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R. Thomas, S. Wright, P. Clark, W. Thompson, J. Gooch; Spectral Filter Effects on Night Vision Goggle Acuity and Preservation of Dark Adaptation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2001.
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The high luminance output of night vision goggles (NVGs) can cause a loss of dark adaptation for the viewer that results in sub-optimal scotopic and mesopic vision after using the goggles and could potentially delay visually dependent decision making. Filters have been designed to help mitigate this problem by changing the overall luminance and the spectral output characteristics of the NVGs with the intention of maintaining good visual acuity when viewing through the goggles, while sparing rod function after goggle use.
Acuity through the goggles was measured using a 25% contrast target. Preservation of dark adaptation after NVG use was assessed using a Scotopic Sensitivity Tester (SST) and a low luminance acuity chart. The study was conducted in a repeated measures design with subjects serving as their own control (tested with and without filters). Testing was accomplished at two light levels, roughly corresponding to starlight and 1/4 moon conditions.
Under both light levels there was a statistically significant loss of acuity of about one half of a one line while using the filters. The SST identified improved retention of dark adaptation while using the filters by 47% under the low light level (starlight conditions) and 31% under the high light level (¼ moon conditions). However, the improved retention of dark adaptation was only significant at the low light level. No significant differences in performance were seen with the low luminance chart.
The slight loss of visual acuity is unlikely to cause a significant safety risk and the improvement in dark adaptation retention may be beneficial in the operational environment. The filters, though, cannot be endorsed for all operational conditions. Use of the filters should be left to the discretion of the commanders and NVG users to weigh the slightly decreased visual acuity against the potential for improved preservation of dark adaptation in the context of their duties.
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