A Photo Multiplier Tube, universally abbreviated to PMT, is a highly sensitive vacuum tube based light detection device that converts incident photons of light into a measurable electrical signal through a process of photoemission and electron multiplication, achieving levels of sensitivity and signal amplification that far exceed those of conventional solid state light sensors such as the CCD and CMOS devices used in digital cameras and flatbed scanners. PMTs are the light sensing technology of choice in the highest quality drum scanners, where their exceptional sensitivity, wide dynamic range, and low noise characteristics enable the capture of image information from both the brightest highlights and the deepest shadows of a film original with a fidelity and tonal completeness that solid state sensor based scanners struggle to match.
The operating principle of a photo multiplier tube exploits the photoelectric effect - the phenomenon, first explained by Albert Einstein in 1905, in which photons of light striking certain materials cause the emission of electrons. In a PMT, incoming photons strike a photosensitive cathode coated with a material - typically a caesium based compound - that has a high quantum efficiency for photoemission, releasing photoelectrons in proportion to the intensity of the incident light. These primary photoelectrons are then accelerated by an electric field towards the first of a series of secondary emitting electrodes known as dynodes, where each arriving electron causes the emission of several secondary electrons through a process of secondary emission. These secondary electrons are in turn accelerated to the next dynode, where each produces further secondary electrons, and so on through a cascade of typically eight to twelve dynode stages, each multiplying the electron count by a factor of several times.
The cumulative effect of this cascade multiplication across multiple dynode stages is an overall electron multiplication factor - known as the gain - of between one hundred thousand and one million times the original photoelectron count, depending on the number of dynode stages and the accelerating voltage applied. This enormous amplification of the original photon signal allows PMT based sensors to detect and accurately measure extremely low light levels that would be lost in the electronic noise of less sensitive detection systems, giving drum scanners equipped with PMT sensors an exceptional ability to extract image information from the densest shadow areas of transparency film and other high density originals.
In a drum scanner, the film original is mounted on a rotating transparent cylinder - the drum - and illuminated by a tightly focused beam of white light or separate red, green, and blue light beams as the drum rotates. The light transmitted through the film is captured by three PMT sensors - one each for the red, green, and blue channels of the image - whose output signals are digitised to produce the high resolution colour image file. The combination of the PMT's wide dynamic range, low noise floor, and high sensitivity allows drum scanners to achieve dynamic ranges typically exceeding 4.0 Dmax - the ability to differentiate between very dark tones whose optical density differs by less than 0.0001 - a level of shadow detail capture that is essential for extracting the full tonal information from dense transparencies and that far exceeds the capability of most flatbed and film scanners using CCD or CMOS sensors.
Despite the pre-eminence of PMT based drum scanners at the highest levels of quality, the complexity, cost, and operator skill required to use drum scanning equipment has meant that flatbed and dedicated film scanners using CCD sensors have dominated the consumer and prosumer scanning market, with drum scanning remaining the province of professional colour separation houses, high end repro studios, and specialist archival digitisation projects where maximum quality justifies the additional investment and expertise required.