Metamerism: Objects that appear to be color matched, under one type of light may appear different when viewed under another source of light. This phenomenon is called as metamerism. if possible the color matching, therefore, should be done two different sources of light, one of them preferably should be daylight. Phenomenon in which porcelain appears different under varying light condition is metamerism Fluorescence: Natural tooth sometimes absorbsthe light of a shorter wavelength in addition to the wavelengths of the normal visible light. The energy the tooth absorbs by ultraviolet radiation is converted into the visible light of larger wavelengths. In such cases, the tooth itself becomes a source of light and this phenomenon is called fluorescence. Fluorescence contributes to the brightness and a vital appearance of the tooth.