Human Engineering for Australia’s F/A-18 Night Vision Capability

Matthew John Squair Senior Safety Consultant

Jacobs Australia GPO Box 1976, Canberra, ACT 2601 Matthew.Squair@defence.gov.au

Abstract The F/A-18 Night Vision Capability project consisted of the modification of F/A-18 displays and cockpit lighting to be compatible with acquired AN/AVS-9G Night Vision Goggles (NVG). Aircraft lighting compatibility has been identified as a significant factor in a number of military aircraft accidents involving the use of NVG. As a result, careful consideration of the interaction of lighting system design, visual perception and human error is essential to assure safe and effective NVG operations. This paper describes the cockpit lighting modification and the design process used to develop an optimally integrated system.

Keywords: Night vision, displays, visual acuity, spatial disorientation, NVG compatibility, NVIS, system integration, human error.

1 Introduction

1.1 Why night vision for the F/A-18? Night Vision Imaging Systems (NVIS) provide an ability to conduct air operations at night. Operations by NATO and Coalition forces in the Balkans, Iraq and Afghanistan have all demonstrated the utility of such devices for both rotary and fixed wing aircraft. Aircrew using NVG type NVIS can view line-of-sight to the horizon, pick-out aircraft lights at greater than 80 km, lit objects beyond 20 km and a lit cigarette further than 30 km.

1.2 Project background The initial development of an NVG compatible lighting modification of the F/A-18 was carried out by the RAAF Aircraft Research and Development Unit (ARDU) during the early 90’s. Progress was slow until the late 1990’s when development of a full modification project was initiated, culminating in flight tests of the initial design in the period May 1998 to Oct 1999. The initial design was assessed by the ARDU Flight Test Squadron as operationally acceptable.

In November 2002 a project team was set up by the Tactical Fighter System Program Office (TFSPO) to take the initial design concept, resolve any residual issues, and

develop a production-worthy final design meeting all relevant airworthiness requirements1. Challenges facing the project were the tight schedule (under two years from project start to an initial operational capability) and the requirement to integrate the modification program with concurrent upgrades of the aircraft’s displays and transparencies.

2 NVG and safety of flight While the loss of NVG aided vision due to equipment failure during a critical phase of night time flight is implicitly hazardous, the normal use of NVG has also be linked to Spatial Disorientation (SD) class aviation accidents. A survey of US Army aviation SD accidents in the period 1990 to 1995 found a statistically significant (p<0.001) correlation between use of NVIS (both NVG and FLIR) and SD accidents (Figure 1.2-1). Similarly a summary by Berkely of USAF NVG related aviation accidents in the period 1990 to 2000 indicates that 57% involved SD (Berkely 2000). Berkely also identified that 30% of USAF NVG related accidents attributed lighting incompatibility as a significant contributing factor to the accident. These studies support the premise that the interaction between NVG and aircraft lighting and displays is a critical factor in NVG aided flight.

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Figure 1.2-1 SD accident rate comparison (Braitwaite et al. 1998)

3 The physics of night vision

3.1 The NVG spectrum Three windows in the electromagnetic spectrum exist where transmitted electromagnetic radiation is not attenuated by the atmosphere; the near infrared, mid infrared and far infrared. NVGs exploit the near infrared part of the spectrum using reflected rather than radiant

1 Primarily MIL-L-85762A and MIL-STD-3009 design standards.

Copyright © 2007, Australian Computer Society, Inc. A version of this paper first appeared at the 12th Australian Workshop on Safety Related Programmable Systems (SCS’07), Adelaide. Conferences in Research and Practice in Information Technology, Vol. 86. Tony Cant, Ed. Reproduction for academic not-for-profit purposes permitted provided this text is included.

energy. While this allows a simple lightweight sensor design, its coincidence with human vision (Figure 3.1-1) also introduces conflicts with cockpit displays emitting light in these wavelengths.

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Current generation NVG, as shown in Figure 3.1-2, address this conflict by incorporating a minus blue filters to reduce the amount of visible red entering the intensifier stage. Type I Class C NVG illustrated in Figure 3.1-2 (as used in the F/A-18) are a modified Class B NVG that uses a notch filter to provide a direct (if slightly blurry) view of the Head Up Display (HUD) through the NVG.

Figure 3.1-2 Gen III Type I AN/AVS-9G NVG

3.2 Lighting and display compatibility The MIL-L-85762A standard (and MIL-STD-3009) specify design requirements for lighting and displays to make them compatible with the various classes2 of NVG. Compatibility is achieved through ensuring that lighting and displays emit less incompatible light in the visible red and near infrared part of the spectrum.

In order to specify compatible lighting a new unit of measurement NVIS radiance (NR) was created to represent the energy emitted by a light source visible 2 Class A NVIS are most sensitive and only allow the use of green and yellow lighting. Class B is less sensitive and allows the normal green, yellow and red colour hierarchy.

through the NVG. NR is defined as the integral of the curve generated by multiplying the spectral radiance of a light source by the relative spectral response of a composite 3rd generation NVG. Maximum allowable NR is defined as no greater than the anticipated worst case terrain feature (a defoliated tree in starlight) giving a value of NRB = 1.6 x 10-10 foot Lamberts (fL) for a Class B (or C) NVG. As aircrew normally set lighting to 0.1 fL or less for NVG aided flight 0.1 fL was specified as the luminance at which NR is measured (MIL-STD-3009).

Using the 1976 International Commission on Illumination (CIE) chromaticity diagram (Figure 3.2-1) five colour loci were defined to establish a consistent NVG lighting colour scheme (see Table 3.2-1) without ruling out any particular filtering/lighting technology. For each class of NVG a crew-station lighting and display colour hierarchy is specified by MIL-L-85762A along with the associated maximum NR for each specified component type.

Figure 3.2-1 MIL-L-85762A NVIS colour locus

Table 3.2-1 NVIS colour loci usage

Green A (General). A less saturated (and less fatiguing) green matching the eyes maximum luminous efficiency.

Green B (Indicators). A more saturated green for greater attensity and daylight readability.

Yellow (Caution). A low red content yellow initially intended for warnings and master caution only.

Red (Warning). A high orange content red developed as an option for use in warning indicators instead of yellow.

White (Chart). Introduced where Green A would interfere with reading coloured maps or diagrams3.

3.3 NVG performance limitations Whilst current generation NVGs have significantly improved (both ergonomically and optically) from

3 See Crowley for reports of difficulty in reading under ANVIS compatible floodlights (Crowley 1991).

preceding generations, they still exhibit the following significant limitations:

1. reduced visual acuity4 and contrast sensitivity,

2. a limited FOV i.e. 40 rather than 180 + degrees,

3. nil colour contrast (i.e. monochromatic) image,

4. lighting halo effects,

5. reduced stereopsis, and

6. an inability to detect light cloud/or fog5.

These factors shape the performance of the user in perceiving both the external environment and cockpit displays. In turn NVG performance can be adversely affected by both environmental and aircraft lighting.

3.3.1 Environmental effects The nighttime environment can affect NVG performance in a number of ways. Low light levels can degrade both visual acuity (VA) and contrast sensitivity (CS) reducing detection ranges and detail discrimination. Low moon angles introduce glare into the FOV of the NVG and can initiate the NVG AGC function also reducing VA and CS6. Halo effects around cultural lighting can present constant visual angles at short range (Craig, et al. 2005) and obscure detail at longer ranges or induce veiling reflections in the canopy. Terrain and meteorological conditions can also introduce visual illusions or degrade aircrew perception of the environment.

3.3.2 Aircraft incompatible lighting effects Incompatible aircraft lighting can degrade NVG VA and CS by the introduction of haloing and veiling glare within the NVG FOV e.g. Figure 3.3-1(a). Where incompatibility exists a natural response is to turn the lighting down to reduce the effect but in practice this can make displays unreadable while not improving compatibility, because visually dim incandescent sources still produce significant energy in the near infrared. Reflections from canopy surfaces e.g. Figure 3.3-1(a) and (c) also enable lighting outside the NVG’s FOV to trigger the NVG’s auto-gain function reducing VA and CS without aircrew necessarily being aware of it. Incompatible lighting effects are further worsened in low light levels because the NVG is operating close to the noise floor in these conditions.

4 While 20/25 or 20/30 under laboratory conditions 20/40 is what is usually achievable in the aircraft and in poor conditions this may be 20/80 or worse. Poor NVG adjustment can also reduce acuity to 20/100. 5 Near infrared energy passes through light moisture more easily than visible wavelengths resulting in a gradual (and subtle) loss of scene detail as a weather system is penetrated. 6 NVG gain is regulated by the Auto Gain Control (AGC) circuit which maintains constant image brightness.

(a) Direct veiling glare, haloing & reflections

(b) External lighting haloing

(c) Canopy reflections (cultural)

(a) Direct veiling glare, haloing & reflections

(b) External lighting haloing

(a) Direct veiling glare, haloing & reflections

(b) External lighting haloing

(c) Canopy reflections (cultural)

Figure 3.3-1 Incompatible lighting effects

4 Human factors

4.1 Visual perception and NVG Human perception can be viewed as being based of two forms, data (e.g. bottom up) processing and theory (e.g. top down) processing. In data driven perception processing is triggered by an input ‘pattern’, while in theory driven perception memory, goals, expectations and context are used to ‘make sense of’ and inform an active search for data to be integrated into a mental model (Endsley, Smith 1996). The act of perception also requires that limited cognitive resources be allocated between competing perceptual goals (e.g. monitoring instruments, scanning the external scene, reading checklist and so on). The use of NVG can significantly effect both types of perceptual processing, as well as the allocation of cognitive resources.

4.1.1 Ambient vision reduction Focal or central vision is primarily responsible for object recognition, while peripheral vision is primarily responsible for spatial orientation. The limited NVG

FOV combined with it’s bright display reduces ambient vision to the point that focal vision must be used for both recognition and orientation tasks. As focal vision is a conscious task this transfer adds both attentional and physical workload to the process of perception7. Loss of ambient vision also eliminates perception of peripheral optical flow (i.e. motion parallax) a strong speed and distance cue. NVG users also tend to perceive (and assume) that the FOV provided represents the whole world rather than a fragment of that world (Brickner, Foyle 1990). These effects can lead to pilots flying lower, faster and closer to obstacles when using NVG.

The NVG FOV also requires pilots to modify their instrument scan to accommodate this vision obstruction by looking under or around it to view displays. The NVG FOV thus has significant implications for both cockpit display layout and attensity requirements (i.e. visibility within or outside the NVG FOV).

4.1.2 Detection and form perception NVG scintillation and lack of colour contrast have been shown to degrade VA (target detection) and CS (form perception) (Uttal et al. 1996). Experiments by Gibb have also found that for high spatial frequency images a small decrease in VA was associated with a significant decrease in CS, without subjects being conscious of this degradation (Gibb 1996). These results indicate that small levels of lighting incompatibility may lead to significant but unperceived reductions in discrimination. In the real world this translates to increased times and reduced distances for target identification.

4.1.3 Depth perception Depth perception beyond a short range does not rely on the stereopsis principal, therefore it is possible to judge depth and distance while using NVG. However the monocular cues available (such as size/shape constancy, optical flow, inter-position and perspective effects) are degraded or distorted to varying degrees by NVG use. For example, reduced NVG visual acuity causes object to appear dimmer (and more distant) increasing the salience of this perspective cue. Experimental evidence (Zaleski et al. 2001) also indicates that with NVG use, size estimates shift away from using size constancy toward the law of visual angle8. As a side effect this reliance on the visual angle law to judge size and distance also increases the vulnerability of distance perception to cultural lighting halo effects. Consequently, the ability to judge distance to a cultural feature or another aircraft if both are strongly illuminated can be further degraded. Interestingly a survey by Crowley (Crowley 1991) found faulty height or clearance judgement (56% and 11% respectively) as the predominant NVG perception error, possibly because of the strong theory component of size and distance perception. 7 In the form of aircrew NVG scan patterns to attend to masked peripheral cues. 8 In practice illustrated by the use of heuristics to gauge distance, e.g. ‘an aircraft that fills the HUD wingtip to wingtip is X metres distance’.

4.1.4 Colour effects and adaptation The naturally brighter NVG display (4.5 fL) relative to cockpit displays luminance (notionally 0.1 fL) introduces the potential for night adaptation delays when transitioning from NVG to cockpit displays. This effect is exacerbated by the use of blue-green lighting (to which the night adapted eye is more sensitive) in NVG. Tests by Howard, Reigler and Martin found measurable increases in response times at log luminance ratios of two or greater between NVG and displays (Howard et al. 2001). These results provide a minimum luminance level (0.045 fL) below which displays should not be set.

4.1.5 Scene display The dissimilar spectral sensitivity of NVG compared to normal vision, combined with the different spectral reflectivity in these bandwidths can result in unusual scene contrasts and brightness (Zaleski et al. 2001), inducing NVG specific visual illusions in some circumstances. For example flight over calm water (which reflects the stars in the night sky) can induce spatial disorientation as to which way is up, while cloud formations can generate false horizons when viewed with NVG. Aircrew’s ability to identify these visual illusions is also reduced by the limitations in NVG imagery. These scene differences add additional (albeit un-quantified) workload to the perception and comprehension tasks.

4.1.6 Physiological effects An individual’s performance with NVG can also vary considerably with approximately 15% of aircrew having substandard acuity with NVG even if their day vision is 20/20 or better (Ivan 2004). Visual performance can be further degraded by hypoxia effects on the cells of the retina.

The physical task of maintaining a continuous scan pattern to offset loss of ambient vision, whilst wearing a helmet mounted display, considerably increases the workload of maintaining a required level of situational awareness. This physical workload in turn contributes significantly to aircrew fatigue across a mission.

Colour and spatial distortion (e.g. ‘magenta eye’, double vision or loss of depth perception) can also be induced in normal vision after protracted NVG use, especially where the NVG are poorly adjusted pre-flight (Crowley 1991). These effects can also be hazardous if aircrew are procedurally required to perform critical tasks ‘off goggle’ during a mission.

4.2 Human error and NVGs There are two principle question that arise when considering human error and NVG aided flight from a design perspective:

1. What human error classes are most likely?

2. For a specific error class how can the design address it?

Endsley’s model (Endsley, Smith 1996) of situation awarenesss was selected to structure the discussion of human error as it focuses on the element of cognition that is prima facie affected by the aviation use of NVG, i.e. the perception of elements of a situation, comprehension of it’s meaning and the prediction of future states (Figure 4.2-1) in order to achieve awareness of a situation. Endsley’s model separates situation awareness from the other cognitive stages of human cognitive processing and assists in the identification and discussion of human errors that occur due to a failure of situational awareness rather than (for example) mistake type errors of decision making or slip type errors of physical action.

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Figure 4.2-1 Endsley’s model of situation awareness

4.2.1 Performance Shaping Factors (PSF) PSF are conditions that affect the likelihood of human error. In section 3.3 and 4.1 a number of PSF were identified that can adversely affect aircrew in the conduct of NVG aided flight. While these PSF can be classified in turn into operator internal states, external conditions and stressors it should also be borne in mind that they are not independent. Figure 4.2-2 illustrates this complex interdependency between task, operational context, operator’s psychological state and equipment (NVG, displays and lighting).

Internal state of the operatorStressSkill/experience with NVG flightCurrency

External conditionsSituational

tactical, low level flight, air combatNight environment

Moon angleCultural lighting effectsambient illumination levelsFog/cloudlow contrast terrain

NVG image limitationsCockpit lighting compatibilityNVG weight and moment arm

Physiological/psychological stressorsNeck, shoulder fatigueVisual (eye) strainHigh task overload (merging, closure, safe escape)High focal vision channel loadingDangerous environmentCircadian rhythm disruption

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Internal state of the operatorStressSkill/experience with NVG flightCurrency

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tactical, low level flight, air combatNight environment

Moon angleCultural lighting effectsambient illumination levelsFog/cloudlow contrast terrain

NVG image limitationsCockpit lighting compatibilityNVG weight and moment arm

Physiological/psychological stressorsNeck, shoulder fatigueVisual (eye) strainHigh task overload (merging, closure, safe escape)High focal vision channel loadingDangerous environmentCircadian rhythm disruption

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Figure 4.2-2 NVG performance shaping factors

4.2.2 The NVG aided flight environment The environment of NVG aided flight includes both the traditional elements of spatial orientation, those environmental factors that can be used to predict NVG performance and the NVG itself (Figure 4.2-1). While in many cases information on the environment is provided redundantly, e.g. visual scene depth estimation versus radar altimeter, in some cases the use of NVG directly degrades these cues e.g. masking visual cues to the presence of cloud or rain.

4.2.3 Error classes and causal factors Based on the error classes identified by Endsley a taxonomy of errors and proximal causes was compiled (Figure 4.2-3). Of the error classes identified perception is considered the most critical as it is fundamental to situational awareness.

Situation awareness errorsPERCEPTION (LEVEL 1) SA ERRORS

Data not available due to:- No colour contrast- No visibility of light cloud, fog through NVG- Shadowing of terrain detail- Peripheral parallax motion cues

Data hard to discriminate due to:- Reduced visual acuity due to low light levels- Reduced NVG FOV & obscured displays- NVG/display/lighting in-compatibility- External lighting haloing- External scene effects (low moon etc)- Poor readability of displays (luminance, shadowing)- Canopy/windshield reflections in NVG FOV

Memory loss due to:- High scanning workload + operational task load

Data misperceived due to:- Halo size illusion- Distance estimation errors

Failure to monitor due to:- Breakdown of instrument scan- Attentional narrowing to NVG visual scene- Failure to maintain NVG scan for peripheral cues- High scanning workload + operational task load

COMPREHENSION (LEVEL 2) SA ERRORSIncomplete mental model due to:

- Unawareness of NVG limitationsIncorrect mental model due to:

- Visual illusions, spatial disorientationPROJECTION (LEVEL 3) SA ERRORS

Over-projection of current trends due to:- Unperceived spatial disorientation

Situation awareness errorsPERCEPTION (LEVEL 1) SA ERRORS

Data not available due to:- No colour contrast- No visibility of light cloud, fog through NVG- Shadowing of terrain detail- Peripheral parallax motion cues

Data hard to discriminate due to:- Reduced visual acuity due to low light levels- Reduced NVG FOV & obscured displays- NVG/display/lighting in-compatibility- External lighting haloing- External scene effects (low moon etc)- Poor readability of displays (luminance, shadowing)- Canopy/windshield reflections in NVG FOV

Memory loss due to:- High scanning workload + operational task load

Data misperceived due to:- Halo size illusion- Distance estimation errors

Failure to monitor due to:- Breakdown of instrument scan- Attentional narrowing to NVG visual scene- Failure to maintain NVG scan for peripheral cues- High scanning workload + operational task load

COMPREHENSION (LEVEL 2) SA ERRORSIncomplete mental model due to:

- Unawareness of NVG limitationsIncorrect mental model due to:

- Visual illusions, spatial disorientationPROJECTION (LEVEL 3) SA ERRORS

Over-projection of current trends due to:- Unperceived spatial disorientation

Figure 4.2-3 NVG error classes and causation

Without correct perception of the system and environment the ability to comprehend what is going on and predict future states is significantly degraded, and the likelihood of human error significantly increased. In Endsley and Garland’s study of loss of situational awareness in aviation 76% of such errors were traced to problems with perception (Endsley, Garland 2000) with the greatest percentage (37%) belonged to the failure to monitor error class. Endsley and Garland’s study also found that availability and discrimination error classes each comprised 11.6% of the total and when added together comprised the second highest number of reported errors. From these general results it can be inferred that lighting and display design can have a significant effect upon the occurrence rate of direct and

availability/discrimination phenotypes of human error during NVG aided flight, while also indirectly affecting memory loss & monitoring failure phenotypes. This inference is supported by Berkely’s analysis of USAF NVG related mishaps (Berkely 2000).

4.2.4 Managing human error From Figure 4.2-1 and Figure 4.2-3 it can be seen that any human error strategy that solely addressed the technical aspects of NVG integration (e.g. lighting and display compatibility) would be ineffective as other PSF could still adversely affect the rate of human error.

For the F/A-18 NVG integration controlling the non-technical PSF of NVG flight was addressed by the operational airworthiness element of the project. Significant operational airworthiness activities included:

1. treatment of NVGs as another sensor to be integrated into night time flight, rather than as a means to fly at night with daytime flight rules,

2. establishing minimum crew currency and proficiency standards for NVG flight,

3. developing class room and operational training curricula to achieve and sustain currency and proficiency targets,

4. restricting the operational employment of NVG to eliminate high risk but low operational benefit activities (e.g. NVG use in takeoff/landing), and

5. rolling out NVG use in a ‘walk before you run’ fashion to minimise operational risk during the transition phase.

The operational restriction of NVG usage to the tactical phases of flight had a significant effect upon the NVG integration design strategy and is discussed in section 5.

5 The NVG and F/A-18 integration From the identified error classes, their causes and the requirement to retain current day and night display capabilities a set of key performance attributes for the design were identified:

1. daylight and night time readability of displays,

2. lighting balance of diplays,

3. NVG visual acuity/contrast sensitivity in the field,

4. display (HUD and Digital Display Interface (DDI) effects,

5. transparency attenuation effects on NVG ,

6. NVIS compatibility of lighting and displays,

7. colour and spectral radiance, and

8. transparency and cockpit surface reflections.

A design strategy and operating concept were then developed to guide decisions as to what would be the most cost-effective way to modify the F/A-18 fleet while addressing these key attributes of design (Figure 4.2-1).

NVG Operating Concept1. Use only during tactical phases2. No use during take-off or landing3. Unaffected day/night operations

NVG Integration Design Strategy1. Safety and tactical cues to be modified

• Colour hierarchy to be maintained• Threat & warning signals visible in NVG FOV• If not illuminating or safety cue leave unmodified

2. All lighting on main instrument panel to be modified3. Modify existing integral instrument illumination4. Control haloing & reflections through:

• Minimal use of floodlights (side consoles only)• Turn unmodified digital displays & HUD to minimum• Use existing integral illumination

5. Turn off unmodified panel lighting6. Small use of NVIS white for colour map/chart reading7. No wiring changes8. Use augmented lighting for marginal night readability9. External strobes and navigation lights are turned off10. P43 phospor CRT/HUD displays will not be modified11. Mechanical display pointers, flags etc will not be modified12. Use common components for uniform chromaticity

Figure 4.2-1 Operating concept & design strategy

The F/A-18 design also provided a number of inherent advantages to the integration. The cockpit layout places the analog instruments low in the cockpit (Figure 4.2-2) ensuring that they are not obstructed by the NVG FOV9. The Class C NVG also allowed the use of the HUD within the FOV of the NVG thereby increasing the attensity of HUD flight data (attitude, airspeed and altitude). The use of internal instrument and panel lighting elements also meant that shadowing or surface reflection from bezel or post lighting of instruments need not be considered10. Finally the matte grey and black surface treatment of the cockpit minimised the contribution that these surfaces made to veiling glare and reflections. The only significant disadvantage of the F/A-18 design was that the lighting control only operated in 0.25V increments thus the general lighting could not be adjusted to the 0.1 fL luminance level of MIL-L-85762A.

Using the strategy of Figure 4.2-1 a final design concept (Figure 4.2-2) was developed from the initial prototype design.

9 As they are in the F-16 or A-10 cockpits (Berkely 2002). 10 As occurred with the floodlit instrument lighting of the initial EA-6 cockpit NVG modification program.

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Figure 4.2-2 Front cockpit modified components

5.1 Readability The purpose of a display is to present data to the user, with display readability being an important indicator of the effectiveness of this data perception process.

5.1.1 Daylight readability Daylight readability enables a display to be readable in direct or indirect sunlight conditions and, when not energised, not display ghost data (termed dead facing). MIL-L-85762A relaxed Lighted Contrast (CL), from 0.6 to 0.4 to compensate for the additional filtering but for visual signals exposed to direct sunlight requires testing under direct 10,000 ft-candle illumination, representing the worst case situation of a pilot seeing a reflection of the sun or white clouds. Daylight readability can be improved by the use of materials or surfaces that attenuate or disperse ambient light but pass display emissions thereby increasing CL and reducing Unlighted Contrast (CUL).

Initial modifications of the caution light indicator panel were found to be exhibit poor readability and ghost signals during daytime test flights. To resolve these issues the panel was redesigned using an integral filtered legend material that provides improved CL and CUL. Legend indicator surfaces were also micro-etched to provide a matte (high Lambert coefficient) surface that improved CL surface.

5.1.2 Night-time readability Night-time readability enables a display to be readable under night time low illumination levels. A minimum level of luminance is required to minimise dwell time when resolving the presented signal. Illumination can be degraded through either poor direct illumination or shadowing effects when instruments are externally lit.

Tests of the initial design found poor night-time readability of the analog Fuel Quantity Indicator (FQI)11 with the bottom portion of the fuel gauge totaliser poorly illuminated when the instrument lights rheostat was set at a comfortable level for other main instrument panel displays and instruments. To view the FQI required the pilot to turn up the lighting circuit to read the gauge, resulting in other lighting being too bright. As a complete redesign of the F/A-18 FQI was impractical an approach of modifying the existing FQI lamps to increase the FQI’s luminance while augmenting external illumination was adopted. The option of displaying the fuel data on the Digital Display Indicator (DDI) displays was also considered.

The initial illumination levels for the Jettison push-button legend (within a protective barrel guard) were also found to be insufficient. The reflectiveness of the barrel guard interior was increased using tape to rectify this issue.

5.2 Lighting balance

5.2.1 Cockpit lighting balance A good cockpit lighting system provides lighted displays that appear to have the same brightness across the whole cockpit. If unevenly balanced, aircrew are presented with some displays that are so bright that they are a source of glare, and others so dim that they require significant ‘eyes in’ time to read. To control these effects the luminance ratio between dimmest and brightest components must be less than 2:112 to ensure readability of all displays at the lower NVG compatible luminance levels. Displays containing multiple light sources should also be uniformly balanced to ensure readability and that specific indicators possess adequate attensity (MIL-STD-1472).

The initial left and right console flood lighting design was found to be poorly balanced (due to variability in the prototype hoods) leading to inadequate illumination of some consoles. Selecting higher illumination to rectify this resulted in veiling glare and the lighting system transitioning to a high intensity daylight mode. To resolve this issue COTS floodlights were substituted for the initial prototype hoods (Figure 5.2-1)13.

The initial filter only design of the chart-light resulted in such a low brightness that it was necessary to increase the brightness of the chart-light-floodlight circuit above 70% again causing the warning and caution light circuit to

11 Similar problems were also found with the F-16’s analog fuel totaliser. (Marasco, Boyer, Boulter 2001). 12 Reduced from the 3:1 ratio for non NVG lighting systems. 13 Floodlight trimming would have required re-wiring these circuits and was therefore rejected.

transition to daylight mode. In order to rectify this issue the chart-light lamp was simply up-rated to provide greater brightness.

Prototype hood(Inconsistent compatibility)

COTS(USN/USAF qualified)

INITIAL FINAL

Prototype hood(Inconsistent compatibility)

COTS(USN/USAF qualified)

INITIAL FINAL

Figure 5.2-1 COTS floodlight design change

5.3 NVG visual acuity or resolution Also called resolving power or resolution, visual acuity is an early optical science concept that is still popular because it is both easy to understand and to measure. Visual acuity is not a direct equipment attribute but an emergent system property of the equipment, human visual processing and noise. There are a number of variables that can affect resolution; subject visual limitations (and variability), fatigue, NVG performance, target illumination/distance can all affect achievable resolution. The test target utilised for establishing visual acuity of the system was the USAF 1950 medium contrast resolution target of Figure 5.3-1. The operator views the target illuminated to a specific luminance at a specified distance and determines the smallest set of tri-bars on the target that can be resolved. However, there are a number of potential sources of bias and random error when using the USAF 1950 target to assess NVG VA including:

1. the chart shape is known causing decision bias,

2. scintillation caused grating ‘fade in/out’, and

3. differences between test subjects (on one program 24%).

To address these issues two aircrew test subjects were used minimise visual performance differences, and the results cross compared to ARDU initial test results.

Figure 5.3-1 USN 1950 Tri-bar Chart

5.4 Digital display effects The current F/A-18 HUD spectral radiance is within the Class C NVG notch filter band. However, during tests of the initial design it was found that as the brightness level increased the compatibility of HUD with NVGs

decreased. NVG performance was optimised by setting the HUD brightness to a low daylight setting that provided an acceptable level of visual acuity while retaining the ability to see the HUD through the NVG.

Testing of the initial design also found that the due to their total area the left and right Digital Display Indicators (DDI) were a principal source of canopy reflected veiling glare in the modified cockpit when set above normal levels. Comparison with other NVG modification programs confirmed that this was a consistent problem with diffuse sources of illumination such as panel displays. This incompatibility was controlled by maintaining display brightness at levels less than the 0.5 fL display luminance level specified but above the minimum of readability of 0.045 fL established by Howard, Reigler and Martin (Howard et al. 2001). Although no further modifications to the HUD or DDI were planned, a series of test points were included into the ground test plan to evaluate the effects of HUD and DDI performance upon visual acuity. This data was intended to provide a performance baseline for follow-on transparency and display upgrade design changes.

5.5 Transparency effects Anything reducing the light level to an NVG will tend to reduce the output luminance and decrease the Signal to Noise Ratio (SNR). While transparencies are designed to transmit visible light some transmit near infra-red poorly due to transparency or coating properties thereby reducing NVG visual acuity. As an example coefficients of transmittivity for installed windscreens can range from 70% to 20% or less depending on the material and viewing angle. Viewing an image through the HUD combiner exacerbates this effect as two transparencies are now interposed between the observer and the external scene, while the scene itself is made less distinct due to slant range effects (Pinkus 2000). Optical haze14 within transparencies can also reduce contrast between target and background, leading to further VA and CS reduction.

WINDSHIELD

HUD COMBINER

CANOPY

DISPLAY GLARE SHIELD

Figure 5.5-1 F/A-18 transparencies

The larger NVG optical aperture also has a greater aperture size (compared to the human eye) over which

14 Haze is normally caused by micro scratches on transparency surfaces or internal transparency imperfections.

windscreen wavefront aberrations are averaged, reducing resolution. Windscreens are also designed around a ‘design eye’ with optical measurements made from this viewing datum. If the NVG objective lens is located significantly outside the design eye, windscreen distortion will also degrade optical quality (Task 1992).

As the original transparencies of the F/A-18 (Figure 5.5-1) were to be replaced by solar tinted transparencies, a set of test cases for transparency effects were included to provide a performance baseline.

5.6 NVG compatibility NVG compatibility is functionally defined as the characteristic of a lighting system that allows the crew to acquire information inside the cockpit without degrading performance of the NVG when looking outside the cockpit. NVG compatibility is achieved through the use of filtered lighting at reduced intensity both within and outside the NVG FOV.

A number of lighting and display systems of the initial design exhibited failures against the requirements of MIL-L-85762A. To resolve these incompatibilities lighting components were procured as pre-qualified COTS items or from manufacturers experienced in producing/modifying items to NVG compatibility standards.

5.7 Chromaticity and spectral radiance

5.7.1 Chromaticity and NVG compatibility Chromaticity (colour) and spectral radiance are of extreme importance for NVIS compatible cockpit lighting systems. Colour also provides a visual hierarchy of warnings, cautions and advisory displays and legends to air crew. For the F/A-18 both NVIS compatibility and a colour hierarchy was required, therefore the colour of the warning, caution and advisory signals are specified as falling within with MIL-L-85762 specified 1976 CIE chromaticity diagram regions (see Figure 3.2-1). Because NVIS Red still affect NVG visual acuity, the traditional warning legend design of a trans-illuminated background and opaque text are reversed to trans-illuminated text on an opaque background (MIL-STD-411). While this reduces attensity the reduced illuminated area improves compatibility by reducing the amount of illuminated red in the direct FOV of the NVG.

5.7.2 Indicator & legend design The initial Advisory Threat & Warning panel design (Figure 5.7-1(a)) achieved a degree of NVG compatibility but also introduced colour hierarchies and symbology inconsistent with MIL-L-85762A and MIL-STD-3009 while only achieving compatibility on average twice the maximum NRB value. RCDR 1, 2 and 3 legends, introduced by a subsequent design change, also exhibited excessive brightness to the point of constituting an annoying distraction to aircrew.

To address these issues all warning legends was reverted to NVIS red trans-illuminated text to comply with MIL-STD-411 and MIL-STD-3009. The FIRE legends also

had additional barber poles Figure 5.7-1(b) added to maintain the original shape coding15 of this signal. Caution signals were implemented in NVIS yellow to maintain the colour hierarchy rather than the MIL-L-85762A specified Green A16. The cockpit video recorder (RCDR) legend luminance was also balanced to the other legends of the panel.

In order to maintain a consistent colour balance across Green A lighting, which comprise the majority of cockpit lighting, a tighter USN developed ‘Intruder Green’ chromatic sub-locus (Figure 3.2-1) was defined as a design goal for these items.

5.7.3 HUD radiance Because of the wavelength specific nature of the HUD it is impossible to achieve the original MIL-L-85762A minimum NR figures with Type I Class B NVG, as the HUD phosphor is out of band. Instead Type I Class C NVIS provide a notch filter that coincides with the HUD primary frequency of approximately 525 nm. However because the F/A-18 HUD does not incorporate an IR filter the peaks to the right of the primary HUD frequency (refer Figure 3.1-1) will degrade NVG performance. This effect can be exacerbated by the presence of dust on the optical surfaces which will re-radiate an appreciable portion of the incident energy in the near infra-red. During initial evaluations of HUD compatibility a reduction in NVG performance was noted when viewing through the unfiltered F/A-18 HUD.

(b) Final RH AT&W design(a) Initial RH AT&W design (b) Final RH AT&W design(a) Initial RH AT&W design

Figure 5.7-1 Advisory Threat & Warning Panel Design

5.7.4 Monochromatic display radiance Spectral radiance requirements for monochromatic displays are the same as those for general crew-station lighting except when the display is required to show grey scale FLIR imagery where the display must be NVIS-compatible at 0.5 fL17 rather than the standard 0.1 fL.

The Horizontal Situation Indicator (HSI) was the only digital display directly modified by the NVC project. As the HSI is fitted with a Fresnel lens that minimises the amount of diffuse light that the display gives off the moving map backlighting was filtered to NVIS white to ensure legibility of the coloured map. For this reason test cases were included for this display.

15 The FIRE signal is coded using text, size and shape. 16 As derived from MIL-STD-3009. 17 Required to display an eight shade grey scale (MIL-STD-3009).

Test points were included in the ground test to evaluate the effect of the DDI in order to establish a performance baseline for displays located in this area.

5.7.5 Multi-colour display radiance To allow the use of color displays, the NR requirement were relaxed in MIL-L-85762A. This relaxation was based on two assumptions, firstly that the display occupies only as small percentage of the total cockpit area and secondly that the display is not located within the FOV when aircrew are looking outside the cockpit. Unfortunately, the assumption that the radiating area of a color display would be small and located out of the FOV has been invalidated by the current generation displays and cockpit layouts. Additionally back-lit LCD displays can radiate infra-red from un-illuminated areas, increasing the total radiated energy even though the display is visually ‘dark’. Consequently, MIL-STD-3009 now includes requirements to verify that display luminance and radiance scale together.

Display incompatibility can be worsened if FLIR imagery must be displayed as aircrews tend to increase the brightness of these displays to enhance scene contrast and useful detail. In one instance a USAF test subject adjusted the display from the NVIS setting (1.0 fL) to 90 fL in order to distinguish finer image detail (Marasco, Boyer, Boulter 2001).

5.8 Reflections An optimal crew-station lighting system will provide sufficient light to support information transfer without causing objectionable glare from light sources, or reflections on the cockpit transparencies. Reflections can be controlled by limiting light levels, shielding, optimising windshield angles or other means.

Reflections from transparencies can obscure the outside scene leading to higher signal versus noise required before detection. The arrangement of HUD combiner transparencies with large panel displays can also lead to reflection of display emissions into the FOV of NVG. As the HUD transparency is pointed at the design eye this can significantly degrade NVG performance as well as requiring higher HUD display contrast. MIL-L-85762A specifies that the specular reflections should not occur within the area subtended by a solid angle of one steradian centred at the pilot’s design eye position and along the pilot’s horizontal vision line. However reflections may occur on various parts of the canopy and windshield. Field tests of the initial design also demonstrated that reflections from the RH A&TW panel warning legends and L&R DDI could induce veiling glare effects when looking through the adjacent canopy.

6 Design verification

6.1 Verification strategy Verification of requirements was carried out in a two tier program of component and aircraft level testing. Operational T&E was not required for service release of

the modification as this had been conducted with the initial ARDU design.

Components tests consisted of NVIS radiance, daylight readability and illuminated panel and legend requirements and a component qualification test program was established to ensure that all items were qualified prior to system level testing. For COTS items product data sheets were used as evidence of compliance. For those items that were unable to be tested during the aircraft level test due to equipment failure, component qualification data was also used to support system verification.

Aircraft tests consisted of crew-station reflections, night time readability, visual acuity and cockpit lighting balance, i.e. those requirements that could not be verified at the equipment level. To verify these requirements a ground test activity was conducted on the first trial aircraft modification. Daylight readability and NVIS radiance tests were also tested to support investigation of performance discrepancies, such as light leaks.

6.2 System level verification

6.2.1 System verification planning A ground test plan and sequence was developed based upon the methodology developed by Reising and others (Reising et al., 1996) to address the requirements for NVG compatibility (Figure 6.2-1).

Figure 6.2-1 Ground test sequence

6.2.2 Aircraft ground test results Daylight readability (T1). An adequate level of daylight readability was demonstrated during the test. These test results were validated by a survey of F/A-18 aircrew involved in operational flights of the modified aircraft.

Lighting balance (T2). An acceptable lighting balance was achieved with the exception of the FQI (Figure 6.2-2) relative to the Attitude Reference Indicator (ARI). Because the FQI is a hybrid illuminated panel, gauge and barrel counter display, when the lighting level was adjusted the illuminated panel increased disproportionately making the barrel counters difficult to see against the glare. Increasing overall Main Instrument

Panel (MIP) luminance also increased the ARI brightness18 excessively. As the analog fuel gauge (attended to during the instrument scan) and bingo bug was assessed as acceptably illuminated no further changes were made to the FQI.

Fuelgauge

Barrelgauge

Illuminatedpanel

Fuelgauge

Barrelgauge

Illuminatedpanel

Figure 6.2-2 Fuel quantity indicator

Night time readability (T3). A good overall level of readability was achieved with the exception of two components which failed due to errors in the modification process. These were subsequently rectified.

Crew-station reflections (T4). At operational lighting levels no NVG reflections were found within one steradian angle about the horizontal eye line19. It was noted that normal reflections were also reduced.

NVIS visual acuity (T5). The cockpit lighting and displays found no impact of cockpit lighting or transparencies upon VA. Because of the slightly lower (1.3 x 10-10 NRB) than MIL-L-85762A levels of test target illumination, the results represent a conservative assessment of achievable VA, albeit with minor differences in reported VA between test subjects. This result was consistent with ARDU test data. When compared to tests results from other F/A-18 operators it was noted that two of the non-aircrew subjects in that test identified a decrease in VA with the canopy down whereas the aircrew test subjects did not. It was concluded that under low light conditions aircrew are better able to compensate for losses in visual contrast and maintain a consistent level of VA.

NVIS radiance (T6). NR was achieved for all modified lighting components and displays as was the absence of any light leaks. Six measurements were made of characters on the unmodified DDI CRT screen with the average value found to be twice the maximum NRB.

18 The ARI was deliberately left at a higher level of luminance to improve the attensity of this optically deep display. 19 The reflection requirement of MIL-L-85762A specifies a solid angle but not the enclosing arc, a conic section was assumed for testing.

Figure 6.2-3 Modified post display upgrade front cockpit

Other human factors issues. During the system test button intermittent stiction problems were experienced with the integrated illuminated pushbutton lighting panel of the upfront control unit (located immediately below the HUD. This problem was referred back to the panel manufacturer and resolved through a design change.

7 Conclusions A compatible cockpit lighting and display modification for the F/A-18 was developed, integrated and demonstrated by the ground test activity. No identified discrepancy was assessed as having a safety of flight implication. It was determined that the effects of veiling glare from unmodified legacy digital and head-up displays could be managed effectively by maintaining them at a lower than the standard luminance level and that this would not increase reaction time. Baseline tests of transparency and display performance were performed and follow on evaluations of the F/A-18 post display and transparency upgrades were recommended to the relevant projects.

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