Optical filter effects on night vision goggle acuity and preservation of dark adaptation.

INTRODUCTION: The high output of night vision goggles (NVGs) can cause a loss of dark adaptation, resulting in suboptimal unaided vision. Optical filters have been designed to mitigate this problem by changing the overall output characteristics of the NVGs. METHODS: Several aspects of visual performance related to NVG use were studied in a repeated measures design, filters versus no filters. NVG acuity was assessed using a 25% contrast chart, while preservation of dark adaptation after NVG use was measured with a scotopic sensitivity tester (SST) and a low luminance acuity chart. Testing was accomplished at two light levels, roughly corresponding to starlight and quarter moon conditions. RESULTS: Use of the filters resulted in a statistically significant loss of acuity of about a 1/2 line (approximately 2.5 letters) at both light levels. The second part of the study identified a 47% improvement in preservation of dark adaptation under simulated starlight conditions and a 31% improvement under simulated quarter moon conditions with filter use; however, only the starlight finding was statistically significant. No significant differences in performance were seen with the low luminance chart. DISCUSSION: Despite a small loss of visual acuity with filter use, the improvement in retention of dark adaptation may be beneficial in certain operational environments. Aviators, airmen, and commanders should evaluate how the potential for slightly poorer visual acuity and improved recovery of dark adapted vision relates to their mission specific requirements prior to implementing use of NVG filters.

Corrective lens use and refractive error among United States Air Force aircrew.

Corrective lens use by military aviators is an important consideration in the design of head-mounted equipment. The United States Air Force (USAF) has periodically monitored lens use by aviators; however, it has been over a decade since the last study. We provide an update on the prevalence of corrective lenses and refractive error among USAF aircrew based on eyeglass orders processed through the Spectacle Request Transmission System (SRTS). Currently, 41% of active duty USAF pilots and 54% of other aircrew require corrective lenses to perform flight duties. Refractive errors are characterized by low to moderate levels of myopia with a mean spherical equivalent power of -1.01 diopters (D) for pilots and -1.68 D for others. Contact lenses, and more recently refractive surgery, reduce the number of aircrew that must rely on spectacles when flying; however, spectacle compatibility remains an important consideration in the cockpit.