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| Principles of Night-vision Imaging |
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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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