other wavelengths. A very few materials are non-linear, and the transmittance depends on the intensity and the combination of wavelengths of the incident light. Transparent fluorescent materials can work as an optical filter, with an absorption spectrum, and also as a light source, with an emission spectrum. Also in general, light which is not transmitted is absorbed; for intense light, that can cause significant heating of the filter. However, the optical term absorbance refers to the attenuation of the incident light, regardless of the mechanism by which it is attenuated. Some filters, like mirrors, interference filters, or metal meshes, reflect or scatter much of the non-transmitted light. The (dimensionless) Optical Density of a filter at a particular wavelength of light is defined as where is the (dimensionless) transmittance of the filter at that wavelength. Absorptive Optical filtering was first done with liquid-filled, glass-walled cells; they are still used for special purposes. The widest range of color-selection is now available as colored-film filters, originally made from animal gelatin but now usually a thermoplastic such as acetate, acrylic, polycarbonate, or polyester depending upon the application. They were standardized for photographic use by Wratten in the early 20th century, and also by color gel manufacturers for theater use. There are now many absorptive filters made from glass to which various inorganic or organic compounds have been added. Colored glass optical filters, although harder to make to precise transmittance specifications, are more durable and stable once manufactured. Dichroic filter Alternately, dichroic filters (also called "reflective" or "thin film" or "interference" filters) can be made by coating a glass substrate with a series of optical coatings. Dichroic filters usually reflect the unwanted portion of the light and transmit the remainder. Dichroic filters use the principle of interference. Their layers form a sequential series of reflective cavities that resonate with the desired wavelengths. Other wavelengths destructively cancel or reflect as the peaks and troughs of the waves overlap. Dichroic filters are particularly suited for precise scientific work, since their exact colour range can be controlled by the thickness and sequence of the coatings. They are usually much more expensive and delicate than absorption filters. They can be used in devices such as the dichroic prism of a camera to separate a beam of light into different coloured components. The basic scientific instrument of this type is a Fabry–Pérot interferometer. It uses two mirrors to establish a