An image of the Amazon acquired from NASA's MESSENGER satellite shows how substituting an infrared sensor for a blue one renders a crisper image of plants and trees from space and offers valuable information about plant health.
In the accompanying image, MESSENGER substitutes infrared light for blue light in its three-band combination. The resulting image is crisper than the natural color version, because our atmosphere scatters blue light. Infrared light, however, passes through the atmosphere with relatively little scattering and allows a clearer view.
That wavelength substitution makes plants appear red, because vegetation reflects near-infrared light more strongly than either red or green. In this band combination, near-infrared is assigned to look red.
Apart from getting a clearer image, the substitution reveals more information than natural color. Healthy plants reflect more near-infrared light than stressed plants, so bright red indicates dense, growing foliage. For this reason, biologists and ecologists occasionally use infrared cameras to photograph forests.
Image courtesy of NASA.