J.S. and S.W. Aber
|Spectrum of ultraviolet (UV), visible (VIS), and near-infrared (IR) radiation. Primary visible colors are blue, green, and red. Wavelengths given in nanometers (nm). Image adapted from Wikimedia Commons.|
Note: this discussion deals only with near-infrared. Thermal (heat) infrared is much longer wavelengths and requires completely different imaging techniques.
Traditional color-infrared (CIR) photography is based on green, red, and NIR layers in the film emusion, and blue/UV light is excluded by use of a yellow filter. As developed into a visible picture, green is shifted to blue, red becomes green, and NIR appears red. This shifting of spectral bands is known as false-color imagery.
|Analog (film) photographs of the Emporia State University campus, Kansas. Color-visible (A) and color-infrared (B) show dormitory buildings, parking lots, trees, and part of the football stadium. Active vegetation appears in pink, red, and maroon colors in the CIR image. Images acquired with a dual-camera rig.|
|CIR view over the Nature Conservancy marsh at Cheyenne Bottoms, Kansas. Black dots are cattle grazing on wet meadow. Ektachrome EIR 35-mm film taken with an SLR camera and yellow filter. Our last analog color-infrared KAP, July 2004.|
|Overview of the Rowley River (right) and its tributaries in the saltwater marsh complex, Plum Island Ecosystems Long Term Ecological Research site, Massachusetts (August 2009). Active vegetation appears in pink, red, and maroon colors, and water is black. Note the strongly curved horizon and slightly fuzzy appearance of the image.|
Starting in 2008, we tested the Tetracam ADC at several sites in Kansas, Pennsylvania, and Massachusetts. Because of its relatively high cost we flew it only under ideal conditions, but the image quality was disappointing, so we quit using this camera the following year.
|Color-visible (left) and color-infrared (right) views of a pond surrounded by trees and grass, Peter Pan Park, Emporia, Kansas. Active vegetation appears in shades of orange, and water is dark blue. Deciduous trees and grass are bright orange, and conifers are dark orange.|
|Spectrum of near-UV, visible (B/G/R), and near-IR sunlight reflected from common objects. Note sugar beet, grass, and tree curves showing weak green reflection, blue and red absorption, and strong NIR reflection. Dead vegetation or fallow fields, however, do not have this spectral signature. Adapted from Short (1982, fig. 3-5B).|
|Type of imagery||False color||Vegetation||Clear water|
|G = blue|
R = green
NIR = red
= near black
|B = blue|
G = green
NIR = red
= dark blue
Color-infrared photos tend to enhance special lighting effects, for example sun glint and the hot spot. Sun glint is a mirror-like reflection from water bodies, metal and glass structures, etc. The hot spot is the position on the ground at the antisolar point in direct alignment with the camera and sun. It appears bright because of shadow hiding (Hapke et al. 1996).
|Sun glint (left) from a lake surface shows wave pattern clearly. A hot spot (right) appears left of scene center in prairie vegetation. Hot spots are most noticeable over relatively homogeneous ground cover. Both photos taken with the modified Sony camera.|
|In oblique views looking toward the sun, bright pink-white streaks may appear in some images taken with the modified Sony camera. This is presumably due to internal lens reflections.|
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Last update: February 2019.