BRDF Applications in Semiarid Grassland Monitoring with the AVHRRs

backscatter direction Examples of BRDF distributions in a single azimuthal plane. Examples of BRDF distributions in a single azimuthal plane fwdscatter direction

Examples of BRDF distributions in a single azimuthal plane
A longer arrow implies a higher radiance value.

Note that diffuse irradiance (light scattered indirectly onto the target by the atmosphere) is neglected in this scenario (the resulting distribution of outgoing radiation is often considered to be approximately isotropic).

In the first diagram, the distribution of outgoing radiance is equal over the hemisphere; this is the reflectance expected from a truly Lambertian surface. Very few - if any - natural surfaces exhibit this property.

In the second diagram (the one with arrows of differing lengths), the outgoing radiance is not equally distributed over the hemisphere; the radiance detected by a sensor would depend on the viewing angle of the sensor relative to the surface, as well as the angle of incidence of incoming radiation. This is what happens in practice! Note that when we take multidirectional measurements of radiance, we are really sampling the HRDF - the hemispherical-directional reflectance distribution function - since the illumination includes radiation scattered onto the target indirectly.

Use Back button | Index | Disclaimer