= Solution
Use a \b[near-infrared/red ratio], or its normalized form, the <normalized difference vegetation index>:
$$
\boxed{\mathrm{NDVI}=\frac{\rho_{\mathrm{NIR}}-\rho_{\mathrm{red}}}{\rho_{\mathrm{NIR}}+\rho_{\mathrm{red}}}.}
$$
Useful bands are approximately $0.63$–$0.69\,\mu\mathrm m$ in red and $0.75$–$0.90\,\mu\mathrm m$ in near infrared, with exact ranges depending on the sensor. <Chlorophyll> absorbs strongly in red, whereas leaf structure scatters near infrared efficiently; healthy green vegetation therefore has a large near-infrared/red ratio and positive <NDVI>. Soil and water have different spectral contrasts.<Measuring vegetation by NASA Earth Observatory>[Normalized Difference Vegetation Index: visible absorption, near-infrared scattering and the normalized ratio]{ref}
A single value is not a direct census of vegetation biomass or species. Soil background, canopy saturation, season, atmospheric effects and mixed pixels need consideration when mapping the distribution of vegetation.
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