Vegetation Spectral Reflectance Curves
Vegetation has a unique spectral signature, but different types of vegetation differ in their reflectance. Plants that are stressed or diseased can also be identified by their distinct spectral signatures. The leaf pigments, cell structure and water content all impact the spectral reflectance of vegetation. For example deciduous trees have a higher reflectance in the near infrared compared to conifers.
Image Credit: NASA Jeff Carns
In the visible bands the reflectance is relatively low as the majority of light is absorbed by the leaf pigments. Chlorophyll strongly absorbs energy in the blue and red wavelengths and reflects more green wavelengths. This is why healthy vegetation appears green.
For healthy vegetation, the reflectance is much higher in the near infrared (NIR) region than in the visible region due to the cellular structure of the leaves, specifically the spongy mesophyll. Therefore healthy vegetation can be easily identified by the high NIR reflectance and generally low visible reflectance.
The reflectance in the shortwave infrared wavelengths is related to the water content of the vegetation and its structure. Water has strong absorption bands around 1.45, 1.95 and 2.50 µm . Outside these absorption bands in the SWIR region, reflectance of leaves generally increases when water content in the leaf decreases.