jackm_ustc@... asked:
> So how many measurements do you take for each luminance level of a
> CRT monitor?
Take a look at CalibrateMonSpd. cal.describe.nAverage is set to 2, so
apparently the Brainard lab still finds two measures per device per
setting to be good enough.
Rather than worrying about that, though (because over short time
frames, monitors are fairly stable), you might want to be a bit more
concerned about making sure that you don't run into problems between
the geometries and intensities of your stimuli. As I mentioned here
before and wrote up at:
http://www.lifesci.ucsb.edu/~mrowe/MonitorIssues.html
the behavior of CRTs is non-linear when you try to drive them at high
intensities over a large area of the monitor. Since I created that
web page I've found that this has been mentioned in the vision
literature:
In addition to the well-known gamma-non-linearity, saturation
effects can occur at high intensity due to limitations on the
current output of the high-tension power supply. The saturation
effect is more pronounced for large stimuli than for small ones
and varies widely between different models of CRT, since beam
current can often be sustained for small but not large
stimuli. The beam current limitation raises the general
principle that no single monitor calibration can be adequate for
all possible stimuli and the calibration should use stimuli
similar in average luminance to those that will be used in
practice.
That's from:
Colombo, E., and Derrington, A. (2001). "Visual Calibration of CRT
Monitors", _Displays_, 22:87-95.
I know that you already have a copy of this paper, but I just want to
make sure you understand the significance of that paragraph (and to
draw it to others' attention as well).
I've been replicating an earlier study for one of my projects, and I
used the luminance and chromaticity numbers reported in the original
experiment to create my stimuli. My subjects have universally found
the task easier than did subjects in the original report, and my best
guess as to why is that the calibration of the monitor in the original
report wasn't done properly. So the luminances presented to subjects
were significantly lower than the luminances measured during
calibration. Senior author insists the calibration was done properly,
but I have not been able to track down the person who did the actual
measurements, and the numbers as reported seem unreasonably large.
The only way I was able to achieve the reported luminances was to make
the stimuli smaller (and move subjects closer to the monitor to
maintain the same visual angles).
--
Mickey P. Rowe (
mrowe@...)