An instrument for the observation and study of microscopic specimens that absorb light and emit fluorescence. The ability of substances to emit light when excited by a strong light source is called luminescence, which encompasses the phenomena of phosphorescence and fluorescence. If the secondary emission continues for some time (milliseconds) after the light source has been turned off, it is referred to as phosphorescence. The ability to emit light only very briefly (nanoseconds) after the absorption of light is termed fluorescence. Many specimens obtained from plants, animals, and minerals have the ability to emit a weak to strong fluorescence when excited with appropriate light and without being specifically treated or stained. That phenomenon is called autofluorescence. In most cases, in order to obtain specific and meaningful fluorescence, staining with fluorescing dyes called fluorophores or fluorochromes is necessary. Fluorescence microscopy is a highly sensitive method, since often minute quantities of a fluorophore can be visualized with good microscopic contrast. In appropriate applications, brightly fluorescing images can be observed against a dark background. Individual fluorophores have different absorption and emission spectra and a different quantum efficiency (the ratio between the energy absorbed and the energy emitted), factors that must be considered for optimum fluorescence.