Added 26-6-09

RED or GREEN?

Clearing up Confusion about Coloured Light and Night Vision

         In some circles there seems to be considerable confusion about what colour light should be used if night vision is to be preserved. The current vogue is to advocate green light for everything and some writers imply that the last hundred years or so of using red light was an error. What these advocates are in fact displaying is a lack of fundamental understanding of how the eyes work. As a professional physiologist that teaches medical students about vision, (or at least tries!) I'll set the record straight.

         Vision is the product of two types of cell in the eye.

        The “Cones” are responsible for colour vision but also the perception of fine detail such as reading a map.

        “Rods” only see in black and white but are sensitive to much lower levels of light so are used for night vision.

        The catch is that the Cones need a minimum level of the light to work and the Rods are deactivated by high levels of light.
        If the Rods get bleached it may take as long as 30-45mins for you to recover your night vision.

On the right is a graph showing Dark Adaption.         “The change in sensitivity which occurs as the eye remains in the dark proceeds very rapidly for the first two to three minutes, and then at a slower rate for from five to six minutes. During this first period of adaptation the cones have become dark-adapted. A second period of adaption then begins, at first rapidly then at a decelerated pace. This second phase, which accounts for the major parts of the increase in sensitivity, is attributable to the adaptation of the rods, and is associated with concentration of rhodopsin within those cells.” “Night Vision; The Terminal Visual Thresholds” by William S. Verplanck         “Maximum dark adaptation is reached in 30 to 45 minutes under minimal lighting conditions. If the dark-adapted eye is exposed to a bright light, the sensitivity of that eye is temporarily impaired. The amount of impairment depends on the intensity and duration of the exposure. Exposure to a flare or lightning may seriously impair your night vision. Recovery to dark adaptation could take from 5 to 45 minutes in continued darkness.         Night vision goggles affect dark adaptation. If you dark-adapt before donning the goggles and remove them in a darkened environment, expect to regain full dark adaptation in 2 to 10 minutes.” NIGHT VISION PRINCIPLES FM 21-305/AFMAN 24-306

        The human eye has three types of Cones, and each type is sensitive to a different range of wavelengths. These are know as the Red (R), Green (G) and Blue (B) Cones. Alternately they are termed the L, M and S cells to avoid R being confused with the Rods.

        As a generic group the Cones are most sensitive to green light (c540nm). Using green light lets the Cones see detail at low light levels unlikely to bleach the Rods.

        Rods detect very little light of more than 600nm so Red light (620nm+) lets the Cones work with little stimulation of the Rods.

        Green light is best used when you have some control over the light intensity and can set it as dim as possible. Red light is better for when you cannot control the light intensity.

        In practice, green light is best used for transmitted light sources such as compass needles and instrument displays, while red light is best used for reflected light from sources such as torches or interior lighting.

        Fit a red filter to your torch but be aware that some map features will appear different under red light.

        Another consideration is that green light at night is visible over a far greater distance than red light so anyone misguided enough to use a green light at night is far more likely to be seen and targeted by an enemy.

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