BRDF Explained

BRDF Explained

The BRDF is the "Bidirectional Reflectance Distribution Function." It gives the reflectance of a target as a function of illumination geometry and viewing geometry. The BRDF depends on wavelength and is determined by the structural and optical properties of the surface, such as shadow-casting, multiple scattering, mutual shadowing, transmission, reflection, absorption, and emission by surface elements, facet orientation distribution, and facet density.

Sound technical? Well, it is: the BRDF is needed in remote sensing for the correction of view and illumination angle effects (for example in image standardization and mosaicking), for deriving albedo, for land cover classification, for cloud detection, and for atmospheric correction and other applications. It gives the lower radiometric boundary condition for any radiative transfer problem in the atmosphere and is hence of relevance for climate modeling and energy budget investigations.

However, it should not be overlooked that the BRDF simply describes what we all observe every day: that objects look differently when viewed from different angles, and when illuminated from different directions. For that reasons painters and photographers have for centuries explored the appearance of trees and urban areas under a variety of conditions, accumulating knowledge about "how things look," knowledge that today we'd call BRDF-related knowledge. As modern painters, programmers of virtual reality in computers also need to be concerned about the BRDFs of the surfaces they use.

Below are a few examples. The images are taken from Don Deering's Parabola (a BRDF instrument) web page at the Goddard Space Flight Center. He and his team have over the years made wonderful ground-based BRDF photographs and measurements for various land cover types.

Graphic showing Bidirectional Reflectance Distribution Functions

Black spruce forest Left: backscattering (sun behind observer), note the bright region (hotspot) where all shadows are hidden. Right: forward scattering (sun opposite observer)
This is a black spruce forest in the BOREAS experimental region in Canada.
Left: backscattering (sun behind observer); note the bright region (hotspot) where all shadows are hidden.
Right: forward scattering (sun opposite observer); note the shadowed centers of trees and transmission of light through the edges of the canopies.
Photograph by Don Deering

Soybean field Left: backscattering (sun behind observer), note the bright region (hotspot) where all shadows are hidden. Right: forward scattering (sun opposite observer)

A soybean field
Left: backscattering (sun behind observer)
Right: forward scattering (sun opposite observer); note the specular reflection of the leaves.
Photograph by Don Deering

Side-by-side images of a barren field with rough surface: On the left shows backscattering that happens when the sun is behind the observer and on the right is the image with the sun opposite the observer (forward scattering.)

A barren field with rough surface
Left: backscattering (sun behind observer); note the bright region (hotspot) where all shadows are hidden.
Right: forward scattering (sun opposite observer); note the specular reflection.
Photograph by Don Deering

Professor Crystal Schaaf’s Lab

School for the Environment
University of Massachusetts Boston
100 Morrissey Blvd.
Boston, MA 02125