A true confocal scanning microscope focuses the light for illumination into a single spot. In order to generate a two-dimensional image, this spot must be scanned in x and y directions over the sample. Although the scanning machinery requires some efforts in design and manufacturing, the sensor can be a single device, as compared to widefield camera chips, that must provide as many sensor elements in parallel with picture elements (pixels) that are requested. As no xy-readout electronics are needed, single-spot sensors feature better signal to noise ratio as compared to array-detectors.
Photomultiplier Tubes (PMT)
Probably the best-known sensor for confocal imaging so far, is the classical PMT (Fig 01), which started its career more than 80 years ago in the early 1930’s. It is based on the photoelectric effect, first described by H. Hertz and interpreted by A. Einstein. A photo cathode, usually a layer of alkali-atoms that can release electrons readily upon absorption of a photon, is exposed to the source of radiation (here, the light is focused onto the photocathode). Best performance in terms of quantum efficiency in the visible range is achieved with Gallium arsenide phosphide cathodes (