Did You Know?

A diffuse reflectance material or coating is one which exhibits Lambertian reflectance. A perfect Lambertian reflector (which doesn’t exist in reality, although Labsphere’s Spectralon comes very close to realising this ideal) is one in which the reflected intensity (i.e. flux per unit solid angle) varies with the cosine of the angle subtended between the normal to the surface and the direction of view. So the intensity is at a maximum in the direction of the normal to the surface and drops to zero at 90°.

The parameter normally used to define the degree to which a material approaches the ideal of Lambertian reflectance is called the Bi-Directional Reflectance Distribution Function (BRDF). BRDF is the ratio of incident irradiance to reflected radiance for a given direction of illumination and defined direction of viewing. BRDF has the units of inverse steradians (sr^-1). The BRDF of a perfect diffuser is 1/π.

Coincidentally, the radiance or luminance (flux per unit solid angle per unit area) reflected from a perfect diffuse reflector is constant with direction of view.

Cosine corrected BRDF (CCBRDF) is the BRDF multipled by the cosine of the view angle to the surface normal. This parameter takes account of the increased projected area as you view a surface from high angles.

We use Spectralon as the BRDF standard to calibrate our Imaging Sphere for Scatter and Appearance testing, the IS-SA. The Imaging Sphere is an innovative tool that provides for full hemispheric scatter measurements in a matter of seconds and at a fraction of the price of traditional scatterometer goniometers.

» For further information on the Imaging Sphere, click here.