Night vision involves different visual functions which may vary significantly with changing light levels. Changes in visual function are well-recognized and include decreased visual acuity in central and peripheral locations, as well as reduced contrast sensitivity for all spatial frequencies [77]. Night vision systems, such as image intensifiers and thermal imaging systems, have considerable capabilities and allow the expansion of night operations. Visualization through night vision systems has improved situational awareness and spatial orientation which in turn enhances navigation, threat detection, target acquisition and weapons deployment [32].

According to the Night Vision and Electronic Sensors Directorate , the development of image intensification systems began in the 1940s with the use of Sniperscopes during World War II (http://​www.​cerdec.​army.​mil/​inside_​cerdec/​nvesd/​history). While limited, Sniperscopes initiated more advanced night vision technology. The 1970s saw development of the first night vision goggles (NVGs) and use of forward looking infrared (FLIRs) systems for ‘seeing’ at night as well as through smoke, fog and other obscured conditions. Night vision systems progressed significantly in the subsequent years, expanding military capabilities, including aided target detection and recognition. Night vision technologies including NVG and FLIR have been crucial in recent conflicts, significantly enhancing operational capabilities and performance. Exploitation of night vision systems has significantly impacted night operations which is vital for around-the-clock combat readiness.

Under favorable ambient illumination such as a full moon and clear sky, the output luminance of NVGs is in the low photopic range whereas under more challenging ambient conditions such as overcast moonlight or starlight, the output luminance of an NVG is under mesopic range [12]. NVGs are designed to amplify ambient light (Fig. 4.2). Long wavelength visible and near infrared light (600–900 nm) are captured through an objective lens of the image intensification device and are sent to an infrared-sensitive photocathode which converts photons to electrons. The released electrons are then amplified within a microchannel plate. Once the electrons from the microchannel plate hit the phosphor screen, the electrons are converted back to photons creating a green visible image that can be viewed through an eyepiece (Fig. 4.3) [64]. Previous research has shown that NVG -aided visual acuity exceeds unaided visual acuity under the same ambient luminance conditions [39]. This, however does not imply that visual acuity will be as good as in the daytime. Under optimal night conditions, an individual with 20/20 daytime vision can expect no better than 20/50 vision with second-generation NVGs and 20/40 vision with third-generation NVGs [63, 70]. Furthermore, as NVG works by amplifying available light, NVG-aided visual acuity may be dependent on the display luminance of the NVG. This relationship may be mainly attributed to the quantal fluctuations in light intensity rather than optical factors such as accommodation, pupil size and/or high order aberrations [64]. The effectiveness of NVGs may be significantly reduced under conditions such as rain, snow, dust, haze, fog, and smoke [17].

Contrast sensitivity (CS) , which assesses the ability to detect and/or recognize low contrast stationary or moving targets, can be decreased through NVGs. Both spatial and temporal contrast sensitivity were lower through NVGs compared to CS in response to stimuli presented without the NVGs in place but at brightness (luminance) and chromaticity (green color) levels which matched those of the NVG display. Hence CS can diminish through NVGs due to optical attenuation and electro-optical noise, even under optimal ambient levels of illumination [48, 49].

NVGs must be properly adjusted for optimal clarity and aligned to the visual axis to approach the best level of acuity. Aviators who are experienced in using NVGs have been shown to achieve visual acuity ranging from 20/50 to 20/55 with their ‘usual’ adjustment method. Visual acuity significantly improves to a range of 20/45 to 20/50 when adjustment is performed using the standard NVG resolution chart whereas visual acuity of 20/30 to 20/40 may be achieved with proper adjustment training such as in an NVG test lane [15]. Moreover, the prolonged use of improperly adjusted interpupillary distance on NVGs (e.g., PVS-5A) may induce shift in lateral phoria as a byproduct of additional convergent or divergent efforts. This has been implicated in reports of aviators failing stereoscopic depth perception tests after a prolonged flight training employing NVGs [61].

As part of the possible NVG adjustments, NVG systems have built-in spherical lens oculars allowing spherical refractive corrections ranging from +2.00 diopters of hyperopia to −6.00 diopters of myopia. Astigmatism of up to ±1.00 diopter may be corrected by spherical equivalent [19]. Prior to the availability of refractive surgery in the military, older models of NVGs such as AN/PVS-5 and 5A were incompatible with aviators with refractive error. These “full frame” goggles have an occluding face-plate (Fig. 4.4) which precluded the use of spectacles for individuals with refractive errors exceeding the corrective limits of the goggles [19].

Another challenge presented to users of night vision systems is display luminance and the impediments associated with switching between two types of imaging devices from higher luminance to lower luminance. Luminance of the NVG display is typically in the mesopic to low photopic range (0.3–2.0 foot-lambert) and remains relatively constant in any one night sky condition whereas luminance of the FLIR display can be adjusted to be nearly 100 times brighter than the NVG display (Fig. 4.5). In a simulated experiment, the investigators demonstrated that shifting from higher to lower luminance may increase adaptation demands causing transient visual loss of up to 4 s, reduction of visual acuity and contrast sensitivity [50]. Recent developments in next generation devices which combine image intensification and forward looking infrared systems will attempt to highlight the benefits and reduce the limitations of both systems.

Night vision, whether aided or unaided, is susceptible to several physiologic factors. It is known that certain ocular diseases such as retinitis pigmentosa as well as lack of essential nutrients such as Vitamin A and zinc can decrease night vision. Moreover, in healthy individuals various factors including age, pupil size, astigmatism [6, 46, 67] as well as oxygenation [11] and acceleration can impair visual performance at night [68]. Dark adaptation gradually slows down and mesopic and scotopic visual performance decline with age during adulthood [26, 27, 46]. Healthy older adults require significantly more time to dark adapt compared to younger adults [27]. Pupil size generally varies between 2 and 8 mm when shifting from bright light to very dark conditions. Under low light, increased pupil size reduces retinal image quality due to increased higher order aberrations, such as spherical aberration and coma, as well as light scatter in the eye which in turn, decreases visual acuity and contrast sensitivity [4]. Uncorrected astigmatism may lead to non-optimal NVG visual acuity. Astigmatism <1.00 diopter have minimal impact while greater amounts of uncorrected astigmatism significantly decrease NVG acuity [31]. Consistent with this finding, astigmatism exceeding 0.75 diopters may result in greater mesopic visual function complaints in post-refractive surgery patients [30].

Another challenge to vision is high altitude. Aviators as well as ground forces operating in a high altitude environment may be exposed to hypobaric hypoxic stress . A mild hypoxic state, like breathing air at an altitude of 10,000 ft (3048 m) for a short duration, can diminish visual performance progressively with decreasing light conditions. Furthermore, reduced mesopic visual acuity and contrast sensitivity may also be observed in an even milder hypoxic state (equivalent to breathing at 8000 ft or 2438 m). On the other hand, supplemental oxygen appears to improve low contrast acuity and can extend functionally useful vision to lower light levels suggesting that visual performance is oxygen-dependent [10, 11]. Continued impairment of night vision may be manifested during sustained hypoxic state such as during a sojourn at high altitude. The effects of high altitude to dark adaptation are rapid and sensitive. Hypoxia can have a critical and systematic degrading influence on the efficiency of dark adaptation. This effect was observed during a course of high altitude exposure which was more apparent during the first 10 min. Recovery also appeared to be rapid and substantial upon return to sea level or oxygen supplementation [33].

While NVGs are normally used when visual capabilities are unimpaired by oxygen deprivation, use of NVGs at high altitudes could potentially impair performance. However, NVG-aided contrast sensitivity appears relatively unaffected by oxygenation state. The preservation of the NVG-aided contrast sensitivity is possibly due to its dependency on the goggles’ gain [12, 13, 72].

Visual changes are well-recognized consequences of exposure to high-speeds. With modern weaponry and vehicle performance, service members, such as fighter pilots, may also be subjected to high accelerations. In this environment, there is a significant rise in mesopic luminance threshold (i.e., reduced sensitivity) at +2 Gz, +3 Gz (head to foot acceleration) and +2 Gy (lateral acceleration). Reduced contrast sensitivity may be due to decreased retinal blood supply secondary to blood shifting to lower extremities, reduced cardiac output and/or cerebrovascular constriction associated with hyperventilation. Blood shift may be the primary factor contributing to the changes in contrast sensitivity in +2 Gz and +3 Gz environments but not in a + 2 Gy environment [68]. Specialized trainings such as anti-G straining maneuvers [20] have allowed military pilots as well as astronauts to increase tolerance to high acceleration and safely operate under such conditions.

Marksmanship is a common task for all military service members. For these individuals, effectively engaging a target undoubtedly calls for adequate visual performance. One study observed that individuals with visual acuity of 20/32 or better were more likely to hit a target [22]. Another study showed that the number of missed targets doubled at 20/50 resulting in 71% decrease of marksmanship performance [75]. Both studies indicate that marksmanship is affected by visual acuity at daytime and at nighttime [75] and corroborate the validity of the vision classification system and the importance of the aforementioned vision readiness standards of the U.S. military. These studies also highlighted the importance of vision correction to maximize vision performance, especially for deployed military.

Night driving entails mesopic vision as ambient light sources are usually available during operations. Under limited illumination, driving at night is considerably more visually demanding than driving during the day. Recognition of road signs, obstacles and pedestrians while driving at night can be significantly degraded under low light conditions, and this problem may be more pronounced in older drivers [77]. Overall nighttime driving performance is significantly impacted by reduced visual acuity compared to daytime conditions [76]. There is also a greater risk of nighttime accidents with reduced mesopic vision and increased glare sensitivity [3]. In the military, there are certain instances when service members are required to drive during periods of darkness and limited visibility. Such missions are carefully planned in order to avoid catastrophes as more accidents and incidents are encountered during night operations than their daytime counterparts [28]. Availability of image intensification systems offers wheeled vehicle operators the ability to accomplish their mission even in conditions that would not otherwise be possible. Military manuals are available to address techniques and procedures and specific considerations for using NVGs for driving. Driving using NVGs safely and effectively depends on the operator’s overall proficiency in NVG use and habituation under these conditions, to include a limited field of view and ambient light adjustments.

The daily tasks of military service members include the ability to discriminate objects of military relevance. An object of interest may provide visual clues to help discriminate between friendly forces, combatants and civilian/non-combatants. The hierarchy of target acquisition begins with a target being detected as militarily relevant which may be followed by an action to perform higher levels of discrimination, such as classification, recognition, and identification. Typical target acquisition tasks may include detection and identification of combat vehicles, detection of personnel, and discrimination between handheld objects and weapons. The U.S. military rules of engagement generally require positive identification of a target prior to action, such as use of force against an enemy target [38].

Before the identification of a target can take place, one has to locate the target first. Locating a possible target occurs during a search task. Generally, a search task consists of an operator viewing a specific portion of a scene and, if an imaging system is used, the operator adjusts the device to get a better look at any “target like” areas encountered to determine if a target is, in fact, present [38]. During scotopic search , studies have shown that scotopic contrast sensitivity is significantly reduced with a functional scotoma in the fovea. When the fovea is not functional, the brain adjusts to meet some properties of rod vision. This implies that humans are able to modify their search behavior as ambient light level decreases [45]. In a search and rescue exercise at sea, visual acuity and color vision did not seem to correlate with life raft detection using NVGs, which seems reasonable since NVGs use only a single green color to identify objects and life raft detection likely involves detection of low spatial frequencies which are less acuity dependent. The combination of life raft light and NVGs in night searches seemed to be as effective as daytime searches with unlit rafts. Night searches for unlit rafts were influenced by weather conditions and not by individual differences. As NVGs amplified life raft light, detectability was not limited by human performance and weather conditions [16]. Once a target has been detected and localized, it can be identified. The ability to identify military targets at night has been shown to directly correlate with dark adaptation , scotopic retinal sensitivity and contrast sensitivity under mesopic condition but not visual acuity. However, only scotopic retinal sensitivity and mesopic contrast sensitivity metrics are reproducible, making them suitable for assessing night vision ability in pilots and military personnel [35].

Military personnel rely on visual performance and a reliable way to improve this performance is to enhance the warfighter’s vision with refractive procedures that reduce or eliminate dependence on corrective lenses. There are several benefits that refractive surgery offers over glasses or optical inserts in a military context. It optimizes vision without the constraints of glasses, which may break, get scratched, or degrade due to environmental conditions (fog, rain, salt spray, sand etc.). Optical inserts which correct vision in masks can become dislodged or may limit peripheral vision. Sophisticated weapon systems can require goggles, headgear, or ballistic protection whose functionality may be impeded by glasses.

Contact lens wear can eliminate some of the interface obstacles inherent with spectacles, but aside from exceptions for authorized Air Force Personnel, use during deployment is prohibited by Department of Defense policy. This prohibition is a consequence of numerous cases of contact-lens related issues including keratitis, ulcers and infiltrates (DOD policy, [14, 69]). Refractive procedures avoid these aforementioned shortcomings. In fact, given operational demands and environmental extremes, there is no other profession that has benefited as greatly from refractive surgery as the military [7, 21, 58, 64, 65, 71].

Not only is refractive surgery an operational amplifier, refractive surgery enhances the quality of life and individual readiness of service members. A survey of Naval Aviators who underwent LASIK showed 95.9% of aviators felt improvement in their individual readiness and overwhelmingly would recommend the surgery to others (99.6%) [65]. These findings were similar to a review of the Army’s Warfighter Refractive Eye Surgery Program (WRESP) of Soldiers returning from deployment [55]. Respondents were asked to rate how their capabilities changed in relation to the following tasks as a result of their refractive surgery: weapon sighting ability, night operations, ability to weather extreme environmental conditions, and use of personal protective equipment. Their capabilities in accomplishing the following tasks were rated as “better” or “much better” in weapon sighting ability (91.4%), use of night vision goggles (86.2%), operations in extreme environments (74.2%) and use of personal protective equipment (88%). When asked to rate the impact of refractive surgery on their deployment, 95.2% of soldiers felt their individual readiness was “better” or “much better”, while 93.4% felt they were better able to contribute to their unit’s mission [21, 55]. Refractive surgery also provides an opportunity for Service members to apply for occupational specialties for which they were previously not eligible due to restrictions of uncorrected visual acuity or refractive error, augmenting the pool of applicants to specialty programs, such as Aviation and Special Forces [21, 65].