Image Processing Reference
In-Depth Information
Figure1.1 The near-UV, visible, near-IR, and SWIR wavebands.
Figure1.2 Sunlight split into some of its components by a prism.
region of the spectrum. Our eyes are unable to perceive this light, although we can
feel its heat if the source is bright enough.
We cannot see near-IR light because near-IR photons do not have enough energy
to trigger the chemical reactions in the eye that lead to the visual perception of
light. This characteristic of near-IR light underscores an important fact about light:
the longer the wavelength, the lower the energy of its photons. In fact, we cannot
see any light with wavelengths longer than red—that is, a wavelength of about 750
nm—nor can any animal discovered so far. This is because all known animal visual
systems are based on the same light-sensitive chemical, known as rhodopsin. The
chemical reactions necessary to produce a signal in the retina have a threshold
energy that is higher than the energy imparted by light with wavelengths longer
than 750 nm. We can otherwise detect near-IR light as the molecules in our skin
readily absorb it, producing the warm sensation we feel when we bask in sunlight.
The near-IR waveband was the first portion of the electromagnetic spectrum
outside the visible waveband to be discovered, by astronomer Sir William Herschel
(1738-1822) in 1800. He devised an apparatus with a prism and three blackened
thermometers, and measured the transfer of heat from various color components of
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