<P> Most fluorescence microscopes in use are epifluorescence microscopes, where excitation of the fluorophore and detection of the fluorescence are done through the same light path (i.e. through the objective). These microscopes are widely used in biology and are the basis for more advanced microscope designs, such as the confocal microscope and the total internal reflection fluorescence microscope (TIRF). </P> <P> The majority of fluorescence microscopes, especially those used in the life sciences, are of the epifluorescence design shown in the diagram . Light of the excitation wavelength is focused on the specimen through the objective lens . The fluorescence emitted by the specimen is focused to the detector by the same objective that is used for the excitation which for greater resolution will need objective lens with higher numerical aperture . Since most of the excitation light is transmitted through the specimen, only reflected excitatory light reaches the objective together with the emitted light and the epifluorescence method therefore gives a high signal - to - noise ratio . The dichroic beamsplitter acts as a wavelength specific filter, transmitting fluoresced light through to the eyepiece or detector, but reflecting any remaining excitation light back towards the source . </P> <P> Fluorescence microscopy requires intense, near - monochromatic, illumination which some widespread light sources, like halogen lamps cannot provide . Four main types of light source are used, including xenon arc lamps or mercury - vapor lamps with an excitation filter, lasers, supercontinuum sources, and high - power LEDs . Lasers are most widely used for more complex fluorescence microscopy techniques like confocal microscopy and total internal reflection fluorescence microscopy while xenon lamps, and mercury lamps, and LEDs with a dichroic excitation filter are commonly used for widefield epifluorescence microscopes . By placing two microlens arrays into the illumination path of a widefield epifluorescence microscope, highly uniform illumination with a coefficient of variation of 1 - 2% can be achieved . </P> <P> In order for a sample to be suitable for fluorescence microscopy it must be fluorescent . There are several methods of creating a fluorescent sample; the main techniques are labelling with fluorescent stains or, in the case of biological samples, expression of a fluorescent protein . Alternatively the intrinsic fluorescence of a sample (i.e., autofluorescence) can be used . In the life sciences fluorescence microscopy is a powerful tool which allows the specific and sensitive staining of a specimen in order to detect the distribution of proteins or other molecules of interest . As a result, there is a diverse range of techniques for fluorescent staining of biological samples . </P>

What kind of light source is on a microscope
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