What is relative diffuse reflectance?

Relative diffuse reflectance is measured by using an integrating sphere to measure the diffuse reflected light with specular reflected light excluded, as shown in the top diagram. This is used to measure samples with a rough surface, such as paper. The picture at bottom left shows an integrating sphere attachment used to measure relative diffuse reflectance. When using an integrating sphere, a material such as barium sulfate or specialized fluorine-based polymer is used as the reference plate (sometimes referred to as a white reference plate).