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post #1 of 47 Old 09-13-2014, 11:13 AM - Thread Starter
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APL primer

Can someone explain the Average Picture Level (APL) patterns, and how to use the patterns for grayscale/gamma calibration? It's mentioned everywhere here, in bits and pieces and has my head spinning. I have a 55" OLED display.
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post #2 of 47 Old 09-13-2014, 03:28 PM
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add me to the list for an explanation of APL with regard to calibration
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post #3 of 47 Old 09-13-2014, 05:07 PM
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Average picture level is just what its name implies.
averaging the white areas and the dark areas to achieve an average.
normal pictures on a TV is made up of super bright areas and dark areas.
very rarely is the screen 100% white or 100% dark.

Tech like Plasma and OLED have Auto brightness limiter which cause window patterns to not
be brighter than the ABL allows so you can get skewed calibration info as the TV limits the brightness of the pattern.

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.

these APL patterns are rated by percentage of brightness.
so a 22% APL pattern will maintain 22% brightness from the 0 to the 100 stimulus.
this causes the abl circuit to be constant and not change pattern brightness.

some calibrators swear by the APL pattern.

.

Loving D65
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post #4 of 47 Old 09-13-2014, 06:39 PM
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Quote:
Originally Posted by CalWldLif View Post
Average picture level is just what its name implies.
averaging the white areas and the dark areas to achieve an average.
normal pictures on a TV is made up of super bright areas and dark areas.
very rarely is the screen 100% white or 100% dark.

Tech like Plasma and OLED have Auto brightness limiter which cause window patterns to not
be brighter than the ABL allows so you can get skewed calibration info as the TV limits the brightness of the pattern.

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.

these APL patterns are rated by percentage of brightness.
so a 22% APL pattern will maintain 22% brightness from the 0 to the 100 stimulus.
this causes the abl circuit to be constant and not change pattern brightness.

some calibrators swear by the APL pattern.

.
Say i am calibrating a plasma tv should i use a 10% window ( this is the way i have been doing it) or a APL 10% window.
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post #5 of 47 Old 09-13-2014, 07:01 PM
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Quote:
Originally Posted by umass66 View Post
Say i am calibrating a plasma tv should i use a 10% window ( this is the way i have been doing it) or a APL 10% window.

Which plasma?

Samsung 64F8500, Panasonic 65VT50, Oppo 95, Tivo Roamio for OTA, Dish VIP722, Marantz AV8801 preamp, Rotel Amps, Atlantic Tech 8200 speakers, Seaton Submersive HP, Calman 5, Chromapure, Accupel DVG-5000, i1Display3pro, i1pro2, eecolor colorbox.
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post #6 of 47 Old 09-13-2014, 07:05 PM
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Originally Posted by jimp View Post
which plasma?
lg 60pb6900
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post #7 of 47 Old 09-13-2014, 07:21 PM - Thread Starter
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Quote:
Originally Posted by CalWldLif View Post
Average picture level is just what its name implies.
averaging the white areas and the dark areas to achieve an average.
normal pictures on a TV is made up of super bright areas and dark areas.
very rarely is the screen 100% white or 100% dark.

Tech like Plasma and OLED have Auto brightness limiter which cause window patterns to not
be brighter than the ABL allows so you can get skewed calibration info as the TV limits the brightness of the pattern.

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.

these APL patterns are rated by percentage of brightness.
so a 22% APL pattern will maintain 22% brightness from the 0 to the 100 stimulus.
this causes the abl circuit to be constant and not change pattern brightness.

some calibrators swear by the APL pattern.

.



Great explanation -- just what I was looking for and simple and to the point!


Is a 22% APL pattern normal for Plasma displays? How do you determine the level of the brightness pattern for your set, try them all and see when ABL kicks in?


Edit: what I mean is, is there a chance a 50% APL pattern may trigger ABL?

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post #8 of 47 Old 09-13-2014, 07:49 PM - Thread Starter
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Just checked out my Lumagen Mini3D and it offers a Medium 50% APL and Large 50% APL.


Edit: that's Small 50% and Medium 50% APL

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post #9 of 47 Old 09-13-2014, 08:00 PM
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Quote:
Originally Posted by Pres2play View Post
Great explanation -- just what I was looking for and simple and to the point!


Is a 22% APL pattern normal for Plasma displays? How do you determine the level of the brightness pattern for your set, try them all and see when ABL kicks in?


Edit: what I mean is, is there a chance a 50% APL pattern may trigger ABL?
there is no standard. Different manuf and different years of tech seem to have
slightly different "recommendations".
I use widows not APL.

I have seen charts attempting to show ABL across different manufs and models.
no real consistency.

I believe you want some ABL affect to better represent viewing.
I have also seen charts that show little differences between window sizes.

this is the biggest reason why plasmas and now OLEDs have debates on accuracy of calibrations.

the most consistent APL size I have read recommended was 26%.

Sorry there is no specific answer.
Gamma and color saturation are the things hit by patterns sizes/types.

I will say that a large window works well for color work.

Loving D65
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post #10 of 47 Old 09-13-2014, 08:02 PM - Thread Starter
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I also have the Mascior's calibration disc which offers 2%, 4%, 5% and 10% APL Window patterns. I'm playing a few of the patterns right now and wow, very impressive amount of work went into this disc.


I get it now. Thanks CalWldLif!!!
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post #11 of 47 Old 09-14-2014, 04:16 AM
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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. Typical long term average brightness (I prefer the term average relative luminance - ARL) is similar to a "gray world" assumption (if you are familiar with calibrating exposure meters in photography you will recognize that term) of about 15%. Every plasma I have measured exhibited very little ABL with a 15% ARL but it's transfer function was not stable at very low ARL typical of small windows on a black background. The ARL of a 10-step 10% window pattern ranges from 0.006% - 10% for a 2.2 gamma display, and is only 0.2% at the 50% level.

Also to clarify when most people refer to APL they are talking about average input video level (stimulus) and not average relative luminance. For example an APL of 22% means the average video level of each pattern is 22%, this will create a range of average luminance of ~4%-11% during a 10-step grayscale calibration. (see here for example).

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, personally I use 8% area windows with 25% APL for all calibration work.
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Last edited by zoyd; 09-14-2014 at 05:17 AM.
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post #12 of 47 Old 09-14-2014, 02:32 PM
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Quote:
Originally Posted by zoyd View Post
Typical long term average brightness (I prefer the term average relative luminance - ARL)
I like the term ARL, because it describes the 3 facts envolved. Where as the term brightness is subjective and generalised.

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post #13 of 47 Old 09-14-2014, 03:39 PM
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Originally Posted by zoyd View Post
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. Typical long term average brightness (I prefer the term average relative luminance - ARL) is similar to a "gray world" assumption (if you are familiar with calibrating exposure meters in photography you will recognize that term) of about 15%. Every plasma I have measured exhibited very little ABL with a 15% ARL but it's transfer function was not stable at very low ARL typical of small windows on a black background. The ARL of a 10-step 10% window pattern ranges from 0.006% - 10% for a 2.2 gamma display, and is only 0.2% at the 50% level.

Also to clarify when most people refer to APL they are talking about average input video level (stimulus) and not average relative luminance. For example an APL of 22% means the average video level of each pattern is 22%, this will create a range of average luminance of ~4%-11% during a 10-step grayscale calibration. (see here for example).

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, personally I use 8% area windows with 25% APL for all calibration work.
which calibration disc contains 8% windows with 25% APL
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post #14 of 47 Old 09-14-2014, 03:51 PM
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None that I know of, you need a programmable pattern generator. But either the 22% APL or 4% equal energy patterns on Mascior's disk should give similar results.

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post #15 of 47 Old 09-14-2014, 04:04 PM
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Originally Posted by zoyd View Post
None that I know of, you need a programmable pattern generator. But either the 22% APL or 4% equal energy patterns on Mascior's disk should give similar results.
i used the 5% apl window from the mascior disc. do you know what the apl is in percentage for this disc. thanks.
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post #16 of 47 Old 09-14-2014, 05:55 PM
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I don't remember a 5% window with constant APL, the three I remember are labeled:

2% APL windows = 2% area window on variable background to create constant 22% average video level patterns.

4% APL windows = 4% area windows arranged in a equal energy geometry (equal energy = constant average relative luminance). These also have a ~22% average video as well as 13%-15% average relative luminance.

10% APL windows = 10% area window on variable background to create constant 22% average video level patterns. As mentioned above the average relative luminance of these will vary from around 4% to 11% for a 2.2 gamma display.

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post #17 of 47 Old 09-14-2014, 06:31 PM
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zoyd,

Ryan's final version has 2, 5, and 10% variable background APL patterns and a 4% equal energy set.

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post #18 of 47 Old 09-14-2014, 06:34 PM
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Quote:
Originally Posted by zoyd View Post
I don't remember a 5% window with constant APL, the three I remember are labeled:

2% APL windows = 2% area window on variable background to create constant 22% average video level patterns.

4% APL windows = 4% area windows arranged in a equal energy geometry (equal energy = constant average relative luminance). These also have a ~22% average video as well as 13%-15% average relative luminance.

10% APL windows = 10% area window on variable background to create constant 22% average video level patterns. As mentioned above the average relative luminance of these will vary from around 4% to 11% for a 2.2 gamma display.
you are right. i must have used the 6.5% apl windows. will this be equivalent to your recommendation.
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post #19 of 47 Old 09-14-2014, 06:42 PM
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post #20 of 47 Old 09-14-2014, 07:30 PM
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Originally Posted by umass66 View Post
you are right. i must have used the 6.5% apl windows. will this be equivalent to your recommendation.
as Larry pointed out the final version includes 5% windows/22% APL. All of these should give you very close to the same settings when used to calibrate, 10% windows/22% APL would be closest to what I use.

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post #21 of 47 Old 09-14-2014, 08:51 PM
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Originally Posted by CalWldLif View Post
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.

http://www.poynton.com/notes/colour_...FAQ.html#gamma

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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.

Quote:
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
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post #22 of 47 Old 09-14-2014, 10:18 PM - Thread Starter
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Originally Posted by zoyd View Post
I don't remember a 5% window with constant APL, the three I remember are labeled:

2% APL windows = 2% area window on variable background to create constant 22% average video level patterns.

4% APL windows = 4% area windows arranged in a equal energy geometry (equal energy = constant average relative luminance). These also have a ~22% average video as well as 13%-15% average relative luminance.

10% APL windows = 10% area window on variable background to create constant 22% average video level patterns. As mentioned above the average relative luminance of these will vary from around 4% to 11% for a 2.2 gamma display.

I'll try the 10% APL window tomorrow and compare with the 10% Gray Window test patterns. Up to now, I've only used 6.5% Gray windows for all my measurements.


Gamma is set at 2.22 in ChromaPure.
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post #23 of 47 Old 09-14-2014, 10:28 PM - Thread Starter
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Originally Posted by LarryInRI View Post

I have REV_009 which has the same APL patterns, but I like the look of the Advanced disc. Will put on my buy list.
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Originally Posted by alluringreality View Post
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.

So the moment we make a gamma adjustment, the average brightness can change. Is this the reason luminance can vary 4% to 11% with a 2.2 gamma display, as Zoyd pointed out above?
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post #25 of 47 Old 09-15-2014, 04:08 AM
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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.
Generally speaking plasmas have 3 distinct operating ranges based on ARL, < 5%, 5%-20%, > 20% and the transfer function is most stable between 5%-20%. This range also happens to be what typical video material stimulates and I don't think that is a coincidence (at least not the upper level wrt ABL). Within the "stable" range there are variations in gamma response depending on which APL pattern levels you use but they are smaller than the differences with respect to patterns that keep the display at the lower end of the ARL. If you wanted to be super rigorous you would find which settings are needed to calibrate to several different ARL levels and then pick a midpoint for each setting.


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So the moment we make a gamma adjustment, the average brightness can change.
Average luminance changes based on both which video levels are being stimulated and the gamma of the display

Quote:
Is this the reason luminance can vary 4% to 11% with a 2.2 gamma display, as Zoyd pointed out above?
ARL will vary at constant APL if you only use 2 video levels (window and background) in the pattern, to maintain constant ARL you need the multiple video levels of the MCD 4% patterns or AVHSD small/large APL patterns.

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post #26 of 47 Old 09-15-2014, 04:43 AM
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Zoyd,

My head is spinning with this APL issue.
I don't have a signal generator but have access to some calibration disc available on avsforums ie AVSHD, GCD, Masicor.
I want to do gray scale, gamma ( BT1886 ) and color gamut calibration on a LG 60pb6900 ( 2014 LG plasma TV )
Can you make some recommendations based on the above disc based on your experience. thanks.
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post #27 of 47 Old 09-15-2014, 08:29 AM
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Don't get too wrapped up in the available pattern geometries, the end results are really not that sensitive to which ones you use as long as you stay away from anything above about 15% in area for the measurement window to avoid strong ABL. This is especially true for the color gamut calibration and grayscale chromaticity. The main effect of windows vs. APL will be in average gamma and that tends to only be on the +/- 0.1 level. Environmental factors such as room lighting and wall coloring will have much larger effects than which pattern you use to calibrate with.

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post #28 of 47 Old 09-15-2014, 08:44 AM - Thread Starter
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Generally speaking plasmas have 3 distinct operating ranges based on ARL, < 5%, 5%-20%, > 20% and the transfer function is most stable between 5%-20%. This range also happens to be what typical video material stimulates and I don't think that is a coincidence (at least not the upper level wrt ABL). Within the "stable" range there are variations in gamma response depending on which APL pattern levels you use but they are smaller than the differences with respect to patterns that keep the display at the lower end of the ARL. If you wanted to be super rigorous you would find which settings are needed to calibrate to several different ARL levels and then pick a midpoint for each setting.



Average luminance changes based on both which video levels are being stimulated and the gamma of the display



ARL will vary at constant APL if you only use 2 video levels (window and background) in the pattern, to maintain constant ARL you need the multiple video levels of the MCD 4% patterns or AVHSD small/large APL patterns.

I must ask, what is the advantage of maintaining a constant ARL? The gamma response in the thumb nails (of Chad's APL) look awful.
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post #29 of 47 Old 09-15-2014, 10:32 AM
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I must ask, what is the advantage of maintaining a constant ARL? The gamma response in the thumb nails (of Chad's APL) look awful.
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The philosophy behind APL patterns is to put the display in an operational state that more closely emulates how it behaves during actual programming...
While some of the ChadB comparisons of different APL values might make it look like there is a lot of variation, nobody has shown any measurements that would imply favoring say 22% APL patterns over 15% ARL patterns from a colorimetric standpoint. Whereas there is a clear separation in color rendering performance when using small windowed patterns vs. APL type patterns. Please see this post for an example.

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post #30 of 47 Old 09-15-2014, 10:37 AM
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I must ask, what is the advantage of maintaining a constant ARL?
An APL series with a constant ARL generally means that what you see on-screen is what you are measuring. The luminance you measure from each video level is expected to be relative to the other on-screen video levels, since constant ARL likely removes possible luminance variation due to changing ARL. Constant ARL takes some guess work out of what exactly you are measuring and brings luminance measurements more in line with the basic concept of gamma. As described in the prior Gamma FAQ link, gamma is fundamentally intended to relate video levels with light output in the on-screen image. By holding both APL and ARL constant the luminance variation that can happen when APL or ARL change is effectively removed. Instead of possibly measuring luminance differences that result from other factors, the only item that significantly changes in the constant ARL series is the video level, so the constant APL and ARL measurement series effectively describes the on-screen image.

LCD with a fixed backlight is an example of a display technology that typically fits very well with the basic concept of gamma. On an LCD with a fixed backlight it's generally reasonable to expect each video level to correspond to a fixed luminance. Regardless if you alter APL or ARL, the fixed-backlight LCD will typically return a very similar luminance measurement based entirely on the displayed video level. Of course if you take that same LCD and introduce backlight dimming, then APL or ARL changes can cause the same video level to measure a different luminance. Using an APL pattern with a constant ARL on the dimming-backlight LCD generally defeats the dimming so that measurements again represent the on-screen image that the display delivers. While this forum generally tends to muddle what exactly gamma or EOTF represent, the basic function of gamma is to differentiate video levels in the on-screen image, and that's effectively the measurements you get from a constant ARL series of APL patterns.

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The gamma response in the thumb nails (of Chad's APL) look awful.
Since the gamma measurement on the "21% APL" pattern is almost entirely at reference, I would guess the display may have been set according to that reference. If the display had been set according to a different series of patterns, then all of the other measurements would likely be altered to some extent. The patterns that hold a constant ARL generally indicate that the on-screen image likely tends to have a lower gamma near black and a higher gamma near white. In spite of the forum's preference for other measurement patterns, personally I'm not sure of what exactly the other gamma charts really represent for displays that can vary luminance depending on APL or ARL, such as plasma or OLED or dimming-backlight LCD.


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