For example, when chlorophyll is illuminated at 440 nm, i.e. in the blue part of the spectrum, it emits much dimmer light at 665 nm, that is, red light. Many chemical stains are available to show particular cellular features. Acridine orange, for example, stains chromosomes, so that they will fluoresce as green. A fluorescence microscope illuminates the specimen from above. The so-called excitation light passes through an appropriate filter, through the objective.
Fluorescence microscopes are normally very expensive.
Sometimes it is necessary to preserve an image of the specimen. Although the specimen can itself usually be preserved, an image may be needed for publication, reproduction, or forensic purposes. The obvious method is photography. Very specialized microscopes intended solely for photography are available, but although convenient, they are rarely needed. Photography requires simply that the image be focused on the film-plane rather than the retina.