THEORY GUIDE
10DEC05
Changing Light Energy to an Analog Signal
TG4825-1
Page
The following components of CR systems change the light energy in the exposed SCREEN to
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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.
Radiography Theory