser-speckle size was also estimated as part of the display calibration. For a chosen camera f-number, speckle-size data are necessary to determine the sensor pixel density required to photometrically estimate speckle contrast, and they may be useful for comparison with the data obtained using the perceptual procedure described here. Mean speckle size was estimated by scaling and normalizing the laser-speckle luminance and then determining the width of the associated autocorrelation function. It is proposed that a laser-display standard be developed based in part upon a comparison of task performance on the laser display to be evaluated and on a standard display.