It is often necessary to diffuse light, either through transmission or reflection.
Diffusion can vary with wavelength.
Quartz is required for UV diffusion. These coatings are, however, quite expensive and fragile. Some lamps use collimating lenses or reflectors to redirect light into a beam of parallel rays. If the lamp filament is placed at the focal point of the lens, all rays entering the lens will become parallel. Similarly, a lamp placed in the focal point of a spherical or parabolic mirror will project a parallel beam.