The Effects of Cockpit Lighting on Human Visual Performance

An in-depth look at the challenges that are faced in the design of aircraft cockpit lighting systems in relation to the structure and function of the human eye.

Eliza Weber

Structure of Human Eye

• Eye contains two types of receptors: Cones and Rods.• Cones are located in the center of the retina.

• Resolving fine detail• Seeing color• Function under high illumination levels (Photopic vision)

• Rods are located in the periphery of the retina.• Poor visual acuity• Seeing shades of black and white• Function under low illumination levels (Scotopic vision)• Enhanced sensitivity & low detection thresholds under reduced

illumination

• Light adaptation – Upon exposure to light, photopigments in cones and rods undergo a chemical reaction.

• Converts light energy into electrical impulses that are sent to the brain

• Photopigments are decomposed• Reduction in retinal sensitivity to dim light

• Dark adaptation – Regeneration of photopigments.• Eyes adapt for optimal night visual acuity under conditions of low

ambient illumination• Cones regenerate in 5-7 minutes• Rods regenerate in 30-45 minutes or longer and achieve a much

greater sensitivity than cones• Scotopic sensitivity is greater than photopic sensitivity at all except

the longest light wavelengths (the “red” end of the spectrum)

Dark adaptation

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Cockpit Lighting

• Red light was used for illumination of the cockpit in post-World War II aircraft because it did not degrade dark adaptation.

• Under conditions of total or nearly total darkness, red lighting preserves visual sensitivity for outside viewing to a greater extent than does white lighting, while still providing some illumination for central foveal vision.

• Low intensity, white cockpit lights are often used now because they afford more of a natural visual environment within the aircraft, without degrading the color of objects.

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Cockpit Lighting Design Factors

• Dark Adaptation• Visual Acuity• Contrast Sensitivity• Luminance Sensitivity• Color Discrimination

Visual Acuity• Eye’s ability to resolve spatial detail• On a typical flight deck, pilots must be able to read

dials, controls, switches, displays and all associated information from a working distance of between 2 ft and 3 ft.

• Red lighting on the flight deck requires more focusing power than white light for near objects to be observed clearly.– Longer wavelengths require more elastic accommodation of

the lens in order achieve focus on the retina. – This may cause difficulty for pilots in their 40s and older with

presbyopia – the most common age-related change in vision

Visual Acuity

• In a test of near visual acuity under low-level red and white light by Richard Dohrn (USAF):– younger subjects with normal color perception

did not exhibit a difference in near visual acuity under red or white light

– Older subjects experienced reduced near visual acuity under red light

– The questionably better dark adaptation provided by red light compared to white light may be negated by the decrement in near visual acuity

Contrast Sensitivity

• Contrast sensitivity is the eye’s ability to perceive differences in luminance between the information being presented and the background against which it is presented.

• Threshold Contrast Detection• Spatial resolution• Display luminance• Viewing duration• Adaptation level

Contrast Sensitivity

• Requirements for minimum contrast values may vary among lighting and display configurations.

• Generally, higher contrast values are easier to achieve in dark conditions (night) than in high ambient lighting conditions (day).– Red lighting appears to be better suited for best

contrast sensitivity

Luminance Sensitivity

• Luminance sensitivity is the eye’s ability to process images in different levels of illumination.

• Flight deck lighting systems must provide sufficient illumination for each instrument and its associated controls or switches to be readable. – near total darkness and daylight-illumination

levels

Luminance Sensitivity• Glare/reflections

– The shape of the canopy, placement of lights, and the presence of an instrument sunshield place the highest probability for glare and/or reflections in the areas on either side of the pilot where they would most likely be picked up by peripheral vision.

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Luminance Sensitivity

• Since peripheral vision predominantly involves the rods, the likelihood that the pilot will perceive and/or be distracted by the reflections of red lit instruments and consoles is less than if they were lit by white lamps.

• Legibility of displays – Red vs. White– At equal levels of luminance: Red better than white– At low levels of luminance: Red better than white– At high levels of luminance: Red equal to white

Luminance Sensitivity• Luminance of any display is a function of both the

spectral nature of the luminance and the reflectance characteristics of the display surface.

• White light is produced by radiant energy • Red light is confined to energy above 580 millimicrons

(monochromatic)

• The spectral reflectance of the white characters among various displays must possess almost identical spectral reflecting qualities or the resultant appearance of the total display will be very non-uniform under white lighting.

• Under red lighting, color differences are far more difficult to perceive and any spectral/reflectance differences due to non-uniformity will be seen only as minor differences in luminance and not as wide color variations.

Color Discrimination

• Color discrimination is the eye’s ability to differentiate between colors.– begins as the cones begin to contribute to vision as an

element of mesopic vision and is optimized with photopic vision.

• White lighting system makes map and chart features, which are often color coded, more distinguishable.

• Red lighting system gives maps a monochromatic appearance in which red ink has little contrast against white paper.

Night Vision Goggles (NVGs)

• Using NVGs for night flight provides the flight crew with improved methods of orienting the aircraft and avoiding terrain and obstructions.– multiply very low levels of existing light and present the

operator with a scene in front of his or her eyes that closely resembles a daytime scene

• NVGs cannot differentiate between light originating outside the cockpit (i.e., the desired response) and light originating inside the cockpit (i.e., light from the display instruments).

NVGs

• Conventional cockpit lighting is made with incandescent lamps, which have a considerable part of their emissions in the near infrared region (700nm to 900nm), a region to which the NVGs are very sensitive.

• A blue-green lighting system has been developed that uses the unique spectral response of NVGs.– Blue-green light is visible to the human eye beneath the

NVGs, allowing the pilot to view the instruments, but is virtually invisible to the NVGs and does not adversely affect performance.

Conclusions

• White and red cockpit lighting has their merits and their disadvantages.

• There are several factors that must be considered in the design of cockpit lighting for optimal human visual performance.

• The reliability of NVGs is significantly decreased if they are not compatible with the interior cockpit lighting system.

• Areas for improvement:– Light diffusion across instruments– Auxiliary lighting– Reduction of reflections and glare– New techniques for Cockpit/NVG compatibility

References• American Optometric Association. (2006).• Dohrn, R.H. (1968). Near visual acuity under low-level red and white light (Report No. SAM-TR-68-119). Brooks Air Force Base, TX: USAF School of Aerospace Medicine

Aerospace Medical Division (AFSC). • Hitchcock, Jr, L. (1976). F-18 Interior lighting analysis: red vs. white (Report No. NADC-

76177-40). Warminster, PA: Crew Systems Department Naval Air Development Center.• Poston, A.M. (1974). A literature review of cockpit lighting (Report No. Technical Memorandum 10-74). Aberdeen Proving Ground, MD: U.S. Army Human Engineering

Laboratory. • Rash, C.E., Manning, S.D. (2003). On the flight deck, lighting must satisfy a variety of

needs. Human Factors & Aviation Medicine, 50(5), 3-10.• Task, H.L. (1992). Cockpit/NVG visual integration issues. Wright-Patterson Air Force

Base, OH: Air Force Research Laboratory