Cockpit Lighting: Part II: The effect of lighting color on night vision

Dark adaptation is the process by which the eyes adapt for optimal night visual acuity under conditions of low ambient illumination. The eyes require about 30 to 45 minutes to fully adapt to minimal lighting conditions. The lower the starting level of illumination, the more rapidly complete dark adaptation is achieved.

If dark-adapted eyes are exposed to a bright light source (searchlights, landing lights, flares, etc.) for a period in excess of one second, night vision is temporarily impaired. Exposure to aircraft anti-collision lights does not impair night vision adaptation because the intermittent flashes have a very short duration of less than a second.

The debate over the merits of red lighting vs. white lighting has persisted for years, ever since the search for new methods of crew-station lighting began during and after World War II. While red lighting often was used in military planes at the time, pilots were sometimes required to wear red goggles for a certain period of time before embarking on nighttime sorties. These precautions for night adaptation were thought necessary because pilots needed to be able to spot enemy aircraft after departing from inadequately lighted airports and navigating most often by vision, rather than by instruments.

During his work with film development in the late 1930s and early ‘40s, H.K. Hartline, a physician and physiologist, found that he adapted well to darkness under red lighting conditions. Later, working for the U.S. Navy, Hartline demonstrated that red-lighted instruments were readable at low-light levels. Some of his other work with the human retina had shown that the rods are almost totally insensitive to red.

As a consequence of his recommendations, the U.S. Army and U.S. Navy began using red light in their cockpits in the 1940s.

To produce red lighting during an era when incandescent lamps were the primary light source, the light from the lamps was filtered. However, this increased the cost of the lighting, generated heat in the instrument panel, and prompted manufacturers to question whether there really was an advantage to using red lighting in place of white lighting.

Hartline’s conclusions, though, have been supported by numerous other studies on dark adaptation. For instance, a 1982 U.S. Army report compared the effects of red lighting and blue-white lighting (which uses a blue filter to compensate for an incandescent lamp’s tendency to turn yellow as it is dimmed) on dark adaptation under operational conditions. According to the report, “under conditions of total or nearly total darkness, red lighting preserves visual sensitivity for outside viewing to a greater extent than does blue-white lighting. This is true even when instrument lights are set at the low levels ... at which (U.S. Army) aviators normally set their instruments.”

Regardless, the report also said that with a full moon illuminating a clear sky, the difference between the two lighting schemes “vanishes.”

Other studies have examined the advantages of white light. In a 1987 book, Frank Hawkins cited a number of advantages, including that white light reduces eye fatigue, improves instrument and display contrast, provides better illumination in thunderstorms and daylight, and permits effective color coding. In red light, the color coding on some aeronautical charts and some flight instruments disappears—that is, the information is readable, but color differentiation among symbols cannot be seen.

The American Optical Association said that red lighting on the flight deck requires more focusing power than white light or blue-green light for near objects to be observed clearly. This may cause difficulty, especially for pilots in their 40s and older with presbyopia—the most common age-related change in vision—in which the eyes become less able to focus on nearby objects.

Nevertheless, red lighting became the standard for military aircraft and some non-military aircraft, and functioned well until the introduction of night-vision goggles (NVGs), multicolored CRT displays, and active-matrix LCD displays, which were found to be incompatible with red lighting.

Studies determined that ambient red lighting does not provide true dark adaptation, but instead provides color adaptation.

In the human eye, the retina is the inner layer of the eyeball that contains photosensitive cells called rods and cones. The retina functions similarly to the film in a camera: to record an image. The cones are located in higher concentrations than rods in the central area of the retina known as the macula, and exact center of the macula has a very small depression called the fovea that contains cones only. Cones are used for day or high-intensity light vision and are involved with central vision to detect detail, perceive color, and identify far-away objects.

Rods, meanwhile, are located mainly in the periphery of the retina—an area about 10,000 times more sensitive to light than the fovea. Rods are used for low-light intensity or night vision and are involved with peripheral vision to detect position references including objects (fixed and moving) in shades of grey, but cannot be used to detect detail or to perceive color.

The rods and cones adapt to the red wavelengths; consequently, the pilot may have difficulty discriminating between some colors on the color display. Partly to address this issue, the U.S. Air Force decided to use blue-white lighting on its flight decks, but blue-while lighting on an instrument panel requires about 30 percent more lamps, which requires a bigger power supply, which in turn requires more weight, which decreases useful load. So, until the advent of multi-function displays (MFDs) in the 1980s, most commercial aircraft use unfiltered white lighting to reduce costs.

In the early 1990s, the Aerospace Lighting Institute suggested the following guidelines for selecting a lighting system based on color:

If the primary visual task is inside the flight deck, consisting of monitoring display instrumentation and controls, and the outside visual task of scanning for other aircraft takes a secondary role (without compromising safety), then a lighting system comprised basically of white lights is recommended;
If the primary visual task is scanning for lights and other aircraft (but night-vision devices are not being used), then a lighting system comprised basically of red lights is recommended; and,
If night-vision devices are required for flight, then both white light and red light are prohibited. A blue-green lighting system has been found to be effective in military aircraft.These basic guidelines, although useful, have been difficult to apply because of the use of MFDs in aircraft with glass cockpits.

Today’s airliners generally utilize unfiltered white light at crew stations for both panels and instruments (except flat-panel displays). For example, all current Boeing airplanes use unfiltered white light. Pilots are able to dim area lighting and instrument lighting to “appropriately low levels to allow sufficient dark adaptation for nighttime operation,” Dr. Alan Jacobsen, technical fellow, flight deck engineering with Boeing Commercial Airplanes, said.

Those levels were determined by human factors evaluations, Jacobsen explained. The aircraft also are equipped with storm lighting “in which the flight deck lighting can be driven to fairly bright levels with the flip of a switch” to counter the loss of dark adaptation resulting from lightning flashes, he added.

John Lauber, vice president for safety and technical affairs at Airbus, said that his company also uses unfiltered white light on the flight decks of its aircraft. “[Using red light to protect] night vision may have been important at one time,” Lauber said, “but is probably not so significant now, with modern lighting systems, both airborne and ground-based.”

The U.S. Air Force uses blue-white incandescent light for both panels and instruments (except flat-panel displays) at crew stations that do not require utilization of a night vision imaging system (NVIS). A blue filter sometimes is placed over incandescent lamps to compensate for a yellowing that occurs when they are dimmed.

The U.S. Navy and U.S. Army use red incandescent lighting for both panels and instruments (except when flat-panel displays are used) in aircraft where an NVIS is not used. In aircraft in which an NVIS is used, blue-green NVIS-compatible lighting is used. The blue-green lighting is required because an NVIS has a spectral sensitivity that favors the red end of the electromagnetic spectrum, including both the red region of the human visible spectrum and the invisible infrared region. A blue cutoff filter that prevents virtually all blue light from being seen enhances this characteristic.

Modern corporate aircraft have white electroluminescent (EL) panels and incandescent instrument lighting (except when flat-panel displays are used). Most smaller general aviation aircraft are equipped with incandescent post lighting for instruments and post lighted indicia (plates) for legends and circuit breaker panels.

For years, military aviators have operated at night using night-vision-viewing devices with image intensification. The better known of these night-vision aids consists of a pair of image-intensifier tubes mounted in a binocular configuration on a helmet. While using this system the pilot looks through it to view the outside world and looks beneath and around it to view flight instruments. Originally called NVGs and later the aviator’s night-vision imaging system, the device now is referred to as NVIS.

The military used night-vision aids for ground operations during the late 1960s. Aviation-developed NVGs have been used in military aircraft since the 1970s and now are being used in civil aviation—both in rotary-wing and fixed-wing aircraft—especially in law enforcement and emergency medical services (EMS) operations. While not likely to become an option for airline pilots, the U.S. Federal Aviation Administration (FAA) issued the first supplemental type certificate in January 1999 to authorize use of night-vision devices by civilian EMS helicopter operators.

Using NVIS for night flight provides the flight crew with improved methods of orienting the aircraft and avoiding terrain and obstructions. Disadvantages, however, include reduced depth perception, neck strain, fatigue, a decrease in visual acuity, absence of color discrimination, and a reduced field of view.

Image intensifiers amplify reflected or emitted light so the eye can more readily see a poorly illuminated scene. These devices depend on the presence of a minimum amount of light to produce a usable image. This is analogous to using a microphone, amplifier, and speaker to allow the ear to more easily hear a faint sound. The intensified image resembles a black and white television image but in shades of green (caused by the selected display phosphor) instead of shades of gray. Using the principle of image intensification, an NVIS multiplies (amplifies) the few photons present at low ambient light levels into a larger number seen by the user. The multiplication factor is typically 6,000 to 8,000.

The use of an NVIS on the flight deck presents lighting designers with a dilemma. The primary purpose of air NVIS is to allow the pilot to see the outside world. An NVIS has an automatic gain control that reacts to the ambient light level, so if the ambient light level decreases, the gain control increases the multiplication factor; if the ambient light level increases, the gain control decreases the multiplication factor.

The dilemma is that an NVIS must respond to the ambient lighting level outside the flight deck, but cannot differentiate between light (photons) originating outside the cockpit (the desired response) and light originating inside the cockpit (i.e.. light from the display instruments). Therefore, the lighting designer must illuminate the cockpit with light that will allow the pilot to clearly view the instruments through the NVIS, but that will not cause the NVIS to lower its performance in amplifying the low-light scenes outside the aircraft.

The military has attempted to overcome this problem by developing a blue-green (no red) lighting system that uses the unique spectral response of NVIS. The blue-green light is visible to the human eye beneath the NVIS—allowing the pilot to view the instruments—but is virtually invisible to the NVIS and does not adversely affect performance. The pilot can optimally perform both the required internal visual tasks and external visual tasks.

Until the recent introduction of alternative light sources, flight deck lighting consisted totally of incandescent lamps. A substantial part of emissions from these lamps is in the near-infrared region of the electromagnetic spectrum—the most sensitive portion of the NVIS response. The acceptable blue-green lighting system was achieved by the use of special filters capable of blocking almost all of the red and infrared energy of the incandescent lamp.

The blue-green lighting scheme was a satisfactory solution to the main function of crew station lighting (visibility of dials, switches, and other items); nevertheless, red warning lights and yellow caution lights remain an additional NVIS lighting challenge. To retain the color function of these lights, they cannot be made completely compatible with an NVIS.

If an NVIS is used, the pilot’s eyes usually function in the low photopic-mesopic region of vision. After the NVIS is removed, complete dark adaptation is regained in just three to five minutes. This is because the average light levels associated with NVIS do not completely bleach the eye’s rhodopsin.

Although glass cockpits retain a few dedicated instruments requiring separate lighting, LED light sources or EL light sources increasingly are replacing traditional incandescent lamps.

Newer light sources also are being developed, including the organic (carbon- based) light-emitting diode (OLED), which consists of a series of organic thin films between two conductors. Bright light is emitted when electrical current is applied, in a process called electrophosphorescence.

OLEDs, which could be available on flight decks within three years and could become commonplace within a decade, are self-luminous, require no backlights, and will provide high luminance and low-power displays that are only thousandths of an inch thick. OLEDs also have the potential to be used as flexible displays that can be bent, twisted, or rolled into various shapes. Lighting specialists believe that OLEDs may someday make panoramic flight deck displays a possibility.

The types of lighting used on the flight deck differ according to a number of factors, including the requirements of the human visual system and the purpose of the flight. The color of flight deck lighting, and its intensity, should be chosen to ensure that flight crewmembers are always able to obtain information from instrument panel displays and navigational charts and to perform other visual tasks.

OPTIMIZING NIGHT VISION

Regardless of flight deck design, the following actions can help ensure that lighting is optimized for night flying:

Ensure that all aircraft lighting (both interior and exterior. and including dimming controls) is functioning properly prior to flight;

If a night flight is planned, wear sunglasses during the day (15 percent light transmission is recommended). This will increase the rate of dark adaptation and improve night-vision sensitivity;

Avoid bright lights for 30 minutes immediately before a night flight;

Just before takeoff, after the eyes have adapted to darkness, adjust the instrument lighting level so that all displays are readable. Use the minimum setting required to preserve dark adaptation levels while maintaining the ability to see outside the flight deck;

If a map light or utility light must be used, keep the light as dim as possible and use it for the briefest possible period;

Because dark adaptation is an independent process in each eye, close one eye when being briefly exposed to a bright light (for example, while reading a map). This protects that eye and eliminates the need for re-adaptation; and,

While flying an aircraft near storm clouds, increase the level of instrument lighting to its maximum level in anticipation of lightning flashes.

— Rash & Manning