Service manual

LUMISCAN ACR-2000 SERVICE MANUAL - SECTION 4.0 THEORY OF OPERATION

4-10

In all modes the acquisition process ends with the writing of data into the output register and assertion of

the signal DATAVAIL to the DCB. The DCB then transfers the data into the image memory depending on

the following; the Mode, the state of ISFILM, the state of the Pixel counter and the state of the Line

counter.

Calibration and Table Lookup

The three possible Lookup table functions are: Correction LUT, Calibration LUT and Output LUT.

The Correction LUT is used to correct any deviation of the logarithmic amplifier from a true logarithmic

characteristic. It is applied immediately after the A/D results are read.

The Calibration LUT is used to compensate for variations in the sensitivity of the system with respect to

the horizontal scan. It is generated by scanning a uniformly exposed 14” wide image plate, averaging a

number of lines and normalizing the data. This forms a reference look up table with a value for each pixel in

a line. This data is truncated and scaled the provide the calibration curve for any size image plate scanned at

any allowable number of pixels per line. During acquisition each pixel value from the A/D is added to its

corresponding calibration LUT value. Since this addition is of the log of the signals the effect is the same as

multiplying the pre-log signal by a scaling factor.

The Output LUT is for user use and depends on the application. The default table is a 1:1 Lookup Table.

It can be replaced by an inverse 1:1, a 12-bit to 8-bit mapping, or any other desired function.

Automatic Gain Adjustment

Automatic Gain Control is used to compensate for reduction in sensitivity of the PMT detectors due to age.

Automatic gain adjustment (AGC) is performed whenever a DACQ mode 5 command is received. The

DACQ Mode 5 is a special mode exclusively for the automatic adjustment of the output gain of the PMTs,

as well as automatic adjustment of the log amplifier input offset voltage. The only data generated are the

results of the adjustments. The appropriate enable bits must be set in the DACQ CSR for the adjustments to

take place, or else the adjustments will be set to null or minimum. When the AGC enable bit is set the HV

(2000 counts).

Because there is no signal without the presence of a exposed phosphor image plate a special blue reference

LED is incorporated into the collection cylinder which is under firmware and software control. During setup

the LED is adjusted for a calibrated, constant light output. Then, during the AGC process the blue LED is

turned on and the PMT high voltage is changed so as to produce the desired A/D output. Because the laser

produces no signal the galvanometer need not be scanning.