The reflectance characteristics of earth surface features may be quantified by measuring the portion of incident energy that is reflected. This is measured as a function of wavelength (l) and is called spectral reflectance, rl.
rl = ER (l) /EI (l)
Where ER is reflected energy and EI is incident energy.
A graph of the spectral reflectance of an object as a function of wavelength is termed as spectral reflectance curve.
Spectral Reflectance of Vegetation
The spectral characteristics of vegetation vary with wavelength. A compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflect green wavelength. The internal structure of healthy leaves act as diffuse reflector of near-infrared wavelengths. Measuring and monitoring the infrared reflectance is one way that scientists determine how healthy particular vegetation may be.
Leaves appear greenest to us in summer and become red or yellow with decrease in chlorophyll content in autumn.
Spectral Reflectance of Water
Majority of the radiation incident upon water is not reflected but either is absorbed or transmitted. Longer visible wavelengths and near-infrared radiations are absorbed more by water than the visible wavelengths. Thus water looks blue or blue-green due to stronger reflectance at these shorter wavelengths and darker if viewed at red or near-infrared wavelengths. The factors that affect the variability in reflectance of a water body are depth of water, materials within water and surface roughness of water.
Spectral Reflectance of Soil
The majority of radiation on a surface is either reflected or absorbed and little is transmitted. The characteristics of soil that determine its reflectance properties are its moisture content, texture, structure iron-oxide content. The soil curve shows less peak and valley variations. The presence of moisture in soil decreases its reflectance.
By measuring the energy that is reflected by targets on earth’s surface over a variety of different wavelengths, a spectral signature for that object can be made. And by comparing the response pattern of different features we may be able to distinguish between them.