Question about the link between gamma and 100% white reading (peak light output) - AVS Forum
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post #1 of 16 Old 02-02-2013, 10:40 AM - Thread Starter
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Let me start by mentioning my viewing environment is typically dim the whole day (curtains closed and bias light on... though curtains are only moderately effective at blocking daylight) and sometimes at night when watching a movie with a lot of dark scenes in it, I turn off the bias light behind the TV and the room is completely dark. CalMAN 5 recommends a peak light output (100% white reading) of anywhere from 30 fL to 60 fL for a direct view display like my Samsung EH series LED-LCD and a gamma range from 2.0 to 2.4 based on viewing conditions (ambient light, including a bias light). I have been experimenting with a high light output like 60 fL with a dark gamma like 2.4 to get both high contrast in dark scenes and overall while still keeping the picture bright enough to see shadow detail properly and to also keep the midtones from looking too dim.





My question whether you can pair a high light output with a high point gamma or if there are some good reasons why you shouldn't. So far, I like this combo better than one that employs 2.2 gamma and a light output near 40-50 fL for a dim room OR 2.4 gamma and 30-40 fL for a dark room (both of which I have also tried before). Also, the TV has a native on/off CR of 3,000:1, if that matters.


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post #2 of 16 Old 02-02-2013, 01:43 PM
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I think you're on to something there. I like to use that technique with many LED-LCD calibrations. The higher gamma means average picture level is less bright than one might expect for a given peak light output. Increasing peak light output beyond the typical recommended readings for dim rooms will then bring that average picture level brightness back up, with highlights having more punch. biggrin.gif
IMO plasmas don't look realistic with that high of a gamma, but LED-LCDs often do.

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post #3 of 16 Old 02-02-2013, 08:39 PM - Thread Starter
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Originally Posted by Chad B View Post

I think you're on to something there. I like to use that technique with many LED-LCD calibrations. The higher gamma means average picture level is less bright than one might expect for a given peak light output. Increasing peak light output beyond the typical recommended readings for dim rooms will then bring that average picture level brightness back up, with highlights having more punch. biggrin.gif
IMO plasmas don't look realistic with that high of a gamma, but LED-LCDs often do.

thanks, good to know

regarding the bit about plasmas, is the difference due to ABL and the effect it has on gamma readings?


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post #4 of 16 Old 02-03-2013, 03:03 AM
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i always did follow the peak white output @ 40 ftl during calibration but always miss the punch in the average midtones picture during waching a movie or any other content with backlight set at 65 and contrast at 94

Lately because i calibrate from SM the backlight during this calibration is always 100% so that keeps me busy thinking that maybe that is my correct setting for backlight during fine tuning the SM in any other user menu picture mode (thx,movie or expert).

setting the backlight to 100 and reduce the eyestrain with contrast setting @70-80 give me better PQ with my LG edge led 47LW4500.

Calibration is done with spectro i1pro and dtp94 and ChromapurePro

Pro's have any thoughts on this backlight setting?

thanks

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post #5 of 16 Old 02-03-2013, 11:25 AM
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You will loose some contrast ratio with the contrast in that range.

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post #6 of 16 Old 02-03-2013, 12:22 PM
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Originally Posted by Chad B View Post

You will loose some contrast ratio with the contrast in that range.

thank for replying, still it is difficult to find the right setting for backlight versus contrast although there are better settings flying around for setting it at 65 in bright room viewing a LED screen.

So far i like the 100% backlight but i could set it to 90% and bump up the contrast a bit to 85% to see if that give me better/same PQ and higher contrast ratio.

i will try that, thanks

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post #7 of 16 Old 02-03-2013, 12:33 PM - Thread Starter
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Originally Posted by Killerbeenl View Post

thank for replying, still it is difficult to find the right setting for backlight versus contrast although there are better settings flying around for setting it at 65 in bright room viewing a LED screen.

So far i like the 100% backlight but i could set it to 90% and bump up the contrast a bit to 85% to see if that give me better/same PQ and higher contrast ratio.

i will try that, thanks

It's always better to set contrast to the highest setting that produces no clipping or discoloration at/near reference white (digital 235 is what I target, not 255 or some point in between). Then, adjust backlight to get the desired amount of light output. This maximizes dynamic range/contrast ratio and minimizes black level for a given light output.
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post #8 of 16 Old 02-04-2013, 03:58 AM
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Quote:
Originally Posted by PlasmaPZ80U View Post

Quote:
Originally Posted by Killerbeenl View Post

thank for replying, still it is difficult to find the right setting for backlight versus contrast although there are better settings flying around for setting it at 65 in bright room viewing a LED screen.

So far i like the 100% backlight but i could set it to 90% and bump up the contrast a bit to 85% to see if that give me better/same PQ and higher contrast ratio.

i will try that, thanks

It's always better to set contrast to the highest setting that produces no clipping or discoloration at/near reference white (digital 235 is what I target, not 255 or some point in between). Then, adjust backlight to get the desired amount of light output. This maximizes dynamic range/contrast ratio and minimizes black level for a given light output.

Hi PlasmaPZ80U,

the clipping of white starts @ 94% contrast and higher so 90 % could be a safe setting with my LG.

the problem is how it looks with contrast set @ 90-94 and backlight @ 65 which set it at a peak white output 45 ftl but it looks better with backlight @ 90-100 and contrast @ 80-85 watching film content.

peak white output is somewhere @ 55 with this setting.

Comming from a Philips CRT tv i see the LG edge led behaves totaly different during calibration because of the many settings the LG has.

to bad the CMS is 2D with this type LG rolleyes.gif

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post #9 of 16 Old 02-04-2013, 05:41 AM
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thanks, good to know

regarding the bit about plasmas, is the difference due to ABL and the effect it has on gamma readings?
Yes, almost certainly.
I will add that with front projectors the surrounding wall color can cause a similar discrepancy with perceived gamma.

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post #10 of 16 Old 02-04-2013, 05:52 PM
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Sorry if not on topic but was wondering when calibrators say they can get (example 30 foot lumens out of a Samsung e series).Are they talking about the brightest whites/white box,or something else?
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post #11 of 16 Old 02-04-2013, 06:12 PM
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Originally Posted by PlasmaPZ80U View Post

regarding the bit about plasmas, is the difference due to ABL and the effect it has on gamma readings?

The gamma measurements from the PDF at http://www.avsforum.com/t/1446386/abl-effects-measured-for-comparison support the idea that LCD-based displays with a fixed backlight or iris can measure somewhat differently from plasma displays. Generally LCD-based displays using a fixed backlight are expected to measure gamma with little variation, regardless of the measurement pattern, so with LCD the typical expectation is that a tiny window or a full field will return a nearly-identical gamma graph. On the other hand plasma can have more variation in the resulting gamma graph depending on the displayed images, as shown in the linked PDF. In the link Light Illusion suggests that studio monitors measure more like LCD with a fixed backlight, but personally I don't have any experience with studio monitors to say how well they avoid some of the measurement differences found on consumer displays.

To keep things simple, I'm going to somewhat arbitrarily pick two graphs from the PDF and compare the gamma measurements. I'll select one window measurement run, because windows are the most common historical reference when it comes to discussing gamma measurements. I'll also select a constant on-screen measurement, because grayscale bar patterns have also been around for a long time, and because a constant image reflects the basic purpose of gamma as summarized in http://www.poynton.com/PDFs/GammaFAQ.pdf question 5. A lot of people seem enamored with using small windows on plasma so I'll choose the 5% window gamma measurement, and for another pattern that's also generally darker than typical video content I'll select the AVS S APL measurement. The 5% window and AVS S APL also happen to have Y Max fL numbers that are somewhat close on the given display.

The general trend in the PDF is that the 5% window gamma measures lower than the AVS S APL gamma. I would suggest that, on the given plasma, the gamma you observe on-screen (AVS S APL measurement) likely tends to be higher than the gamma represented using a 5% window measurement. If you were to measure both the 5% window and AVS S APL patterns on an LCD with a fixed backlight, you would instead expect to get only one gamma graph with either pattern. So with the given plasma you can generally expect the on-screen gamma to be higher than the window measurement, but with the LCD the expectation is for the on-screen gamma to match the window measurement. From this premise let's say you measure both the given plasma and a fixed-backlight LCD using 5% windows, and you "calibrate" both displays so that they each precisely give a 2.2 gamma measurement. I would suggest that the measurements in the linked PDF suggest that if you view these two "calibrated" displays side by side on a somewhat dark scene the on-screen gamma for the plasma would probably tend to be higher than on the LCD. You could instead "calibrate" both displays to the same gamma using the AVS S APL pattern, and they might come closer to looking more similar in regards to gamma on a dark scene, but the other measurements in the PDF suggest the two displays would still have performance differences.


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post #12 of 16 Old 02-05-2013, 10:39 AM - Thread Starter
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Sorry if not on topic but was wondering when calibrators say they can get (example 30 foot lumens out of a Samsung e series).Are they talking about the brightest whites/white box,or something else?

it's foot lamberts (fL) and refers to the Y reading of a 100% white window... of course, the larger the window on a plasma display, the lower the Y reading will be (and vice versa)

on a LED/LCD, you can simply use full fields instead of windows since there is no ABL to interfere with the Y readings


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post #13 of 16 Old 02-05-2013, 10:45 AM - Thread Starter
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Originally Posted by alluringreality View Post

The gamma measurements from the PDF at http://www.avsforum.com/t/1446386/abl-effects-measured-for-comparison support the idea that LCD-based displays with a fixed backlight or iris can measure somewhat differently from plasma displays. Generally LCD-based displays using a fixed backlight are expected to measure gamma with little variation, regardless of the measurement pattern, so with LCD the typical expectation is that a tiny window or a full field will return a nearly-identical gamma graph. On the other hand plasma can have more variation in the resulting gamma graph depending on the displayed images, as shown in the linked PDF. In the link Light Illusion suggests that studio monitors measure more like LCD with a fixed backlight, but personally I don't have any experience with studio monitors to say how well they avoid some of the measurement differences found on consumer displays.

To keep things simple, I'm going to somewhat arbitrarily pick two graphs from the PDF and compare the gamma measurements. I'll select one window measurement run, because windows are the most common historical reference when it comes to discussing gamma measurements. I'll also select a constant on-screen measurement, because grayscale bar patterns have also been around for a long time, and because a constant image reflects the basic purpose of gamma as summarized in http://www.poynton.com/PDFs/GammaFAQ.pdf question 5. A lot of people seem enamored with using small windows on plasma so I'll choose the 5% window gamma measurement, and for another pattern that's also generally darker than typical video content I'll select the AVS S APL measurement. The 5% window and AVS S APL also happen to have Y Max fL numbers that are somewhat close on the given display.

The general trend in the PDF is that the 5% window gamma measures lower than the AVS S APL gamma. I would suggest that, on the given plasma, the gamma you observe on-screen (AVS S APL measurement) likely tends to be higher than the gamma represented using a 5% window measurement. If you were to measure both the 5% window and AVS S APL patterns on an LCD with a fixed backlight, you would instead expect to get only one gamma graph with either pattern. So with the given plasma you can generally expect the on-screen gamma to be higher than the window measurement, but with the LCD the expectation is for the on-screen gamma to match the window measurement. From this premise let's say you measure both the given plasma and a fixed-backlight LCD using 5% windows, and you "calibrate" both displays so that they each precisely give a 2.2 gamma measurement. I would suggest that the measurements in the linked PDF suggest that if you view these two "calibrated" displays side by side on a somewhat dark scene the on-screen gamma for the plasma would probably tend to be higher than on the LCD. You could instead "calibrate" both displays to the same gamma using the AVS S APL pattern, and they might come closer to looking more similar in regards to gamma on a dark scene, but the other measurements in the PDF suggest the two displays would still have performance differences.

so, in a nutshell, you're saying that 2.2 measured gamma gamma on a plasma would look more like a darker/higher (perhaps 2.4 or so) measured gamma on a LCD with a fixed backlight?

is there any window size or apl pattern type that would make the plasma measure gamma exactly like it looks with real program material? in other words, so a 2.2 measured gamma on a plasma looks like a 2.2 measured gamma on a LCD with a fixed backlight?


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post #14 of 16 Old 02-05-2013, 06:07 PM
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so, in a nutshell, you're saying that 2.2 measured gamma on a plasma would look more like a darker/higher (perhaps 2.4 or so) measured gamma on a LCD with a fixed backlight?

That's essentially the point I was making regarding window measurements. I'm trying to avoid too many generalizations based on limited information, so I'll point out that the linked measurements are specific to one single plasma display, and it's entirely possible that other plasma displays could exhibit different tendencies. Anyway, the linked measurements from Chad B appear to support his comment about gamma in the first reply.
Quote:
is there any window size or apl pattern type that would make the plasma measure gamma exactly like it looks with real program material? in other words, so a 2.2 measured gamma on a plasma looks like a 2.2 measured gamma on a LCD with a fixed backlight?

I'm not sure there's any simple clear-cut answer to this sort of question, especially once RGB balance is included in the discussion. While a fixed-backlight LCD is expected to exhibit a fixed gamma, the linked measurements suggest that the 55GT50 plasma likely varies on-screen gamma to some extent depending on the image displayed. For example the 20% measure on the AVS S APL pattern is around 2.2 gamma, and the 20% measure on the AVS L APL pattern is closer to 2.4 gamma. I would expect that's enough difference where it would probably be possible to notice some change in gamma between dark and bright scenes on real program material. Other people have posted similar AVS S APL and AVS L APL measurement comparisons in the HCFR measurement thread, and I don't think that much variation is necessarily expected for all plasma displays, but of course it's also possible that some plasmas could show even more variation in on-screen gamma depending on the image displayed. Since different display types and display settings do not necessarily react similarly as overall screen brightness varies, I still think it's a good idea to at least look at some sort of a pattern with a fixed screen brightness (grayscale bars, Chad B's APL, etc.) when adjusting gamma.


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post #15 of 16 Old 02-06-2013, 03:50 AM
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Thanks plasmapz
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post #16 of 16 Old 02-09-2013, 11:11 AM
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http://www.avsforum.com/t/1441407/grayscale-variances-different-patterns-plasma-calibration/150#post_22941498
Above is a link to another graph that may suggest it's possible common consumer measurement procedures might result in higher on-screen gamma for plasma than on fixed-backlight LCD. If the measured display had been a fixed-backlight LCD the linked graph would be expected to show only one horizontal line at whatever gamma had been chosen for calibration. Instead the measured plasma shows approximately a 0.1 increase in gamma from the left side of the graph to the middle of the graph. Windows are the most common measurement type, and when the video levels are averaged in the way the linked graph shows the measurements are generally darker than most typical video content. For example a 15% area window series would be only 7.5% average video level, or 7.5 "APL (stimulus)" shown in the link above. The linked graph seems to suggest that the measured plasma could be expected to have a higher gamma with typical video content than indicated by a window measurement, while the fixed-backlight LCD would generally be expected to have the same gamma with both the window measurement and typical video content.

For an idea of how bright typical video content is in comparison to the graph and window measurements, below are some random Blu-ray images. Even with black bars on the screen, all the Blu-ray example images still have higher averages than the 7.5% average video level from 15% area windows.

http://www.avsforum.com/t/1361140/star-wars-comparison-pix-and-vid#post_20965206
Post #3 - approx. 33% average video level
Post #4 - approx. 41% average video level
Post #5 - approx. 9% average video level
Post #6 - approx. 13% average video level
Post #7 - approx. 27% average video level
Post #8 - approx. 30% average video level


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