In the electromagnetic spectrum, our eyes only respond to wavelengths between 400 and 700 nanometers (nm), which we normally experience as colors (figure 1). The less light, however, the less we distinguish color and detail. On a dark night, we lose color perception entirely, and objects become shadowy, somewhat brighter or dimmer shapes
Figure 1. The electromagnetic spectrum, with a focus on the ultraviolet (250-400 nm), visible (400-700 nm), and infrared bands (700-1,500 nm).
There are two ways of improving night vision. The first is to increase the amount of light reaching the eye, as with a telescope or flashlight. The second is through light amplification, by creating a visible, phosphor-screen image from normally imperceptible radiation, as in a night vision imaging system (NVIS) or forward looking infrared (FLIR) camera.
An NVIS usually looks like a pair of goggles or binoculars, is worn by the viewer, and responds to visible and near-infrared wavelengths up to 930 nm. FLIR is built into aircraft, with sensors below the nose cone and a display screen in the crewstation. It responds to long wavelengths in the 800-1600 nm infrared range, forming a visual representation of heat. Compared to NVIS, it has two main disadvantages; it doesn't track well with the pilot's head movements, and it is not real-time imaging. This brochure deals only with NVIS and NVIS-compatible lighting.
Besides perceiving only one part of the spectrum, our eyes respond to some wavelengths more intensely than others. For example, we see green light as much brighter than the same amount of red light. In daylight, our visual response describes a bell curve-known as the photopic or standard observer curve - that peaks in the green area, at 555 nm (figure 2). When our eyes adapt to the dark, their response curve shifts towards blue, and is known as the scotopic curve (figure 2).
Figure 2. Human visual response at night (scotopic) and in the day (photopic).
Just like the eye, every type of NVIS has a response curve describing its sensitivity to different wavelengths. Early forms of NVIS (Gen 2 and earlier) were more sensitive to visible light than to IR. More recently, Gen 3 was developed with greater response to IR, in both relative and absolute terms.
The response or sensitivity of NVIS to IR makes it far more useful for night vision than a device that only amplifies visible light. At night there is much more available IR than visible light (figure 3). Also, many materials - foliage, concrete, or stone, for example - reflect IR more efficiently than they do visible light. Foliage, in particular is such a high reflector of red and IR that is our visual sensitivity for red were higher, trees would appear red, not green (figure 4).
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