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  • Human Engineering for Australias 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 90s. Progress was slow until the late 1990s 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 aircrafts 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

  • 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.

    400 500 600 700 800 900 1000

    20

    0

    40

    60

    80

    100

    VISIBLE INFRARED

    WAVELENGTH (nanometres)

    Sky ambientradiation

    Generation III (Class B/C)NVIS response

    Class Bminus bluefilter line 665 nm

    Human eyeresponse

    RELA

    TIVE RESPO

    NSE (%

    )

    Display/HUD P43 phosphor

    Figure 3.1-1 Near infrared spectrum usage

    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 NVGs FOV to trigger the NVGs 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