From Zach Lewis:
"I’m not sure if this is just Sony, but I do know our BVM-X300s utilize non-CIE-1931 colorimetry (i.e., Judd-Vos); and are thus calibrated to an offset white-point. The net result is a (theoretically) closer perceptual match to CRT displays. I’ve been wondering – in the context of ACES in general – how to express and account for this elsewhere in the pipeline (I presume this wouldn’t affect much outside dictating how any non-mastering / preview X300s in the pipeline need to be set and calibrated). But I have to imagine this would cause a discrepancy between the expected output values listed in the tables here, and actual measurements from such a duly-calibrated display, no? And, at the same time, if the display itself is converting from CIE-1931, and then calibrated to perceptually compensate for the difference, the academy-provided ODTs are ideal as-is. If I’m correct in my presumptions – that the standard ODTs are appropriate for these devices, but / and a correctly calibrated device elicits values different from those listed – how should this be addressed?”
I’ll take a stab at this part…
ODTs can be thought of as outputting the ideal colors for a display, but it assumes you also have an ideally calibrated device that receives the ideal color space signal for the current settings. There is another transform sometimes needed to calibrate the physical devices, and in the ACES system, it is called the ODCT - Output Device Calibration Transform (you can never have enough acronyms sic.) If a device has internal controls for calibration, then the device could be setup properly and no code value would be harmed in transmitting the ideal color to the display. If the device has no controls at all, then you would have to apply an ODCT to change the code values into exactly what that device needed to show the correct color. Then you would potentially have a colorimetric match. The Judd-Vos correction fits in an uneasy place in the model – because it is a perceptual transform. Some ODTs have perceptual adjustments. Because of the narrowness of OLEDs compared to CRT, the correction is an attempt to minimize observer metamerism (and effectively to give a better white). This may have to be tuned individually for different devices as the same offset might not be the best choice for different spectrum coming from a display. It is probably best to treat the Judd-Vos correction as more of a unique device calibration rather than worry about needing a whole new ODT for it. It follows the same principle in many post houses that making the signal be correct is the important thing, and minor perceptual differences should not be baked into the signal (because then some devices would be getting a correction they don’t need). The real test is whether displays being compared are showing roughly the same result. A consideration must then also be, if I am setting up OLEDs with a Judd-Vos correction, does an LCD need the same or is it already broad enough in the three components that it doesn’t need it. I do know that there is still work going on within the industry at looking at the Judd-Vos correction as it applies to the Sony X300 – and at the same time, the X300 is largely the reference monitor of choice. I think you are correct that you may not get colorimetric readings from a JuddVos corrected display, so it helps to know what type of calibration was used, but it may be perceptually closer to the requested color – so a colorist’s judgements are more likely to be correct with the JuddVos correction.
but i think some people might think that ODCT is something which is exclusively ACES workflow related, but it is not. This white point modification is not something new and there is nothing different to do whether you work in a traditional workflow or ACES. And it is true: when you want to closely match 2 Displays of different technologies to one another you might need different white point modification transforms for the different displays and you will have to obtain these modifications mostly by eyeballing the monitor you want to match to the reference monitor (X300 for example or a Barco DCI Projector). So this white point modification should under no circumstances be part of the ODT! i try to imagine the confusion that would start. You would need one ODT with the modification for Viewing and one ODT without for rendering - that is just a recipe for confusion, maybe even disaster. You would also need these modified versions of ODTs for every target color space…