Switching from Forward-Looking Infrared to Night Vision Goggles: Transitory Effects on Visual Resolution

Helmet-mounted displays under development for rotary- and fixed-wing aircraft will allow the user to switch electronically between forward-looking infrared (FLIR) and night vision goggle (NVG) sensors. These sensor transitions potentially involve large changes in display luminance which could transiently impair visual resolution and performance. The purpose of this study was to identify the display luminances which produce a transient reduction in vision when switching from a higher luminance (i.e., FLIR) to a lower luminance (i.e., NVG) display. A letter recognition task was used to assess the effect of luminance adaptation on visual resolution in five subjects. A significant reduction in letter recognition was observed in the first second after switching from simulated FLIR to simulated NVG's when the FLIR luminance was > or = 10 fL. By varying letter size, contrast, and exposure time, the magnitude and duration of visual loss after switching from a bright (49.2 fL) FLIR display were determined. The visual loss lasted up to 4 s, and included a 2x reduction in visual acuity, and a 3x reduction in contrast sensitivity. Large differences in sensor display luminance should be avoided to maintain high levels of visual performance and aviation safety. Design features or training may be necessary to achieve a proper balance between FLIR and NVG luminances which optimize performance and safety without sacrificing the quality of the sensor image.