Bidirectional Reflectance Distribution Function (BRDF)
The Bidirectional Reflectance Distribution Function (BRDF) is a function that describes how a surface reflects light in different directions. The BRDF is defined as the ratio of the reflected radiance in a particular direction to the incident irradiance from that direction, as measured at the surface of the material.
The BRDF of a surface is determined by several factors, including its geometric structure, surface roughness, and the optical properties of the materials comprising the surface. In general, the BRDF is a complex function that can vary over different wavelengths of light and across different viewing and illumination geometries.
One example of the effect of BRDF can be seen when looking at a freshly mowed soccer field. The lawn is mowed in lanes. The blades of grass in one lane are bent in one direction, and in the other lane they bend in the opposite direction. This variation in reflectance values is not related to changes in the underlying surface properties, but rather to the geometry of the surface and the direction of the incident light.
In Earth observation, the BRDF has a significant impact on the accuracy of remote sensing measurements. Because the BRDF of a surface can vary depending on the viewing geometry and illumination conditions, it can cause variations in the measured reflectance values that are not related to changes in the underlying surface properties. This effect is known as the BRDF anisotropy, and it can be particularly significant in regions with highly variable surface cover, such as forests, urban areas, and snow- or ice-covered regions.
To correct for the effects of BRDF anisotropy, several models and algorithms have been developed. These algorithms typically use mathematical models of the BRDF to estimate the surface reflectance values for a given observation geometry. The models are often based on empirical measurements of surface reflectance at different viewing and illumination angles, and they may also consider other factors such as atmospheric effects and sensor calibration.
One commonly used approach for atmospheric correction of Earth observation data is to use a set of precomputed BRDF models, such as those provided by the MODIS BRDF/Albedo product[1]. These models are based on a series of look-up tables that describe the BRDF for different surface types and viewing geometries. During processing, the algorithm retrieves the appropriate look-up table based on the observed surface type and illumination geometry and uses it to correct the measured reflectance values for the effects of BRDF anisotropy.
In summary, the Bidirectional Reflectance Distribution Function (BRDF) is an important factor that affects the accuracy of remote sensing measurements. It describes how a surface reflects light in different directions, and it can cause variations in the measured reflectance values that are not related to changes in the underlying surface properties. To correct for the effects of BRDF anisotropy, a number of models and algorithms have been developed, which typically use mathematical models of the BRDF to estimate the surface reflectance values for a given observation geometry.
