Reflectance is based on the physical principle of reflection. Basically reflection is the change in the direction of a wave at an interface between two different media so that the wave returns into the medium from which is originated. In the particular case of the light, depending on the nature of the surface, the reflection can be specular or diffuse.
In the case of specular reflection the first Snell´s law says: the angle that the incident beam makes with the normal to the surface is equal to the angle of the reflected beam, also measured from the normal. This type of reflection occurs on surfaces that are similar to mirrors. The diffuse reflection is more complex because when the beam is incident on the surface there are multiple beams reflected in various directions, due to the irregularities in the surface.
Reflectance measures the fraction of incident electromagnetic power that is reflected from a surface.
Typical applications for reflectance measurements are color measurements and materials (surface) characterization. Color measurement involves determining the reflection spectrum of a sample and applying it to a standard illuminant.
For reflectance measurements Sarspec offers the Reflectance SPEC Pack Vis/NIR.