Output phosphor

Last revised by Raymond Chieng on 28 Feb 2023

The output phosphor is a component of the image intensifier in fluoroscopic systems that converts the energy from the electrons into light photons. In an II, the large number of light photons produced are subsequently captured by various imaging devices to produce a visible image.


Output phosphor is composed of a fluorescent compound (known as P20) made of silver-activated zinc cadmium sulphide (ZnCdS:Ag) particles 2. The output phosphor layer is very thin (4-8 μm) and plated directly on the glass output window of the image intensifier:

  • electrons that strike the output phosphor result in the emission of a large number of light photons (~2000 luminescent photons are generated for every 25keV accelerated electron)

  • the emission spectrum of these light photons is around 530nm (green light) – this matches well the sensitivity of most orthochromatic film and video targets

  • the luminescence decay time of the output phosphor determines the temporal resolution of the image intensifier

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