[argyllcms] Re: Display measuring at maximum brightness beneficial?
- From: Nikolay Pokhilchenko <nikolay_po@xxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 22 Jun 2011 17:38:56 +0400
22.06.2011, 17:13 от Edmund Ronald wrote:
I think white LED backlight displays are basically switched -on/off- with the
duty cycle determining the brightness.
Yes, in some cases the brightness is regulated by duty cycle. In some cases the
diodes supplied by constant current (through an inductor) to reduce EMI and/or
flickering.
RGB LEDs can do on and off quite fast and in case of duty cycle regulation,
there may be no spectrum changes. It's not the case with "white" LEDs. The
changing of duty cycle or decreasing of DC through diodes can influence on the
spectrum because of "phosphors" in white diodes.
Has anyone looked at spectral drift here? There may be thermal effects involved.
On Wed, Jun 22, 2011 at 2:11 PM, Nikolay Pokhilchenko <nikolay_po@xxxxxxx>
wrote:
22.06.2011, 15:57 Edmund Ronald wrote:
http://photofeedback.blogspot.com/2011/05/calibrating-macbook-pro-display.html
Yes. But Juergen Lilienthere have proposed the spectrum correction for this
case. The spectrum correction of white points may be useful, because the
spectrum of light sources can changing significantly with brightness changing.
For example, while dimming the CCFL.
Please, excuse me for my English.
On Wed, Jun 22, 2011 at 1:46 PM, Nikolay Pokhilchenko <nikolay_po@xxxxxxx>
wrote:
22.06.2011, 14:50 от Juergen Lilien wrote:
Many instruments especially spectrometers seem to have accuracy problems
when measuring at low light levels. Couldn't it be beneficial, if we
would first measure a display at its maximum brightness setting,
subsequently set the brightness to the desired level, remeasure at least
the new white point spectrum (I suppose the white point of the backlight
unit will change when dimmed) and then try to compensate/recalculate the
first measured data based on the new WP/brightness?
I think it's a good idea, nor dumb.
If one have spectrum data, he can just multiply this data by spectrum
difference between white points, calculated after brightness changing. But in
case of spectrum distribution change, this methode will work only for
profiling, not for calibration.
Could this strategy be beneficial at least for measuring the very low
gray/brightness levels?I suppose it could.
Graeme, it's a good idea. We can implement the modes of calibration and display
reading at high brightness.
I propose the next workflow which can be implemented in dispcal and dispread:
1. Calibrating desired white point and desired brightness.
2. Measuring and save desired white point spectrum.
3. Changing the hardware brightness of the display baclit to maximum. There is
the check "Is the sensor of instrument saturated"? needed. If saturated, then
operator should decrease brightness.
4. Measuring and save high brightness spectrum.
5. Calculating spectral "brightness correction coefficient" by devision the low
brightness spectrum by high brightnes spectrums previously saved.
6. Doing the calibration or measurements with applying (multiplying by) the
correction coefficient.
That way the program can predict the spectrum of patch on screen at desired
brightness, while the measurements have done at maximum brightness. I suppose
this way can increase the tolerance of measurements and resulting calibration
and profile.
One reason for error I can see, is the display matrix transparency drift
with temperature. At higher brightness the temperature of matrix will be
higher. And if there is matrix temperature drift of color, there may be an
error. But this error may be less than instrument error at low brightness.
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