Effects of image intensifier halo on perceived layout

Night vision devices (NVDs) or night-vision goggles (NVGs) based on image intensifiers improve nighttime visibility and extend night operations for military and increasingly civil aviation. However, NVG imagery is not equivalent to daytime vision and impaired depth and motion perception has been noted. One potential cause of impaired perceptions of space and environmental layout is NVG halo, where bright light sources appear to be surrounded by a disc-like halo. In this study we measured the characteristics of NVG halo psychophysically and objectively and then evaluated the influence of halo on perceived environmental layout in a simulation experiment. Halos are generated in the device and are not directly related to the spatial layout of the scene. We found that, when visible, halo image (i.e. angular) size was only weakly dependent on both source intensity and distance although halo intensity did vary with effective source intensity. The size of halo images surrounding lights sources are independent of the source distance and thus do not obey the normal laws of perspective. In simulation experiments we investigated the effect of NVG halo on judgements of observer attitude with respect to the ground during simulated flight. We discuss the results in terms of NVG design and of the ability of human operators to compensate for perceptual distortions. Halo in the context on NVGs refers to the phenomenon that a bright light source viewed through NVGs appears to be surrounded by a corona or halo that is much larger than predicted by the point spread function of the device. The phenomenon is superficially similar to the physiological halo reported in normal and diseased eyes and the coronas that can be seen when viewing light sources through the atmosphere. However, NVG halos are generated in the image intensifier tubes. Examination of these halos is important for both NVG design and to understand limitations on their use in operational settings1. Being device artifacts, these halos have characteristics that are significantly different from the associated environmental features in the image. These distinctions are perceptually important and predict specific distortions of perceived environmental layout and movement. We provide quantitative description of the halos formed by light sources as a function of intensity and distance and describe briefly a method and preliminary data from psychophysical experiments designed to analyse halo-induced errors in slope estimation.