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

INTRODUCTION

The ability to conduct effective operations at night has been recognized as advantageous to Armed forces from past experiences. Image intensification technology has been widely used to increase visual capability at night. Image intensifiers amplify portions of the electromagnetic spectrum available in the ambient environment, usually 2000– 7000 times greater than the actual scene.^[1] The amplification depends upon the type of image intensifier and its response to ambient light conditions. 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 envelope can clearly be understood and operational possibilities can be optimally explored within that envelope.^[2] Aeromedical issues of NVG are reduced visual acuity (VA), poor contrast, field of view, less effective through rain, fog, and snow.^[2,3] In addition to issues related to visual perception through NVGs, the effect of NVG on dark adaptation of eyes is a concern. Dark adaptation is an independent process during which eyes adjust from high luminance to low luminance which includes biochemical, physical, and neuronal aspects.^[4] The use of NVG does not require scotopic vision because the output brightness of image intensifier tubes is in the low photopic to mesopic range (600–930 nm). This output brightness will affect the dark adaptation of human eye. The literature on visual adaptation of eye reveals that, as the eyepiece illumination of NVG would have been providing low photopic or mesopic vision, it would take some time to adapt to have good scotopic vision post-de-goggling of NVG.^[4] Aircrew on night operations used to wear NVGs for a substantial period. Numerous studies examined the dark adaptation process after the removal of NVGs. However, the exposure to NVG was not more than 5 min in any of the previous experiments.^[5-7] The evolution of NVGs through different generations has different display brightness levels which affect time to dark adaptation post-removal of NVGs. The display of recent NVG is slightly brighter than older NVGs. Hence, the relation between display brightness and dark adaptation time is a concern post-removal of NVG. Contrast sensitivity (CS) in actual night flying scenarios in different moon-light conditions is one of the visual parameters of enormous importance.^[4] CS defines the threshold between the visible and invisible. Through the years, several different measurement techniques have been available to quantify an individual’s CS as quickly and easily as VA is measured. Hence, it is an operational concern to estimate visual adaptation time in quarter moon (QM), half-moon (HF), and full moon (FM) light conditions post-de-goggling of Gen NVG 2++ using CS and VA as parameters.

MATERIAL AND METHODS

Experimental protocol

This study was approved by the Institute Ethics Committee and a written informed consent was obtained from all study participants. This research was conducted at standard NVG Laboratory conditions at two different locations: Study of non-aircrew participants at the Institute of Aerospace Medicine, Bangalore, and for Aircrew participants at No 1 Aeromedical Training Centre, located at Airforce Station Hindon. The participants were familiar and well indoctrinated on the adjustment and focusing procedures of NVG. The participants were instructed to undergo 30 min of dark adaptation and made to sit at 3 m from the Pelli-Robson CS chart. The participants were asked to read the Pelli-Robson CS chart from the top left. The readings were in triplets in each line. The log CS value of the last triplet for which two letters (two of three) are named correctly was recorded as the log CS at QM light condition without NVG. All participants were initially asked to wear the NVG mounted helmet after proper adjustment and focussing. Then the participants were instructed to keep visualise through the NVG for 10 min without closing the eyes. During this 10-min period, log CS values with NVG were also recorded. After the completion of 10 min, the participants were instructed to flip up (degoogled) the NVG and asked to read the Pelli-Robson chart till the time they were able to read the same triplet letters as before wearing NVG. The time taken to read the baseline log CS value without NVG after de-goggling was recorded as visual adaptation time post-de-goggling of NVG Gen 2++. The above measurements were similarly repeated for HF (0.05 lux) and FM (0.1 lux) and time taken for visual adaptation time post-de-goggling of NVG was recorded [Figure 1]. Similar measurements of visual adaptation time were recorded from 6 m distance using USAF 1951 Tri-Bar Chart with VA as parameter in three moonlight conditions. During the study, Log CS values and logarithm of the minimum angle of resolution (log MAR) VA values were also measured with the naked eye and with NVG usage.

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Figure 1:

Schematic representation of experimental protocol. CS: Contrast sensitivity, QTR: Quarter moon, NVG: Night vision goggle

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RESULTS

Visual adaptation time

The visual adaptation time post-de-goggling of Gen 2++ NVG under FM, HF, and QM conditions is depicted in Tables 1 and 2. It was observed that the visual adaptation time showed an increasing trend with the decrease in illumination [Figures 2 and 3]. This increment in visual adaptation time was statistically significant using ANOVA test (for CS: ANOVA, F [2,126] = 412.72, η² = 0.87, < ≤ 0.001) and (for VA: ANOVA, F [2,126] = 1010.49, η² = 0.94, < ≤ 0.001).

Table 1: Assessment of difference in visual adaptation time (in seconds) for post-de-goggling of Gen 2++ NVG using CS (n=64).

Visual adaptation time postdegoggling of NVG using CS
Ambient light	QM	HM	FM
Mean	248.25 (4’01”)	170.19 (2’50”)	93.66 (1’34”)
SD	67.44 (1’07”)	48.4 (48”)	38.03 (38”)
Repeated measures ANOVA F (2,126)=412.72	p<0.001

Minutes are denoted as’ and Seconds as”. NVG: Night vision goggles, CS: Contrast sensitivity, QM: Quarter moon, HM: Half moon, FM: Full moon, ANOVA: Analysis of variance, SD: Standard deviation

Table 2: Assessment of difference in visual adaptation time (in seconds) for post-de-goggling of Gen 2++ NVG using VA (n=64).

Visual adaptation time postdegoggling of NVG using VA
Ambient light	QM	HM	FM
Mean	332.12 (5’32”)	186.34 (3’01”)	88.84 (1’29”)
SD	48.63 (49”)	62.41 (62”)	28.39 (28”)
Repeated measures ANOVA F (2,126)=1010.49	p<0.001

Minutes are denoted as’ and Seconds as”. NVG: Night vision goggles, CS: Contrast sensitivity, QM: Quarter moon, HM: Half moon, FM: Full moon, ANOVA: Analysis of variance, SD: Standard deviation

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Figure 2:

Box-and-Whisker plot (visual adaptation time post-degoggling of Gen 2++ night vision goggles using contrast sensitivity). CS: Contrast sensitivity, QM: Quarter moon, NVG: Night vision goggle, HM: Half moon, FM: Full moon.

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Figure 3:

Box-and-Whisker plot (visual adaptation time post-degoggling of Gen 2++ night vision goggles using visual acuity). VA: Visual acuity, QM: Quarter moon, NVG: Night vision goggle, HM: Half moon, FM: Full moon.

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CS

It was observed that the contrast discrimination (in log CS values) obtained from Pelli-Robson Chart with Gen 2++ NVG was significantly better than the scores without wearing NVG in all three ambient moonlight conditions (< < 0.001). This is depicted in line diagram [Figure 4] and values are depicted in Table 3.

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Figure 4:

Line diagram depicting the contrast sensitivity under different illumination conditions for Gen 2++ night vision goggle. CS: Contrast sensitivity, NVG: Night vision goggle.

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Table 3: Comparison of contrast sensitivity values among different moonlight conditions.

Comparison of CS (log values) among different illumination conditions
Ambient light	Without NVG (Mean±SD)	With NVG (Mean±SD)	Wilcoxontest
QM	0.49±0.09	0.82±0.10	p<0.001
HM	0.72±0.10	0.99±0.11	p<0.001
FM	0.92±0.10	1.13±0.12	p<0.001
Friedman test	p<0.001	p<0.001

NVG: Night vision goggles, CS: Contrast sensitivity, SD: Standard deviation, QM: Quarter moon, HM: Half moon, FM: Full moon

VA

It was observed that the VA (in log MAR values) obtained from USAF 1951 Tri-Bar chart with Gen 2++ NVG was significantly better than the scores without wearing NVG in all three ambient moonlight conditions (< < 0.001). This is depicted in line diagram [Figure 5] and values are depicted in Table 4.

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Figure 5:

Line diagram depicting the visual acuity (VA) under different illumination conditions for Gen 2++ night vision goggle. CS: Contrast sensitivity, Log MAR: Logarithm of the minimum angle of resolution, NVG: Night vision goggles, QM: Quarter moon, HM: Half moon, FM: Full moon.

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Table 4: Comparison of VA among different ambient moonlight conditions.

Comparison of VA (log MAR values) among different illumination conditions
Ambient light	Without NVG		With NVG		Wilcoxon test
	Log value (Mean±SD)	Snellen’s equivalent	Log value (Mean±SD)	Snellen’s equivalent
QM	1.04±0.03	20/219.2	0.43±0.05	20/53.8	p<0.001
HM	0.74±0.07	20/109.9	0.36±0.04	20/45.8	p<0.001
FM	0.58±0.07	20/76	0.30±0.04	20/39.9	p<0.001
Friedman test	p<0.001		p<0.001

VA: Visual acuity, log MAR: Logarithm of the minimum angle of resolution, NVG: Night vision goggles, SD: Standard deviation, QM: Quarter moon, HM: Half moon, FM: Full moon

DISCUSSION

Night vision equipment for military aircraft significantly improves the ability to operate in low light. After using NVG, most pilots are apprehensive about doing night flying on their own again. At present, available night vision imaging technologies enable pilots to fly in ambient light, an exceedingly risky, if not impossible, situation without assistance. However, the visual information provided by these devices is not nearly as good as what can be seen with the unhindered, unaided eye during daylight hours. The use of night vision devices compromises a number of visual parameters, including depth perception, sharpness, and field-of-view. The intrinsic properties of image intensification-based sensors create additional challenges for the processing of visual data because their spatial and spectral contents differ from those of unassisted vision.^[10]

When an individual is suddenly exposed to a dark environment from daytime or fully illuminated conditions, he/she takes some time before visualizing optimally in the dark. This phenomenon is known as dark adaptation. The ability of visual system to achieve good visibility in low light conditions is because of dark adaptation. The dark adaptation occurs in two stages: The initial stage is commensurate with lowering of stimulus threshold in cone cells and the second stage is a function of the sensitivity of rod cells. On exposure to bright light conditions, the pigment in the rods, i.e., rhodopsin gets completely degenerated. The time taken for cone adaptation is approximately 6–8 min while it takes 20–30 min for rhodopsin to regenerate and optimally function. Although the “Dark adaptation time,” in laboratory conditions varies from 30 to 40 min, in nature, these levels of illuminance do not usually occur, and so, the time to adapt is much shorter.^[2,11] The NVG image is monochromatic in the green spectrum. After an extended period of NVG use, some aircrew complained of an orange or brown after-image. This is called brown-eye syndrome which is a transient physiological effect and occurs due to green photoreceptor fatigue. This phenomenon affects people differently and usually lasts for a few minutes depending upon how long the NVGs were worn. However, this is not likely to affect the VA once the eye is fully dark adapted.

Numerous studies examined the dark adaptation process after the removal of NVGs. However, the exposure to NVG was not more than 5 min in any of the previous experiments.^[5-7]

Multiple studies have been extensively used VA to evaluate and describe through night vision image intensification devices. The resolution limit of night vision systems under varying ambient illumination and contrast levels was found by multiple investigations. The use of image intensifiers to test CS has been less common. Such data would be helpful because CS can offer a more complete estimate of visual function across a variety of stimulus sizes, whereas acuity merely indicates the limit of resolution.

In a prospective repetitive measure design, in the present study, a simpler approach was used to measure CS and VA using NVG Gen 2++ and expressed as log values. The time taken to dark adaptation after the removal of NVGs was measured for all three moonlight conditions. The results obtained were analyzed to find the changes in dark adaptation time and log values of CS and VA under different illumination conditions. The average recovery time (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 [Table 1]. 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 [Table 2].

The results of this study brought out that visual adaptation time post-de-goggling of Gen 2++ NVG increased substantially with decreasing ambient moonlight illumination (< < 0.001). This study also brought out that log value of CS [Table 3] and log MAR values of VA [Table 4] under three ambient light conditions decreased significantly with decrease in illumination both during with NVG and without NVG usage (< < 0.001).

These findings of visual adaptation time, log CS, and log MAR VA values are consistent with the results of other studies.^[5-7,12-15] Post hoc analysis revealed that there was a significant difference in visual adaptation time using both VA and CS and other parameters such as log CS and log MAR VA with and without NVG between all three ambient light conditions (< < 0.001). This finding was also considered an important finding in the present study. Furthermore, the estimated dark adaptation time falls within the cone adaptation threshold curve limits.^[10]

The evolution of NVGs through different generations has different display brightness levels which affects the time-to-dark adaptation post-removal of NVGs. The display of recent NVGs is slightly brighter than older NVGs. In this study, the average display brightness level of Gen 2++ NVG was 1.06 lux for QM, 1.25 lux for HF, and 1.47 lux for FM conditions [Table 5]. The increase in display brightness with increased illumination is attributed to increasing gain properties of I2 tubes. In addition to ambient illumination, the display brightness of NVG affects the time required for dark adaptation after the removal of NVG. Hence, the relation between display brightness and dark adaptation time is a concern post-removal of NVG.

These findings in this study have operational implications in night flying with the usage of NVG, where a particular level of light would be required to get differentiate contrast between the objects seen through NVG and CS function is essential for target recognition by pilot. Furthermore, visual adaptation from unaided dark night or NVG light to bright light conditions was almost instantaneous, due to photopic light conditions. However, adaptation from NVG vision (mesopic) to ambient moonlight conditions requires few minutes which is a serious concern for aerospace safety during approach/landing and taxiing the aircraft in night flying operations. Hence, the aircrew flying with NVG needs to be cautioned about the period for dark adaptation required for optimum detection and recognition of any objects or obstacles post-degoggling of NVG.

CONCLUSION

This study was undertaken at standard NVG labs to estimate the duration of visual adaptation time for various light conditions during night operations using CS and VA as parameters. It was observed that the adaptation time to ambient moonlight conditions after de-goggling of NVG was found to be statistically significant. Furthermore, the visual adaptation time was found to increase with decreasing moonlight intensities. The visual adaptation post-degoggling of NVG Gen 2++ based on CS can be considered in terms of the time required for the detection of objects while that of VA can be attributed as time for recognition of the objects. Notwithstanding the above, the aircrew undertaking an NVG mission should be aware of the occurrence of this phenomenon.