Psychophysics of Night Vision Device Halo

We provide quantitative measurements of night vision device (NVD) halos formed by light sources as a function of intensity and distance, describe a method to simulate their effects in the lab and present results from psychophysical experiments designed to analyze haloinduced errors in slope estimation. The effective halo generating potential of a point source is presumed to be a function of ambient illumination, source intensity, spectral content and distance. We designed a study to compare perceptual and objective measures of NVD halo size directly using identical laboratory conditions. NVD halo size is effectively invariant of light source intensity and distance when the halo is perceived. Source intensity and distance affect the likelihood that a primary and secondary halo will be perceived and the vividness of the halo but have little effect on halo image size when halo is present. The fact that primary halos do not change angular size with distance of the generating sources however lends them interesting perceptual properties. A given halo should appear to shrink as one approaches the light source and an isolated bright halo should appear nearer than a dim one even if further away. We have verified these predictions in the lab. The effects of halo on judgments of slope were studied during simulated helicopter approach and landing. Three-dimensional computer graphic simulations of flight over modeled terrain were rendered by a cluster of PC workstations. In one set of experiments observers made judgments about their attitude with respect to the ground. In another, observers watched a simulated approach to the runway and estimated their aimpoint given their current heading. Observers perceived increasing slant with increasing simulated slant in the daylight conditions and in night conditions with regular patterns of lights in the absence of halo. In the presence of halo or with irregular patterns of lights there was a poor correlation between perceived and simulated slant. When slant was seen in the regular light arrangement and halos were present, observers reported a strong increase in the perceived size of the halos with simulated distance although halos were constant size over the image (appropriate size constancy as found in Emmert’s law). Anecdotally observers reported they could ‘see through’ the halo to the slanted surface suggesting they can segregate the slant of the surface from the frontal slant specified by the halo. We discuss the results in terms of NVD simulation and of the ability of human operators to compensate for perceptual distortions.