NIGHT VISION
Vision under low light is a different physiological activity than vision under bright light. A chemical change within the cells of the retina processes light energy. During the day and in well-lighted conditions, most light rays are focused on the fovea, which is packed with cone cells along with some rods.
Such cells have immediately available a chemical called iodopsin, which allows the eye to be immediately stimulated with the high-intensity light.
Another chemical (rhodopsin, or visual purple) is found in the rods and needs less light; however, the chemical must be made by these cells at the time light levels are low. This requires time for this chemical to be formed, which means it takes longer (up to 45 minutes) to adapt to low light than to bright light (about 10 seconds); therefore, the retina has the ability to adapt to different light conditions by changing the amount of low-light-sensitive chemicals found in the rods.
Types of vision
Because of the varying resources the eye and retina have to adapt to chang- ing light conditions, our vision is divided into three types, depending on the amount of light available (Fig. 7-8).
Photopic vision
This is daylight with relatively sufficient light to activate the foveal cells (cones), either from sunlight or from artificial illumination. Here color is easily discerned and images are sharp because of the use of the fovea (central vision). Only cones are found in the fovea, but the visual purple chemical already present is bleached out by the light; thus, photopic vision is primarily through use of cones and central vision.
Scotopic vision
This occurs in low-light conditions, either at night or in darkened environ- ments; cone cells are ineffective, resulting in poor resolution of detail. Visual acuity decreases considerably, to as much as 20/200. Color vision is totally lost. Because central vision depends almost solely on cones and not the now ineffectual rod cells, there is another blind spot in the fovea. This is in addi- tion to the other blind spot, always present, and a result of the entrance of the optic nerve into the retina. This is why one must look off center to see specific stars (or air traffic) in the night sky. As a result, peripheral vision is the only real source of visual input in darkened conditions and is often based, in part, on seeing silhouettes against a contrasting background.
Mesopic vision
This is a situation of light availability being between adequate and inad- equate, such as during dawn or dusk or at full-moon levels. As expected, visual acuity and color discrimination diminish as light levels decrease and cone cells become less effective. A form of “night myopia” or 20/40 vision
occurs for ages of 20–30 simply for being in low light. One might think he/
she is seeing clearly because of the gradual change. Awareness of impend- ing light-deficient visual impairment is crucial during these midrange light levels.
Dark adaptation
Dark adaptation is the process by which the retinas increase their sensitivity to decreasing levels of available light. The chemical (rhodopsin, or visual purple) change occurs in the rods as the cones become less effective. The lower the starting level of available light, the more rapidly the adaptation will occur (Fig. 7-9).
114 Vision Figure 7-8
Figure 7-8 Characteristics of night and day vision.
Night vision 115 It takes about 30–45 minutes for this adaptation in minimal light conditions.
The lower the target’s source of light in the dark, the longer it will take to adapt. This also is dependent on ideal conditions, during which there is no exposure to bright light, even for brief periods. This bleaches the changing chemical in the rods, which then has to be readapted. A very bright light, such as a flashlight directly into the eye for even a second or strobe lights on an aircraft, can seriously impair night vision. Recovery could take several minutes.
Prolonged exposure to very bright light for as little as 3–5 hours, such as sun glare off sand, water, or snow, can hinder dark adaptation later on. The more severe the intensity and the longer the exposure, the more diminished initial night vision can be. Some of these impairing effects are cumulative and can per- sist for days after the exposure. During wartime, pilots on alert at night would sit in dark rooms so their eyes would be already dark adapted for their flight.
The ability of the eye to see at night is enhanced by the technique of scanning to overcome the blind spots and poorer acuity. Under scotopic levels, the eye has a central blind spot about two degrees in diameter; thus, the eyes must continually scan to keep the object to be located or examined off this central area. Scanning also prevents the periphery, or rod area, from adapting to an image, which occurs if the eyes are kept stationary for more than a few seconds.
Preserving night vision
The obvious technique to preserve night vision is to avoid bright lights and to stay in a darkened environment for the time it takes for the rod cells to adapt.
There is another method to protect your night vision, and that is by using red light for illumination.
Figure 7-9
Figure 7-9 The eye in night vision.
116 Vision
Rod cells in the retina are least affected by light wavelengths in the red range (longer wavelengths). Red lenses block light at wavelengths of less than 620 millimicrons, effectively keeping the rods in the dark while minimal cone action, sufficient for vision, is still maintained.
This means that a red light source will not bleach the chemical rhodopsin as do other colors of shorter wavelengths. It follows that the use of red light, once vision is adapted to night, would not alter the night-adapted status of the retina and the rods. For this reason, wearing red goggles in certain situ- ations prior to actual flight and in preparation for a trip at night is advisable, as is using red as a light source for acuity (Fig. 7-10). This is also effective when preparing for night work by wearing these goggles in brighter light con- ditions, again prior to flight, allowing the rods to begin their adaptation. Dim white light works the same as red light as far as acuity is concerned.
There is a trade-off, however. Red light refracts less than the other colors of white light (remember that white light is a combination of all colors). Objects are no longer in focus using only red light, and it is more difficult to accom- modate from near to distant vision. Red light also masks any red-colored objects, instruments, or map markings.
As more and more liquid crystal display (LCD) and cathode-ray tube (CRT) displays are used in “glass cockpits” with multicolored and multihued charac- ters and backgrounds, red light (or any light source other than white) becomes counterproductive and should not be used. The other extreme is blue light (shorter wavelengths), to which the retina is least sensitive. The same prin- ciple of color’s effect on acuity is true with sunglasses with designer-type colors (every color of the rainbow), as will be subsequently discussed.
Figure 7-10
Figure 7-10 Luminosity curves for rod and cone vision.
Night vision 117 A good compromise would be to use red light only when trying to enhance the adaptation process, but not during actual flight. Low-level white light on the instruments and within the cockpit is the best, provided the intensity can be controlled. As darkness outside increases and the eyes adapt more to low light, this white-light source should be reduced so as not to interfere with acuity outside the cockpit.
Techniques for improving night vision
In review, there are significant changes in how the eye perceives any level of light as darkness increases. Central vision is compromised because of the dependence on cone cells. Peripheral vision becomes the primary source of visual cues (Fig. 7-11); therefore, off-center vision must be used, which is looking at an object about 10 degrees away from the object. This allows the peripheral vision to be the source of light signals to the brain. The tendency is to look straight at the object, and it takes a conscious effort to look beyond the central-vision habit.
Figure 7-11 As darkness outside increases, off-center vision must be used.
118 Vision
Another challenge is that if an object is visualized for more than 2–3 seconds, the retinal cells can become bleached out, which can result in the object not being seen. Continuous scanning or changing position of the eyes on an object can prevent this occurrence.
Color vision becomes less dependable at night or cloudy low-light conditions.
Colored lights from beacons, runways, and wingtips are discernible if the light source is strong enough to activate the cone cells; however, other colors outside (and often inside) a dimly lit cockpit will appear gray. Contrast with the background will be the primary way of defining one object from another in flight.
Reaction time of visual accommodation, recognition, and action is also impaired (Fig. 7-12). This reaction time is defined as that necessary to first recognize that an object is present. Additional time is necessary for this signal to get to the brain cells dealing with vision, and still more time is necessary for the eye and head to turn toward and focus on the unknown object. The brain must then determine the importance and significance of the object’s presence, and further time is necessary for the body to react, such as by moving the muscles necessary to take corrective action at the controls. This
Figure 7-12
Figure 7-12 Visual reaction time.
Visual scanning 119 whole process can take up to 4–6 seconds, depending on the condition of the environment and the pilot. A keen awareness of this discrepancy is neces- sary in night vision as well as daylight activities to allow for this greater risk of slower reaction time.