Principles of fluorescence 1 | 2

When irradiated with short wavelength light, fluorescent substances emit light of a longer wavelength; non-fluorescent objects, such as the background, remain dark. This property, possessed by many different materials, is known as primary or auto-fluorescence. The majority of microscopically interesting specimens, however, do not have this property.
In order to achieve the aim in fluorescence micro-scopy of rendering certain structures visible or of highlighting details for specific analysis, such specimens must first be stained or, in an immunochemical reaction, labelled with a fluorescent dye, known as a fluorochrome.

The light emitted from a substance stained with a fluorochrome is called secondary fluorescence. Important biomedical applications of fluorescence microscopy include the classic immuno-fluorescence technique for detecting infectious diseases or, as examples of recent molecular-genetic developments, fluorescence in situ hybridisation (FISH) or comparative genomic hybridisation (CGH).
The FISH method is used for direct localisation of genes and other DNA/RNA sequences in chromosomes or tissue (e.g. antenatal karyotyping), while CGH involves examining complete genomes for genetic changes, a method of screening which provides valuable information, particularly for tumour pathology, on all un-balanced genetic changes of the examined DNA.