Development of Contemporary Laser Protection Glasses for the Bundeswehr Flying Personnel: Results of Visual Testing

Frank M. Jakobs^a, Peter Hank^b, Diana Hering^a, Frank Weber^a1, Dietrich Pertsch^b, Lothar Bressem^a

¹ Colonel (ret.). Priv.-Doz. Dr. Frank Weber has been a member of the German Air Force Center of Aerospace Medicine until Sept 2021.

^a German Air Force Center of Aerospace Medicine, Cologne/Fuerstenfeldbruck, Germany

^b ESG Electronic Systems ans Logistics Inc., Fuerstenfeldbruck, Germany

Summary

Background: Laser illumination of aircraft continues to constitute a problem in national and international air traffic, requiring effective protection measures for pilots and accompanying personnel. The protective glasses used by the German Air Force to date, do not meet current requirements in terms of visual demands and extent of protection, especially for military pilots. The present study addresses the visual and operational testing of a contemporary laser protection eyewear for flying personnel, developed in cooperation with industry.

Methods: Based on the optophysical properties of commercially available laser protection glasses, a combined dye and coating process was developed allowing simultaneous blocking of laser irradiation at different wavelengths in visible and invisible ranges of the light spectrum. The referring filters were modified until the desired attenuation was achieved and the first prototype could be provided. Compatibility with visual requirements for flying personnel was verified at the German Air Force Centre of Aerospace Medicine; testing included visual acuity, color discrimination, contrast sensitivity, and subjective visual comfort. Glasses were developed by ESG Inc., Fuerstenfeldbruck, with involvement of test pilots from the GAF Test Facility (WTD) 61 in Manching, and under expert supervision of the Federal Office of German Armed Forces Equipment, Information Technology and In-Service Support (BAAINBw) in Koblenz.

Results: Simultaneous blocking of three wavelengths in RGB mode resulted in acceptable visual acuity values while largely maintaining color discrimination in all three color spaces. The relative change in AQ values was 14–15 % in the deuteranomal and 10–38 % in the protanomal range, depending on transmission and protection level. The probability of error in color discrimination was 4–9 % under photopic and 19–32 % under scotopic conditions. In contrast to selective attenuation in conventional devices, this resulted in a saturation loss of about 30 % and a contrast reduction of about 25 %. Additive blocking of UV-A, UV-B and Near-Infrared did not introduce any effect on visual acuity and color vision of tested study subjects.

Discussion and conclusions: Our results show that it is technically possible to design laser safety eyewear that provides effective protection in three visible and three invisible wavelength bands without compromising visual perception and color vision relevant to flight safety. Increasing color saturation and contrast sensitivity will be subject to further research, along with increasing levels of protection. The prototype developed here is expected to be available for distribution from the third quarter of 2022, following final testing by WTD 64 and operational approval by BAAINBw.

Keywords: Bundeswehr, laser strikes, laser protection glasses (LPG), laser glare protection (LGP), laser eye protection (LEP), aerospace medicine

Scientific Background

Epidemiology

Laser attacks on aircraft remain to be a serious concern in terms of flight safety. While regarded a peccadillo in earlier times, they are meanwhile classified as dangerous interactions with air traffic and penalized by harsh law sentences in the whole world. Nevertheless, FAA data as of 2020 demonstrate a rise of respective incidences, with a 2020 increase of 10 % as compared to 2019, and a 20 % increase as compared to 2018 [11]. For the year 2021, the preliminary statistics are indicating the highest number of events ever recorded since 2005 – despite of largely reduced number of flights due to the Corona Pandemic [11][17]. This tendency is confirmed by the German Federal Aviation Administration (LBA) as well as by the German Air Force General of Flight Safety [14].

Laser Dazzle Model

The specific reference to aviation results from different grades of psychological, physiological and pathological dazzling phenomena to be observed under laser exposure and eventually leading to in-flight incapacitation of the referring pilot. The scheme in figure 1 outlines the interrelationships between laser dazzling and its potential operational impact.

Fig. 1: Laser Dazzle Model (Jakobs)

The most important part of this model relates to psychophysical incapacitation, constituting as a result of sensoric degradation of visual perception at 3 distinct levels:

• Level 1 is glare which is defined as a transient visual impairment by low-grade or distant overexposure with light that causes psychological aversion and discomfort.
• Level 2 is related to flashblindness, i.e., the inability to see visual objects or structural details following high-grade or near-range overexposure with light.
• Level 3 is related to flashblindness with persisting afterimages in central visual fields similar as directly viewing into a photographer’s flashlight by night.

in 2003, the FAA initiated a study focused on investigation and visualization of these dazzle patterns to evaluate flight performance and visual acuity dependent on the extent of individual laser exposure [25]. The respective simulator pictures are displayed in figure 2. The main concern in this study was the conclusion that in principle, a 5 mW laser was able to introduce visual degradation at all three levels.

Damage at Distance

In our experience, one of the most frequent concerns of pilots is the anticipated risk of irreversible retinal damage in-flight. It is well-known that coherent irradiation in principle is capable to induce such damage. Evidence comes from multiple case reports in Pediatric Ophthalmology [5][15][19][21][28]. In any case, however, this will require foveal fixation of the irradiation source being retained for instance at very close distances from the eye. The question of whether similar injury could be induced by a laser from distances typical in aviation is discussed controversially. Most specialists interviewed in this context argue that the energetic power of an incoming laser would be too low due to attenuation effects by atmospherical turbulences and scatter effects at the cockpit windshields [13][23]. At this point, the FAA insists to deny any evidence of ocular damage in a pilot by laser illumination of aircraft.

At Risk: Take-off and Final Approach

This implicates that the actual hazard for aviation is rather caused by different grades or severities of glare which interferes with flight safety even when no physical injury might be expected. Analysis of risk factors indicates that primarily aircraft flying at low altitude and with reduced speed are being illuminated [27]. This is particularly true for helicopters (in the Bundeswehr environment in about 60 % of cases). Second, every starting and landing aircraft is at risk.

The main concern is the effect of laser exposure on the pilot’s ability to maintain control during take-off and final approach when operational skills are critical. According to the Federal Aviation Regulations requirements, aircrew interactions in these phases of flight are restricted on operationally relevant tasks to maximize attention and to minimize the potential of human error. This safety principle is called the ’sterile cockpit requirement’ [36] which meanwhile is agreed to be violated in any case of accidental or deliberate laser illumination.

Fig. 2: Laser dazzle effects:

(A) No laser. Runway from the pilot’s view. (B) Distraction: Exposure with 0,5 μW/cm² or a 5 mW laser pointer at 3,700 ft. The runway is mostly visible. (C) Glare: Exposure with 5 μW/cm² or a 5 mW laser pointer at 1,200 ft. The runway is mostly invisible. (D) Flashblindness: Exposure with 50 μW/cm² or a 5 mW laser pointer 350 ft. The runway is completely obscured.

German Air Force Laser Protection

Current Laser Protection Glasses (LPG) used in the German Air Force (GAF) are dated from a time when visible laser irradiation did not play a significant role in operational air traffic. The providers’ concern was rather due to the hazards specific for invisible laser irradiation. Consequently, the protection glasses developed for this purpose nearly exclusively covered UV and IR related wavelengths of the electromagnetic spectrum (figure 3).

Fig. 3: Spectrometry of current LPG used in the Bundeswehr

This situation has fundamentally changed. The epidemic increase in laser attacks on aircraft observed since 2005 is clearly the result of universal availability and cheapness of visible lasers, attributable to 520–532 nm (green) wavelength ranges in more than 90 % of cases [26]. In addition, lasers in the 425–450 nm (blue) range are of concern, whose relative impact in the FAA controlled airspace has steadily risen from 0.8 % in 2010 up to 9.2 % in 2020; the main reason for this is most probably power increases up to several watts making them much better visible by day and by night [3]. On the other hand, however, first aviation incidents are reported in the military and police-related environments with lasers operating in the invisible range of light.

Taken together, these contemporary trends indicate that laser protection currently available for flight crew needs to be adapted to new conditions. Technically spoken, the rationale for the development of improved laser protection for flight crew derives from a shortfall in personal supplies that is beyond any doubt safety relevant and affects military as well as police and special operations task forces all over the world.

Light Transmission and Color Discrimination

The technical development of such protection devices is not trivial. We as humans live in a visual world, with up to 80 % of our cerebral functions being occupied with the perception, transduction, and processing of visual impressions. The problem with this is that lasers are just another form of light which, in its incoherent mode, is completely natural to us, but renders us incapable of most kinds of action if its intensity falls below a certain residual level. Any increase in optical density of the applied protective technology will predictably lead to a reduction of light transmission, and distinction of single or multiple wavelengths will result in a shift of the remaining light to another spectral range subsequently leading to perceptive color and contrast vision problems.

For this reason, conventional laser protective glasses, such as those used in industry, medicine, or laboratory environments, are not suitable for aviation purposes. The resulting self-induced protanomalous, deuteranomalous, or tritanomalous disorders can be such prominent in single cases that a reliable differentiation of color-coded cockpit displays is not possible anymore. A first-time applicant for flying duties presenting with those findings in his natural vision would inevitably be assessed unfit according to the German (and most other nations’) military regulations [20]. Hence, protective glasses are required for pilots and crew, which leave color discrimination unaffected as far as possible.

Unfortunately, at this point, such glasses do not exist. Although some manufacturers claim to have found the ultimate laser protection strategy, the available samples either cover too narrow a bandwidth or too low an optical density for comprehensive protection, or – in case of multiple wavelength suppression – are incompatible with visual in-flight requirements. In view of this demand, it is not surprising that governments and other authorities have repeatedly tendered millions of dollars for the development of advanced aviation-related LPG [2][4]. The type of LPG developed by the ESG Elektroniksystem- und Logistik Inc. in cooperation with mission test pilots and the Air Force Center for Aerospace Medicine could be a first step into a respective protection technology. The aim of the present study was visual testing of the prototype developed by ESG Inc. to evaluate its aeromedical suitability with special emphasis on color discrimination under photopic and mesopic conditions.

Materials und Methods

The development of the prototype presented here took a total of 5 years. The main challenge was to develop a combined filter technology that would capture about 99 % of the known spectrum of laser attacks on aircraft by attenuating the three most common visible wavelengths and blocking the three main invisible wavelength ranges (UV-A, UV-B, NIR). In parallel, the external design was developed in collaboration with various eyewear manufacturers. Upon completion, 6 initial prototypes with various degrees of visible light transmission (VLT) were submitted to the Air Force Center of Aerospace Medicine for visual testing to establish the criteria for the required fine tuning.

Spectacle Design

Shape and design of the developed LPG consider both occupational safety requirements as well as aviation-related operational aspects. The see-through area was designed to be larger than the usual industrial design to minimize potential restrictions of visual fields. The inner and outer bends were kept customizable depending on the facial contours and the helmet/visor systems worn. The curvature was manufactured to fit close on both temple sides to prevent lateral intrusion of laser irradiation under the protective glasses. With focus on low weight, high temperature resistance and shatter protection, a polycarbonic derivative was used as translucent material. A clip device in the nose pad area was applied for optional use of corrective lenses without altering the basic geometry of the device.

Fig. 4: Design and Geometric Construction of the ESG Prototype

The temple joint areas were shifted inward to narrow goggles in the upper part ensuring compatibility with the visor systems as being used in the German Air Force. Temples were kept as flat as possible to avoid pressure points under tight-fitted helmet systems; furthermore, the typical curvature of temples behind the ears was spared to facilitate sliding them underneath headphones and integrated helmet systems. According to the initial feedbacks, acceptance among pilots and accompanying personnel in terms of aesthetics and functionality appeared to be high.

Optical Filter Technology

Optical filters are glass or synthetic translucent media that select the incident electromagnetic radiation according to certain criteria, such as wavelength or polarization state [8]. In case of laser light protection, the referring filters are blocking devices that either exclude an entire band of wavelengths (invisible radiation) or selectively exclude individual wavelengths (visible radiation). The passband is referred to as transmission, while the spectral range outside transmission is referred to as the off-band or blocking range. The two main types of filters for this purpose are dye and interference filters [22]. In the present case, both types were used.

Dye Filters

The working principle of dye filters is absorption of light by color pigments which are usually embedded in polycarbonate materials. This principle was used in the glasses described here for selective exclusion of wavelengths in the red, green, and blue (RGB) range of the visible spectrum. Physically seen, it is thus a trichromatic absorption filter. The blocking range per wavelength in such filters is about ± 25 nm which means that light attenuation is relatively soft in effect. The disadvantage is that due to superimposition, transmission decreases with increasing numbers of excluded wavelengths, i.e. visual perception darkens comparable with the wear of sunglasses. The advantage is that absorption filters are less sensitive to the incidence angle of directed light, particularly outside the central 30° of visual fields.

Interference Filters

Interference filters are based on the principle of light reflection at the interface of multi-layered films deposited on the surface of the lens. For the pupose described here, a total of 120 layers in the nanometer range were applied. When electromagnetic radiation is incident, it will be split into a transmitted and a reflected portion at each interface. The redundancy of this effect leads to formation of multiple, partial reflections and transmissions, which interfere either constructively (in the sense of mutual amplification) or destructively (in the sense of mutual extinction). This physical principle is named interference and has been used in the present case for additive blocking of invisible wavelength regions. The spread of light attenuation is lower than for absorption filters (± 15 to ± 20 nm), i.e. the filter is more “sharp-edged” in effect. The advantage is a weaker dependency on the number of wavelengths blocked, while the disadvantage (relevant in the context of lasers) is a clear dependency on the angle of incidence.

Optical density and transmission

The two most important metrics in filter technology are Optical Density (OD) and Transmission (VLT). Optical density describes the amount of energy absorbed or reflected by a filter, while transmission describes the amount of non-blocked energy. OD and VLT correlate in an inversely proportional way, i.e. the higher the OD, the lower the VLT, and vice versa:

xVLT = 10^-OD x 100 [%] (unweighted)

Table 1: Correlations between optical density, transmission, and attenuation of light

Further details of the dual filter system presented here are subject to confidentiality.

Color vision testing

Regarding the optical properties of the newly developed glasses, the impact on color vision was of primary interest. Two modified test procedures were used for this purpose: First, a classical anomaloscopy was performed to identify shifts of color perception into deuteranomal or protanomal spectral regions. Secondly, a 15-Hue (D-15) test was used to examine color discrimination under reduced light conditions. The tests were performed on 20 subjects each, who volunteered following informed consent while completing their relicensing examinations as pilots or crew-members. For the test series with conventional protective glasses, which were carried out for comparison, 10 study subjects were tested each.

Anomaloscopy

In general, anomaloscopy is used to be the standard procedure for identifying color vision defects on the base of spectral equations. The best known and most frequently used device for this purpose is the Nagel anomaloscope [24]. In principle, this test relies on subjective comparison of two separated hemicycles, in the upper part of which a mixed color of green and red (548 and 666 nm) is offered, while in the lower part, an orange-yellow color (589 nm) is set, the brightness of which can be varied by the examiner and matched (“adjusted”) by the tested person through operating a red-green screw. The purpose is to determine the anomalous quotient (AQ), i.e. the subjectively required ratio of green and red color shares. Calculations were performed using the Rayleigh equation [6], where P corresponds to the study participant and N to the (normal-sighted) examiner:

Fig. 5: Hemicycles of the Nagel Anomaloscope and Rayleigh Equation

This corresponds to the relationship:

(Green (P)x Red (N)) / (Red (P)x Green (N))

In the formula in figure 5, 73 is the scale value of the mixed color for green-free presentation, and 0 that for red-free presentation. A color normal subject will perceive the two half-fields as equal if the mean standard equation 40/15 (i.e., mixture = 40 and brightness = 15) is set. Since color discrimination is subject to a certain degree of variance (range of adjustment), ratios between 0.65 and 1.32 are considered normal. A deuteranomalous subject will match with too much green (P < 40; AQ > 1.32), a protanomalous one with too much red (P > 40; AQ < 0.65).

Anomaloscopes are designed as viewing devices whose eyepieces largely exclude daylight and therefore are not suited for spectacles worn at testing. For this reason, a top-view device, as usually applied for screening purposes, was chosen for testing. The testing principle is the same in both cases, the differences being a replacement of the color screws by scaled buttons and a stronger dependence on ambient luminance.

15-Hue Test

The 15-Hue or D-15 Test is a reduced version of the Farnsworth-Munsell 100-Hue Test for identification and quantification of color vision deficiencies. The test is based on repeated dichotomous decisions made by the subject between two closely spaced gradations of colors and was available in a saturated [10] and a desaturated [26] version (figure 6). It was selected for testing because, in contrast to the conventional anomaloscopy described above, it covers the blue-yellow (tritan) color spectrum in addition to the red-green (protan/deutan) color spectrum. Testing using the desaturated pattern also allows conclusions to be drawn about the extent of color deficiency under reduced (mesopic) light conditions.

Fig. 6: 15-Hue test saturated (top) and desaturated (bottom)

The test consists of a fixed reference chip (position 0) and 15 removable color chips labeled with position numbers on the back. The subject’s task is to re-sort the removed color chips back into the correct order, starting at the reference cap. This test was performed separately for each LPG. No analysis of potential learning effects was performed.

For evaluation, drawing templates are provided by the manufacturer that allow visualization of the error-related confusion axes. Since this was already determined by anomaloscopy, these templates were not used; instead, an Excel^®-based error quantification on the base of incorrectly positioned chips was used. In summary, the total of incorrectly positioned chips was set in relation to the overall number of correct placement possibilities, allowing calculation of an error rate (or hit probability) in percent for the respective LPG.

Results

Spectral properties of the prototype

Measurement of the preliminary ESG prototype documented successful blocking of the three most commonly used visible wavelengths in commercially available laser pointers. In the invisible wavelength range, both UV-A/B were completely blocked as well as Near-Infrared up to a range of 1080 nm. The spectral curves shown in figure 7 are not native measurement results, but the result of a computer simulation of the targeted filter ranges. The final prototype was adjusted several times according to the respective test results and comparative analysis of laser safety glasses from other manufacturers.

Fig. 7: Simulation of the intended spectrogram in the visible and invisible wavelength range

Fig. 8: Experimental evidence of successful RGB blockade in the prototype (left) as compared to conventional laser protection glasses (right).

Light transmission turned out to be different for blocked wavelengths and, as a total, languished in a range of 30–35 %. Experimental increase of transmission resulted in a decrease of the protection level (optical density) to values below the targeted minimum. Conversely, experimental elevation of optical density introduced a disturbing image obscuration, which, as a result of reduced contrast vision, was rated operationally unsatisfactory by test pilots, especially under twilight conditions. The finally achieved VLT as measured here is thus to be regarded the maximum of protection currently feasible with the technology described in this study.

Results of color perception tests

The commercially available LPG tested for comparison purposes exhibited significant impairment of color discrimination, which apparently resulted less from filter types used than from the wavelengths selectively or cumulatively blocked. In the experiments described here, monochromatic filters had a stronger effect on color discrimination than dichromatic filters, and these again had a stronger effect than trichromatic filters. Selective blocking of blue at 450 nm (Figure 9 C) resulted in formal unfitness to fly in 100 % of subjects.

Figure 9 summarizes the psychophysical properties of the prototype investigated here. Transillumination of the spectacle lens with a common 10 mW laser pointer documented effective elimination of the glare effect caused by the scattered radiation, with blue being blocked most effectively (R<G<B). Two dichromatic filters, as commonly used for laser protection purposes¹, were included as comparison glasses. The LPG shown in Figure 9 A blocked blue and green wavelengths, while glasses shown in Figure 9 B blocked blue and red wavelengths of light. Since blue light was additionally blocked in both cases, it was expected that the primary effect would result in a deutan or protan shift of color perception.

 

Fig. 9: Results of color vision testing. The diagrams show the anomalous quotients (red dots) of the respective glasses imaged underneath, as compared to the individual reference values without glasses (black dots). The transillumination images display the filter capacities of test glasses in a dark room. The residual dot remaining at effective attenuation levels is intentional in order to enable the pilot to realize and to report the respective laser event.
Legend: Mean: Mean plus/minus standard deviation. Avg. ΔRef AQ1/2: Mean deviation from reference value (= AQ without LSB). F-Rate (AQ1/2): Medical fitness rate considering AQ1 and AQ2. U-Rate (AQ1/2): Medical unfitness rate, inverse function of T-rate. PERROR (sat.): Error probability 15-Hue saturated.. PERROR (desat): Error probability 15-Hue desaturated.

In fact, this prediction was confirmed. While color perception shifted into the deuteranomal range (average adjustment range 1.08–1.46) under green/blue blockade and into the protanomal range (average adjustment range 0.39–0.48) under red/green blockade, it remained approximately normal (average adjustment range 0.77–0.94) under simultaneous triple blockade. In case of comparison glasses, this corresponded to a deutan shift of +31.6 to +39.1 % (A) and a protan shift of -52.4 to -54.7 % (B), respectively, whereas the RGB glasses (D) induced considerably smaller changes in color perception with relative shifts between -10.2 and -12.4 %.

This implicates that glasses (A) and (B) would have rendered 70 % of pilots formally unfit, as compared to 10 % of pilots becoming unfit if wearing the newly developed protypes (D).

As a result, D-15 error rates by use of conventional LPG increased to 64.0 to 66.7 % under daylight conditions (saturated test), but stayed below 10 % under RGB blockade in the ESG prototype. Under mesopic conditions (desaturated test), however, this low error rate could not be maintained (increase to 31.9 %), but still was less than half of the error rate as seen in conventional LPG (83.3 to 86 %, respectively).

Variation of the VLT

Transmission is the limiting factor in the development of any laser safety eyewear because it is inversely related to the protective effect. With regard to the glasses presented here, it is primarily a function of the amount of embedded color pigments. When transmission increases, quality of vision improves as well, the prize of which is a decrease in laser glare protection. To evaluate the best cut-off within this dependencies, 6 VLT variants of the developed prototype were tested for their color fidelity and image quality. The tests were performed blinded, i.e., the respective transmission of the provided test glasses was not known at time of testing. Examinations were performed on 20 male study subjects, with each subject evaluating all glasses according to the same proceeding and order. Anomalous quotients and error rates were determined as described before. In addition, a subjective ranking of test glasses was performed.

The first task, subjects were asked to comply, was sorting the glasses based on their quality of vision in order from 1 to 6. For evaluation, an inversely proportional score from 6 to 1 was assigned. From this, a total ranking score was calculated for each of the 6 glasses. After unblinding and assignment of transmission levels, an almost perfect correlation between both parameters was found (R² = 0.98), i.e. the higher the light transmission by glasses, the better the subjective visual impression was rated by the subjects. There was a remarkably low variance in these results.

Fig. 10: Subjective ranking of quality of vision at different transmission levels.

AQ evaluation revealed an asymptotic drift towards weakly protanomalous ranges with increasing transmission. Hence, color perception obviously changed with increasing brightness when glasses with light-absorbing pigments were used, which in turn limited transmission changes within undefined boundaries. Why this in the current setup resulted in a measurable and constant protan shift (see fig. 11), is unclear. Since those glasses inducing the highest degrees of protanomaly were subjectively rated most comfortable at the same time, it can be concluded that under daylight conditions, perception of brightness was considered more relevant or acceptable than the perception of unchanged color spaces.

Fig. 11: Relative change of color discrimination as a function of transmission

This result was confirmed by investigation of error probability using the 15-Hue test. It was found that with increasing transmission, error rates increased in both, the saturated as well as the desaturated tests. This was unexpected since the error probability should have decreased with increasing ambient brightness. At this point, we are interpreting this as a result of protaneous shifts seen in the previous test.

Fig. 12: Error probabilities as seen in 15-Hue tests

Taken together, the results of various VLT testing indicate that color sensation shiftings towards weakly protanomalous ranges are not necessarily perceived as unpleasant or uncomfortable, as might be suggested by crude anomaloscopia values. From this, it can be concluded that in the experimental setup described here, the optophysical thresholds of color fidelity were obviously not completely congruent with the sensory color processing results in the visual cortex of subjects investigated.

Discussion

The development of advanced laser safety eyewear for flying personnel is an ambitious task that requires multiple considerations in terms of safety and efficacy on the one hand and cut-off decisions on technological feasibility on the other hand [235]. This includes, for example, the visibility and discriminability of cockpit displays, the upgrade-ability of glasses with refractive corrections, and the customizability of spectacles fit. Under military conditions, operational aspects such as suitability for combat missions, operability with helmet and visor systems, and compatibility with night vision devices have to be considered.

The protection devices presented here are the result of a coordinated collaboration between the ESG Electronic Systems and Logistics Inc., the German Air Force Center of Aerospace Medicine, and the test and mission pilots of the Air Force Test Facilities, for the primary purpose of military use. Contractor was the German Federal Office of Armed Forces Equipment, Information Technology, and In-Service Support (BAAINBw), preceded by an initiative on behalf of the helicopter combat units in Le Luc (FRA) and Fritzlar (GER).

More than Eye Protection

The developed prototype belongs conceptually to a new generation of laser protection devices whose function is no longer primary protection against organic damage, but minimization of the glare effects known to be dangerous and reducing flight-safety. For these reasons, the previously used terminus of laser protection glasses (LPG) is increasingly being replaced by the differentiation into laser glare protection (LGP) and laser eye protection (LEP) eyewear [12][32]. This not only helps to reduce the pilots’ concerns about ocular injury, but also makes clear that the same protective eyewear used in medical, or laboratory environments can definitely not be used in aviation. We therefore propose an analogous differentiation into glare and full protection glasses in the German language as well.

Figure 13 shows the operating principle of such a glare protection device. The reduction of the glare effect, with its typical scattering and blooming patterns, to a point-shaped pattern in the wavelength of the incident laser is clearly visible. The attenuated spot of light corresponds to the pilot’s mesopic perception in the cockpit. Due to this concept, laser exposure can be detected and reported, but without interfering with the crew’s specific tasks during critical phases of flight.

Fig. 13: Working principle of laser glare protection (A330 cockpit, mesopic conditions). (A) Laser exposure without LGP: The runway is hardly recognizable. (B) Laser exposure with LGP: The runway remains visible (see also Figure 9).

Filter Types used

Technically, the anti-glare effect can be achieved by using different optical filters such as absorption and reflection filters. While the effect of absorption filters is based on multiple color pigments embedded in the translucent supporting material, the effect of reflection filters is based on thin films in the nanometer range that are applied to the surface of the spectacle lens reflecting unwanted wavelengths. In the present case, an absorption filter was chosen for visible wavelengths because this type of filter is, unlike reflection filters, less sensitive to the angle of incidence of coherent light [33]. In addition, absorption filters are more scratch-resistant and thus more durable.

Transmission

The disadvantage of absorption filters is that both the increase of the protection level and the simultaneous blocking of several wavelengths will lead to a measurable reduction of transmission, i.e. darkening of visual impression. This effect, which is induced by intrinsic accumulation of color pigment similarly as seen in sunglasses, was unavoidable and also present in the prototype presented here (see simulation in Fig. 14 D). The total transmission finally achieved was about 30 % and made it necessary to increase contrast vision by implementing an additional K-edge filter.

Color Perception

The main problem of most laser protection glasses is the distortion of subjective color perception. imbalanced filtering of visible light leads to a compensatory shift of the (remaining) color perception into another spectral range. The cause of this is an overrepresentation of red, green, or blue hues induced by selective exclusion of single wavelengths. In extreme cases, this may lead to illumination environments in which a reliable color discrimination is no longer possible. Examples include cockpit displays as color monitors, overhead displays, or warning lights as well as outside safety systems like terrain markings, PAPI systems, or airfield lightings. These color distortions are more pronounced under mesopic than under photopic, and more evident under scotopic than under mesopic conditions. Importantly, they may lead to fatal flight errors depending on the pilot’s vigilance.

 

Fig. 14: Simulation of the blocking effect of laser safety glasses with effective protection in the visible wavelength range (cockpit A330):

(A) View at night without laser safety glasses.

(B) Selective blocking of the green channel by anti-green directed conventional LPG results in over-representation of red and violet hues (deuteranomaly). Reliable differentiation of cockpit displays under these conditions is not possible.

(C) Selective blocking of the red channel by anti-red directed conventional LPG results in over-representation of green and blue hues (protanomaly). Reliable differentiation of cockpit displays under these conditions is not possible.

(D) Simultaneous blocking of the peak wavelengths of common laser pointers in the sense of a triple RGB blockade results in a reduction of contrast vision and color saturation while maintaining a largely normal color distribution. Reliable differentiation of cockpit displays under these conditions is possible.

This risk is largely abandoned with the protection glasses presented here. The principle of RGB triple blocking leads to a much more equilibrated glare reduction than the selective exclusion of individual wavelengths. The effect can be simulated with digital image processing software and results in perceivings as shown above. According to the test pilots interviewed, the final result, as shown in Figure 14D, closely corresponds to the visual impression with the prototype used under mesopic conditions and is considered a major improvement of laser protective glasses available so far. On the other hand, this result has to be regarded the optimum cut-off achievable with the technology described here.

Military Aspects

Under military conditions, additional aspects must be considered that cannot be discussed in detail at this place. By way of example, two of the most important considerations in this context are singled out as follows:

Compatibility with Image Intensifiers (NVG)

The first aspect refers to the military use of Night Vision Goggles (NVG). For safety reasons, the novel LPG are designed in a way that the built-in green blockade does not interfere with the fluorescence of the night vision equipment used in military flight operations. However, in the authors’ opinion, this is not a principal problem. First, NVG imaging is based on the concept of residual light amplification by electrons, which leads to inevitable loss of color information anyway; in a dark-adapted eye, perception will be primarily a function of contrast vision, which is largely independent of retinal cone functions. Secondly, the green hue is due to the (arbitrary) choice of green phosphor in the reconversion of amplified light; likewise, hues outside the usual 532 nm could be used for image reconstruction, providing other, possibly better options in case of interference problems [7][34].

Possible Threats from Invisible Lasers (NIR)

The second aspect refers to the threats posed by invisible lasers. This aspect was taken into account by additive integration of interference filters that completely cover UV-A and B as well as the NIR up to 1080nm. The LPG presented here effectively reflect near-infrared wavelengths by a very high factor. In view of two incapacitating flight incidents involving infrared lasers in the recent past, and with regard of the pending introduction of infrared-based laser weapons [30], the authors believe that there is an urgent need to further enhance NIR protection by optimization of the prototype presented here. Although no protective glasses in the world will be able to withstand the destructive power of a 50 kW laser weapon [18], our pilots must be protected against the reflected radiation of lasered targets (e.g., drones, missiles, ballistic projectiles). Even if only the one-millionth part of such a laser is reflected to a pilot’s eye, this might be sufficient to cause irreversible retinal damage.

Conclusions

Our results show that it is possible to design laser safety eyewear that provides simultaneous protection in 3 visible and 3 invisible wavelength ranges without affecting the color vision of flying personnel in an extent that would be incompatible with actual flight safety requirements. Increasing color saturation and contrast vision will be adressed by further research, along with increasing the protection level for invisible lasers. The prototype developed here is expected to be delivered to the Tiger helicopter units in Fritzlar (D) and Le Luc (FRA) by 3rd quarter of 2022 following ergonomic and operational testing by test pilots and final approval by BAAINBw.

The prototype presented here is far away from perfection. The concept of simultaneous blockade of visible and invisible lasers is no more and no less a first step into a future that will be determined by weapon systems with a destructive power that is unimaginable by today’s means². Nevertheless, these systems already exist [9]. The war in Ukraine has shown how quickly novel weapons such as hypersonic missiles may be deployed in a military crisis [36]. Given the fact that perhaps the only way to intercept such a weapon is a counterstrike at the speed of light, the expected impact of lasers in future military conflicts becomes evident. From this point of view, our concept is to be considered the preparation of a protection technology that will apply to a dimension different from the present one.

Keynotes

• The authors propose a differentiation of ­laser protection glasses into anti-glare and full protection glasses for the field of aviation. Anti-glare devices cannot be used as full protection, and full protection devices not as­ ­anti-glare protection.
• The risk of direct damage to a pilot’s eyes is considered to be rather low, considering the distances involved in flight operations. ­Anti-glare devices are therefore generally sufficient in civil aviation.
• Under operational conditions (military, special forces, police), elements of full protection have to be implemented. This applies particularly to invisible radiation components. This principle has been realized in the prototype presented here.
• Our prototype is capable of attenuating the glare of the three laser wavelengths most commonly used for aircraft illumination by means of an RGB filter. In addition, the invisible wavelengths of UV-A, UV-B and IR-A are blocked with high protection factors.
• The final product ready for series production will be available by Q3 2022 and will then be delivered to flying units.

Abbreviations

Table 2: Abbreviations and Units

References

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Credits

We have to thank Sergeant Yannah Rauchman, Sergeant Chris Weierter, and, from the civilian staff, Mrs. Nina Kolloch and Mrs. Manuela Schmidt who were particularly committed to the implementation of the test series of the prototype presented here. They are all members of the Department II 3 c (Ophthalmology and Optometry) of the German Air Force Center of Aerospace Medicine.

Disclaimer

The authors of the German Air Force Center of Aerospace Medicine declare no financial or other types of interest associated with the research results presented here. The results presented are not a recommendation to purchase the products developed by ESG Inc..

Manuscript data

Citation

Jakobs FM, Hank P, Hering D, Weber F, Pertsch D, Bressem L Development of Contemporary Laser Protection Glasses for the Bundeswehr Flying Personnel: Results of Visual Testing. WMM 2022; 66(5): e4.

DOI: https://doi.org/10.48701/opus4–16

For the authors

Colonel (AF, MC) Dr. Frank M. Jakobs

Air Force Centre of Aerospace Medicine

Department II 3c – Ophtalmolgy

Straße der Luftwaffe, D-82256 Fürstenfeldbruck

E-Mail: frank2jakobs@bundeswehr.org