Apparatus for exciting a fluorescent or phosphorescent molecule applied to a specimen in a microscope, the molecule having a known excitation wavelength, comprises a semiconductor light source capable of emitting an output light within a preselected wavelength band correlated with the excitation wavelength of the molecule, an electronic controller coupled to the light source for controlling the intensity of the output light, and an optical system for converting the output light into an excitation beam having a pre-selected distribution of light flux suitable for illuminating the specimen. The semiconductor light source is preferably a light emitting diode or superluminescent diode. The subject invention provides for a fluorescence excitation illuminator having a long lifetime and relatively low cost. Intensity modulation or attenuation can be achieved electronically, without the need for shutters.