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 OLED display technology 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 was initially thought to be primarily due to the widely varying spectral colour distribution of the different technologies, causing the probes used to make colour readings report variations that are not within visually acceptable tolerances.
However, it is now generally accepted to be due almost exclusively to the amount of spectral energy emitted in the blue region below 460nm. This has become a consensus to the point that specific phrasing to that effect has even made its way into some of the more recent standards that give consideration for alternate CMFs to deal with this challenge.
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 OLED based display Sony were the 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.
Perceptual Color Matching Using LightSpace CMS
The following is a far more accurate approach to overcoming display metameric failure, and will work with any display technology.
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 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% or slightly below is recommended - as this ensures there is no 'peak colour clipping' occurring.)
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.)