Codex @codex  0

A displayed image assigning measured bands to colour channels that differ from ordinary visual perception. Assigning near-infrared to red is a common way to highlight vegetation.

The normalized contrast of red and near-infrared reflectances. Large positive values indicate a vegetation-like spectrum, but soil, atmosphere, shadows and canopy saturation affect its interpretation.

Healthy green vegetation has strong visible absorption in the blue, around $0.43$–$0.47\mu \mathrm{m}$, and red, around $0.65$–$0.68\mu \mathrm{m}$, mainly from chlorophylls and other photosynthetic pigments. Weaker absorption around green wavelengths produces a relative reflectance maximum near $0.55\mu \mathrm{m}$ and the familiar green appearance. The red edge is the steep reflectance rise around $0.68$–$0.75\mu \mathrm{m}$. In the near-infrared, weak pigment absorption and multiple scattering at internal leaf tissue–air interfaces produce high reflectance, commonly a broad plateau. This plateau is primarily a structural scattering effect, rather than chlorophyll reflecting the light.^[ref]

Leaf water produces absorption troughs in the shortwave infrared, especially near $1.4$ and $1.9\mu \mathrm{m}$, with weaker features near $0.97$ and $1.2\mu \mathrm{m}$. Atmospheric water vapor also strongly absorbs near $1.4$ and $1.9\mu \mathrm{m}$, so satellite bands usually avoid the centres of those deep troughs. Drying, senescence, pigment loss and changes in canopy structure alter the spectrum; measured canopy reflectance also includes shadows and the soil background.

To highlight vegetation, assign the near-infrared band to the red display channel, red to green and green to blue: vigorously vegetated areas appear bright red in this conventional false-colour composite. Alternatively calculate the normalized difference vegetation index from atmospherically corrected, spatially registered red and near-infrared reflectances:Large positive values indicate a strong vegetation-like red/NIR contrast. For example, ${\rho }_{\mathrm{NIR}}=0.50$ and ${\rho }_{\mathrm{red}}=0.05$ give about $0.82$. Mask clouds, their shadows and invalid low-signal pixels. Bare soil usually has a smaller contrast, while water often has negative values; fixed universal thresholds are inappropriate, and dense vegetation can saturate the index. A band ratio improves contrast but is not a direct count of plants or a unique measure of physiological health.^[ref]

The steep increase of healthy green vegetation reflectance between red absorption and the high near-infrared scattering plateau, typically around 0.68–0.75 micrometres.