Originally Posted by CalWldLif
The APL pattern maintains a constant "brightness" from the 0 pattern to the 100 pattern by using a surround
color that is averaged against the window pattern stimulus.
An Average Picture Level (APL) pattern series would be expected to maintain a constant average for the video information. The video information is generally expected to be adjusted according to gamma in order to produce light at the screen. The average luminance ("brightness") at the screen will not maintain a constant average in an APL pattern series with two on-screen video levels, due to the gamma adjustment. Spears and Munsil chose the term "equal energy" to describe their patterns that intend to maintain a constant average luminance, according to an idealized gamma curve, but I don't think the Spears and Munsil "equal energy" patterns maintain a constant APL. It is possible a pattern series could maintain both a constant average for the video information (APL) and a constant average for the light at the screen (average luminance), but such a pattern would require more than two on-screen video levels. Basically, the average of the video information is constant with an APL series, but the average "brightness" (luminance) at the screen is not necessarily constant, since average luminance can vary while maintaining a constant APL.
Originally Posted by zoyd
The philosophy behind APL patterns is to put the display in an operational state that more closely emulates how it behaves during actual programming, in that sense it has nothing to do with which brand you are calibrating or even which technology the display is.
In order that people don't take the technology part too far, I'm just going to point out that with some display types it's possible to measure the same video level and APL to get two very different luminance (Y) measurement results. For example I could make two images that have the same target video level and APL, but the two images could have a different average relative luminance. Depending on the display in question, those two patterns with different average relative luminance may not measure luminance similarly. Personally I have not measured various OLED and plasma technologies to state how similarly or differently they might measure when varying average relative luminance, but it's certainly possible to return different luminance measurements when varying average relative luminance on some display types.
For plasmas, once you get the ARL above a few percent the transfer function stabilizes and the exact pattern APL makes little difference until ABL kicks in
I'm far more familiar with typical LCD-based display operation than I am with OLED or plasma, but for a practical look at how average relative luminance can potentially affect luminance measurements on plasma it may be worth referring to the measurement example from Chad B. Specifically I would suggest possibly comparing the gamma graph for the "21% APL" against the gamma graph for "Chad B's APL" (approximately 19% APL). While the APL maintained on these two pattern series do not vary drastically, the resulting gamma graphs generally share different trends. I'll suggest a possible reason for the different gamma graphs could be due to average relative luminance. Average relative luminance varies on the "21% APL" series and average relative luminace is constant on the "Chad B's APL" measurement. I suppose that "ABL kicks in" on the "Chad B's APL" based on the "Y Max fL" numbers, yet the general shape of the "Chad B's APL" is somewhat similar to the "AVS S APL" (approximately 7% APL), in spite of the considerable difference in APL. Anyway, my general impression is that various displays do not necessarily react similarly with patterns that vary average relative luminance, and based on the measurements from Chad B I just don't see how plasma is an exception.
ABL effects measured for comparison