Sony OLED Calibration part 1
/Sony OLED Calibration part 1
April 22, 2013
Some time ago I wrote a blog post on how to correctly white balance your monitor and how essential this is to a calibrated viewing environment. I would go on to say that this is the most important component in any set to post workflow as color correction data generated on monitors that aren't displaying 100% chroma free white, is in fact for practical purposes, useless.
So how does one determine if their monitor is displaying white that is truly white?
First a recap -
When we are calibrating a HD monitor, we are adjusting it so that 100% white is reproduced as completely neutral and chroma free within the Rec. 709 color gamut.
Rec. 709 Gamut
Rec. 709 is the standard color space for HD images. It specifies a white point at D65, 6500 degrees kelvin. If this white point is placed correctly, it should ensure that all colors and grayscale within the gamut are accurately reproduced. If it does not, then there are calibration issues that cannot be resolve through a white balance adjustment alone. When we white balance a monitor we start at D65 and then adjust Red, Green, and Blue gains to push 100% into the correct target white point in the gamut. As is exemplified in the above graphic, this is represented by a two dimensional chart with coordinates. For LCD's, CRT's, and other legacy displays the coordinates for white within Rec. 709 are x .313 y .329. It turns out however, these targets aren't well suited for today's OLED monitors.
Sony's Trimaster Series OLED's, both BVM and PVM, have proven to be excellent displays. The BVM E and F panels are reference grade, facility-level monitors that also perform very well in the field. The PVM's, with some calibration, are also quite accurate and for many users have proven to be adequate. Please note that the gain and bias adjustments are much more coarse in the PVM monitors than in the BVM's which is to be expected for a product that is half the price. You can get these monitors very close to their calibration target but if 100% is required, then the BVM monitors are a better tool.
When these monitors first became available there was a lot of misinformation swirling around about how to best calibrate them. They were supposedly "perfect" right out of the box. Later it was recommended that they be aligned to the traditional targets for CRT's and LCD's of x .313 y .329. Turns out neither of these were correct.
Two points - I always felt there was a strong yellowish green cast in all of these Sony OLED displays right out the box. When aligning them to the targets of x .313 y .329, resulted in a cleaner white but one that was still just a touch warm / yellow-green. Apparently Sony came to similar conclusions so they are now recommending the following targets for all of their OLED monitors:
x .307
y .318
Y 100
These numbers are based on the 1951 Judd Modification Color Matching Function aka "Judd-Vos CMF". Sony has released an informative White Paper on the topic >>>
This modification is to address the persistent problem of Metamerism Failure. Flanders Scientific Instruments produced this excellent video explaining it.
These Judd modified targets of x .307 y .318 really helped me in aligning my BVM monitor to facility displays and then closely matching PVM and now PVM-A monitors to the BVM as well. There are so many of these OLED's in use now, it's with Sony's permission I'm making this information public to the benefit of anyone doing color critical work with them.
USING SONY'S AUTOMATIC WHITE BALANCE SOFTWARE FOR OLED MONITOR CALIBRATION:
Using the i1 Probe or one of several other spectrophotometers, Automatic White Balance software, and the Judd targets; you can get your OLED monitor reproducing an extremely neutral gray scale. This will make anyone on the set who is referencing it confident in what they're seeing and will ensure that any color correction data made with it will be useful. Is a calibration using the i1 probe as accurate as one done with a facility level colorimeter? The answer is no but you may be surprised just how close you can get with this relatively inexpensive tool.
Download the software for free >>>
You will need two test signals to do this, 100% white and 20% gray. These can be made in Final Cut Pro easily. To create 100% white, generate a white field and looking at the RGB Parade, all 3 channels should be hitting 100 IRE exactly. If they are not in equilibrium, there is chroma contamination in the video. The process is the same for 20% Gray, generate a gray field and adjust the Luminance level until the red, green, and blue channels in the parade hit 20 IRE perfectly.
Abel Cine has a great primer on how to do this process automatically but it can also be done manually, with the following steps.
1. With USB, plug the probe into your Windows machine and run the Sony software.
2. With the calibration cap (solid, light-tight black cap) firmly on the probe, select the X-Rite i1 from the probe choices in the software. If the probe and computer are communicating, in a moment the CALIBRATE button will become available. Hit it and your probe is now calibrated and ready to read.
3. Remove the cap and put the probe in the cradle that allows it to hang flush on the surface of the monitor. Leaning the monitor slightly back will help gravity hold the probe in place.
4. Send 100% white test signal into the monitor via HD-SDI. In the software, you will get a reading for x and y, which is color temperature and Y which is Luminance, or white level measured in cd/m2 (candela per meter squared) aka "nits". All of these variables are intrinsically related so affecting one will likely affect the other. Hitting all targets requires a good deal of back and forth adjustment.
5. Using your monitor's CONTRAST control, adjust until Y hits 100 nits or as close as possible.
6. Using RGB Gains, you will now affect the x and y point. You shouldn't need to adjust Blue unless your reading is way off from your target. Red gain adjustment largely affects x and Green gain adjustment largely affects y. Adjust your gains until you hit the recommended targets of x .307 y .318. It's likely that doing so will lower your Y reading so adjust Contrast again to hit Y 100. If this affects your x y reading, adjust Red and Green gains accordingly. Rinse. Repeat.
BIAS:
Bias affects the dark tones of the picture. Just as the case with white, we want dark gray and black to be reproduced completely neutral and chroma-free. Conveniently, our x and y targets for bias are the same as for gain, x .307 y .318. Y however is obviously a different level and and one that is dependent on what gamma we're using. In my experience with Sony BVM OLED's, once you get the white point aligned, usually Bias adjustments will be minimal. The PVM monitors are a bit trickier and it is difficult if not impossible to totally hit x .307 y .318 on them. You can dance between gain and bias adjustments and get your targets very close but because the adjustments are coarser than on the BVM displays, 100% alignment does not seem to be possible on them.
1. Send 20% gray test signal to the monitor via HD-SDI.
2. If you're using gamma 2.2, your Luminance target is 2.7 cd/m2. For 2.4 gamma, use 2.4 cd/m2. And for 2.6, 2.1 cd/m2. If you're not hitting your target, adjust BRIGHTNESS until you're there. Then if necessary, adjust Red and Green Bias until you hit x .307 y .318.
3. If you had to do a lot of adjustment here, go back and double check white. If you start swinging things too far in Bias, you'll likely affect Gain and visa versa. On a BVM, the adjustments should be pretty minimal. On a PVM, there may be more of a dance to find a satisfactory level of calibration.
FOOTNOTE:
Sony PVM's ship overly saturated and with a white point around 170 nits which is substantially brighter than what you would typically find on a monitor used for critical evaluation. I've found these displays require a good deal of calibration but using all of the available controls - chroma, brightness, contrast, RGB Gain, RGB Bias - and hitting as close as possible to Sony's new recommended targets, you can produce an image that's remarkably similar to what you find in the BVM series monitors. Not 100% but dare I say, "good enough".
THE CASE OF "A".
There is a big problem with these OLED panels and that is they have a brutally unforgiving viewing angle. There is so much color temperature shift when you're off axis that the only way to safely view is to sit right on top of them. It's a real problem when you have three or four people crowded around one monitor as each viewer will see something different. About 18 months after the initial release of the Trimaster OLED series, we now have the updated "A" line which greatly improves this viewing angle problem.
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