mrVista WorkFlow

luminance profile of a monitor (b/w values) and to create a correcting (linearizing) gamma-table
spectral (color) profile of a monitor to allow calibrated display of a color gamut

What you need
- Photo Research PR-650 spectrophotometer, usually kept in Brian's wing (room 485)
- Monitor and Macintosh computer system to calibrate
- Matlab scripts in toolbox:exptTools:calibrationTools
Set up the spectrophotometer
- Mount it on a tripod, and plug in the power-supply
- Connect it to the Macintosh using two cables:
  - A black cable with a mil-spec bayonette connector on the photometer end, and a special RS-232 connector on the other end. On the RS-232 end, note the small toggle switch with two settings: ctrl and xfer. It must be set to ctrl to do the calibration.
  - A standard Macintosh modem-port cable with an RS-232 connector on one end and a DIN-9 connector on the other. Connect the photometer cable to the former, and connect the latter to the Macintosh modem serial port.
  - NOTE: On older Macs (before G3), a modem serial port was standard. On newer Macs, you will need to convert the internal modem port (with telephone connector) to a serial port using either a Griffin gPort or GeeThree.com Stealth card.
- Special requirements for scanner suite calibrations
  - The monitor/screen-box cart must be rolled near the door of the suite where it is visible to the photometer
  - Use the same video cables for calibration as used to operate the display for scanning; if the cables must be extended for logistical reasons during calibration, the extensions must remain in place for subsequent scanning operation
  - Use an RS-232 extension cord to allow connection of the photometer to the Macintosh
  - Cover the monitor and photometer with blackout cloth, and turn off as much room lighting as possible
Gamma calibration
- The results will be written to the Desktop as the file 'gamma.mat'.
  - If you want something else, set the variable gammaFile to a full file specification, e.g., 'Ochre3:Desktop Folder:Calibration:myGamma.mat'
- Check and set parameters for the script calibGamma.m:
  - We assume a two-monitor setup by default. For a one-monitor setup, set the variable monitor = 0
  - Make sure the frequency and monitor size are set correctly in the script (check the Monitors & Sound control panel). Default values are 66.67 Hz, 640 x 480 pixels.
  - If you have the time, set the variable testLinearity = 1. This will double the time the calibration takes, but it's a useful validation of the calibration, playing back a presumably linearized set of intensity values and checking the linearity with a second set of photometer measurements.
- Run the script calibGamma.m
- Turn the photometer on when the script prompts you to do so
  - The photometer powers up in remote mode by default
  - If it doesn't establish communication with the Macintosh within a couple seconds, it goes into manual mode and will not communicate further with the computer
  - Thus, a cmeter('Init') command must be executed shortly after the photometer is connected to the Mac and first powered on
  - The script issues the 'Init' command repeatedly until it establishes communication with the photometer
- A white target square will appear on the monitor display; aim and focus the camera on the center of this target
- Press any key to start the calibration
- Basic calibration takes about 30 minutes; linearity test requires another 30 minutes
- When the calibration is finished, copy or move the output file (see above) into the MRI:Displays:whatever directory, where "whatever" is the name of the display that you're calibrating. Usually, it will be the "default" display. Regardless of what you named the file at calibration time, it must be named gamma.mat in the desired Displays: directory
Spectral calibration
- Check and set parameters for calibSpectra.m; default output file is "spectra.mat" on the Desktop
- Run the script calibSpectra.m
- If the photometer is not already on and initialized, turn it off by disconnecting the power
- Turn the photometer on when prompted
- Aim and focus the camera as above if necessary
- Spectral measurement takes about 3 minutes
- Copy or move the output file into the desired MRI:Displays: directory, where it must be named spectra.mat

Calibrating monitors