From Light Energy To Analog Signal - Kodak DirectView CR 800 System Theory Manual

From Light Energy to Analog Signal

Illustration showing the relationship of the collector to the
emitted light. another part of the ill. Will also show the filter
and detector and the emergence of the analog PMT current signal as i
2–14
Collector
Blue light is emitted from the phosphor when it is stimulated by the laser beam.
The emitted light goes in all directions. Therefore, a light collector is optimally
positioned to collect as much of the emitted blue light as possible.
Different collector systems are available which use integrating cavities, light
pipes, fiber optic bundles, etc. The CR 800 uses an integrating cavity with
mirrors.
Filter
A blue filter filters out any red light reflected off the screen, allowing only the blue
light to reach the collector.
Detector
The light entering the collector is reflected onto a light detector. Since the emitted
light intensity is very low, the detector must have a good conversion efficiency,
that is, one with high gain and low noise. Typically photomultiplier tubes (PMT)
are used.
The PMTs convert the light photons into electrons as they enter the PMT through
a photocathode. As the electrons are moved through the chambers of the PMT,
they increase in number (gain).
Summing and Filtering the Signal
If more than one PMT is used, the signals are summed and converted into one
output.
The output from the PMTs is an analog signal that may contain frequencies that
are out of the range of the system. In this case an analog filter is used to limit the
out-of-range signals (noise).
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