Light filters They are used to modify the light for ease of observation, for improved photomicroscopy, or to alter contrast. ï A green or yellow-green filter is widely used in black-and-white photography to reduce the effect of lens defects on image quality. Most objectives, particularly the lower cost achromats, require such filtering for best results. ï Polarizing filters are used to produce plane-polarized light (one filter) or crossed-polarized light (two filters rotated to produce extinction) for examination of noncubic (crystallographic) materials. Materials that are optically anisotropic, such as beryllium, zirconium, Î±-titanium, and uranium, can be examined in the crossed-polarized condition without etching. A sensitivetint plate may also be used with crossed-polarized light to enhance coloration.