In optics, a diffuser is any device that diffuses or spreads out or scatters light in some manner, to give soft light. Diffuse light can be easily obtained by making light to reflect diffusely from a white surface, while more compact optical diffusers may use translucent objects, and can include ground glass diffusers, teflon diffusers, holographic diffusers, opal glass diffusers, and greyed glass diffusers.
Perfect reflecting diffuser
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 characterised by scatterometers 
Diffusers in photography
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 camera lens to soften the image of 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 a Diffraction orders 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,
- Fairchild, Mark D. Color Appearance Models. John Wiley & Sons. p. 65. ISBN 0-470-01216-1.
- Walecki, Wojciech, and Peter Walecki. "Robust diffuser and roughness metrology tool for LED manufacturing." SPIE OPTO. International Society for Optics and Photonics, 2015.