Visual adaptation time to different ambient light conditions post-de-goggling of night vision goggle

Objectives: The night vision goggles (NVGs) used for aviation can overcome the limitations of the human eye to see in the dark and augment the night vision well enough to enable safe flying and mission accomplishment. The technology, however, has some limitations that need to be defined so that safe flying envelopes can clearly be understood and operational possibilities can be optimally explored within that envelope. Further, the luminance level of NVG displays which operates in twilight to low photopic range affects dark adaptation of the eye post-de-goggling of NVG. Hence, the assessment of visual adaptation time through Gen-2++ NVG under different illumination conditions using contrast sensitivity (CS) and visual acuity (VA) as parameters was the desired objective of the study. Material and Methods: In a prospective repetitive measure design, a total of 64 volunteered participants were examined for their visual adaptation time through Gen 2++ NVG post-de-goggling using Pelli-Robson CS chart and United States Air Force (USAF) 1951 tribar chart in the NVG Lab at two different locations. The visual adaptation time was measured under three different illuminations: Full moon (FM), half moon (HF), and quarter moon (QM). Furthermore, log CS values and VA of participants were measured under three ambient moonlight conditions with unaided eyes and using NVG. Results: The average recovery time post-de-goggling of NVG for CS (i.e., CS value before wearing NVG) was 1 min 34 s for FM, 2 min 50 s for HF, and 4 min 01 s for QM conditions. The average recovery time for VA (i.e., VA value before wearing NVG) was 1 min 29 s for FM, 3 min 01 s for HF, and 5 min 32 s for QM conditions. The results of this study brought out that visual adaptation time post-de-goggling of Gen 2++ NVG increased significantly for both CS and VA with decreasing ambient moonlight illumination ( P < 0.001). This study also brought out that the logarithm of the minimum angle of resolution value of CS and VA under three ambient light illuminations decreased significantly with a decrease in illumination both during unaided eye and NVG usage. Post hoc analysis revealed that there was a significant difference in visual adaptation time in both CS and VA between all three ambient light conditions ( P < 0.001). Conclusion: This study has estimated the time for dark adaptation required after de-goggling NVG use in different ambient moonlight levels for Gen 2++ NVG using CS and VA charts. The values based on CS can be considered in terms of time required for the detection of objects while that of VA can be attributed to the time for recognition of the object. The visual adaptation time post-de-goggling of NVG is very crucial and critical during approach, landing, and taxiing on the runway under dim light conditions for the accomplishment of mission safety. However, keeping in view the dynamic changes in night sky illuminations during flying operations, the findings of the study need to be validated in operational conditions.