Photographic filters are coloured or textured discs of glass, plastic or gelatin placed in front of the lens in order to modify the colour or quality of light passing through it onto the film plane, thereby altering or enhancing the recorded image. Filters for architectural work fall into four main divisions: the ultraviolet (UV) and the polarizer types; special effects filters; colour correction and compensating filters; and filters for black-and-white photography. UV filters can be simply and cheaply replaced should any damage occur to them, while a scratch to a front lens element is likely to be costly and timeconsuming to repair. The polarizing filter, dark grey in appearance with an exposure factor of around four, can reduce or eliminate polarized light from the subject. The use of most special effects filters should be severely restricted for serious architectural work as their effects tend to be gimmicky and artificial, and lacking in subtlety.