Perceptual Colour Matching
Matching Displays that use Different Illumination Technologies
All colour calibration is perceptual. Just because a probe is often used to take measurements doesn't change that fact!
With the advent of RGB OLED and laser based display technologies (and to a lesser extend WRGB OLED and RGB LED LCD) there has been an industry-wide realisation that the existing 1931 CIE 2 degree observer colour matching function (CMF) is not actually accurate enough for colour matching many modern displays.
The issue is primarily due to the widely varying spectral colour distribution of the different technologies, with relatively narrow spectral bandwidths of the primary RGB channels, causing probes used to make colour readings report variations that are not within visually acceptable tolerances. The resulting perception form a viewer's perspective is that the colours on one display do not match those on other displays with wider spectral distributions.
It is also generally accepted that the amount of spectral energy emitted in the blue region below 460nm of some displays (such as RGB OLEDs) is a contributing factor to this issue.
Note: The information provided here is primarily based on RGB OLED displays, and will be valid for RGB laser projection too.
We are yet to be convinced WRGB OLEDs, or RGB LED LCDs (or any other display illumination technologies) actually suffer such issues, at least with professional narrow bandwidth Spectrophotometers. However, the approach outlined here will work with any display where colour matching issues are observed.
In display colourimetry, metamerism is the ability to match colours on different displays with different spectral power distributions, caused by the use of different display technologies.
Colour Metamerism is based on the fact that the human visual system has only three types of cone photoreceptors making it possible for two stimuli to match in perceived colour without having identical spectral power distributions, and so metameric colour matching occurs.
From this we get the opposite - the apparent metameric failure of displays, where the measured profile data of two displays match perfectly with respect to the numbers reported by the measuring probe, but visually the displays obviously do not match, as seen in the image above.
Sony and Judd Modification
As the first source of RGB OLED based displays, Sony were the one of first to come up against metameric failure, and quickly adopted the Judd 1951 Modification concept, which is proposal for a modification to CIE 1931 for wavelengths that are shorter than 460nm. This modification has been confirmed by other studies performed by Stiles (1955), Ishak and Teele (1955) and Vos (1978), but for practical reasons has never been adopted by CIE standards.
While the actual concept of the Judd modification is valid, in practical terms it can only be applied as a simple offset to the measured white point, and it is here that problems occur - as Sony have found out themselves!
Sony Changing the target
When Sony first attempted to deal with metameric failure of their OLED displays they released a set of chromaticity xy offset values, that ranged from -0.001, -0.009, through -0.004, -0.013, depending on the probe used, and alternative display technology to be matched to.
In later documentation on the White Balance of BVM and PVM displays Sony reduced these values to a single set of xy offsets - x=-0.006, y=-0.011.
The problem is that none of these values are actually accurate, as the concept of simply adding an offset value to an existing CMF is too much of a compromise to correct for metameric failure...
A far better approach is to utilise the in-built capabilities of LightSpace CMS to significantly improve the perceptual colour match variation between two displays of differing technologies, defining a new colour space white point, rather than adding an offset to probe's measured data.
Perceptual Colour Matching Using LightSpace CMS
The following is a far more accurate approach to overcoming display metameric failure, and will work with any display technology,
(Although we deal with a RGB OLED here, the concept is valid for any display showing metameric failure.)
1) Place both displays to be accurately matched in the same line of sight.
2) Chose which of the displays is to be the 'master'.
(In the above the larger display is the more accurate, as the smaller RGB OLED shows the usual 'green/cyan' colour cast.)
3) Calibrate the master display to the target colour space standard required - Rec709 for example - using the normal LightSpace CMS calibration procedure.
4) Display a flat white patch on both displays.
Do not use 100% white - 95% to 85%, or slightly below, is recommended - as this ensures there is no 'peak colour clipping' occurring.
Additionally, if using displays with ABL/Power Saving/Dynamic Brightness/Local Dimming a small patch size should be used to avoid possible associated colour issues.
5) Manually adjust the 'colour' (colour temperature) of the second display to visually match the master display
Using RGB Gain controls is the simplest approach.