Image
intensifier: the phosphor screen
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The
phosphor screen converts the multiplied electrons from the micro
channel plate back into photons. Several kinds of phosphors are
available that differ in the emitted spectrum and in decay time,
i.e. in fluorescence lifetime, as shown in the according pictures.
Typical conversion factors lie between 20 and 200 photons per electron,
depending on the phosphor and the kinetic energy of the electrons, i.e.
the acceleration voltage. In order to increase
the number of photons emitted in the direction towards the CCD sensor,
the backside of the phosphor is coated with an aluminum layer that
reflects photons towards the proper direction, as shown in the enlarged
detail.
An
optimum phosphor screen will be choosen for your specific application,
basically concerning the conversion factor versus the fluorescence
lifetime. It must be assured anyway that the fluorescence of the
phosphor from the last image has sufficiently died down before the CCD
sensor is read out to avoid loss of light and, even more important,
to avoid crosstalk to the next frame.
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P43
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P46
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Conversion
Efficiency
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200
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95
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photons
electrons
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decay time 100% => 1%
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1500
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0.2
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µs
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decay time 100% => 1%
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3200
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20
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µs
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