THEORY GUIDE Radiography Theory
10DEC05
TG4825-1
Page
20 of 120
Changing Light Energy to an Analog Signal
The following components of CR systems change the light energy in the exposed SCREEN to
an analog signal:
• LIGHT COLLECTOR:
– provides the collection of the blue light emitted when the SCREEN is stimulated by the
laser beam
– CR 825/850 SYSTEM uses an INTEGRATING CAVITY with MIRRORS to provide the
collection of the blue light
• BLUE FILTER:
– does not allow any red light reflected from the SCREEN to reach the LIGHT
DETECTORS
– allows only the blue light to reach the LIGHT DETECTORS
• LIGHT DETECTORS:
– are normally PHOTOMULTIPLIER TUBES (PMT)
– receive light that enters the COLLECTOR
– change the light photons into electrons when the photons enter through a
PHOTOCATHODE. When the electrons move through the LIGHT DETECTORS, the
electrons increase in number - “gain”.
– when more than one LIGHT DETECTOR is used in a system, the system adds and
changes the signals into one output. The output from the added PMTs can include
frequencies that are outside of the limits of the system - “noise”. An ANALOG FILTER
limits this noise.
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