In optics, a diffuser (also called a light diffuser or optical diffuser) is any material that diffuses or scatters light in some manner to transmit soft light. Diffuse light can be easily obtained by reflecting light from a white surface, while more compact diffusers may use translucent material, including ground glass, Teflon, holographs, opal glass, and grey glass.
A perfect (reflecting) diffuser (PRD) is a theoretical perfectly white surface with Lambertian reflectance (its brightness appears the same from any angle of view). It does not absorb light, giving back 100% of the light it receives. Reflective diffusers can be easily characterized by scatterometers.
A flash diffuser spreads the light from the flash of a camera. In effect, the light will not come from one concentrated source (like a spotlight), but rather will spread out, bounce from reflective ceilings and walls, thus getting rid of harsh light, and hard shadows. This is particularly useful for portrait photographers since harsh light and hard shadows are usually not considered flattering in a portrait.
A diffusion filter (sometimes called a "shoot-through" diffuser) is used in front of a flash or studio light to soften the light on the scene being shot.
Recently, photopolymers have been used for making holographic diffusers. Photopolymers offer better performance than other materials and have a large viewing angle. Also, the process of synthesizing photopolymers is much simpler.
A diffractive diffuser is a kind of diffractive optical element (DOE) that exploits the principles of diffraction and refraction. It uses diffraction to manipulate monochromatic light, giving it a specific spatial configuration and intensity profile. Diffractive diffusers are commonly used in commercially available LED illumination systems. Usually, the diffuser material is GaN or fused silica with processed rough surfaces. LED diffusers can be characterized online using scatterometry-based metrology.