Perceptual Colour Matching

Narrow bandwidth displays, such as RGB OLED and laser based display technologies (and to a lesser extend WRGB OLED and RGB LED LCD) have shown 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 from a viewer's perspective is that the colours on one display do not match those on other displays with wider spectral distribution.

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.

Colour Metamerism

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 ColourSpace 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 ColourSpace

The following is a far more accurate approach to overcoming display metameric failure, and will work with any display technology.
(Although we deal with RGB OLED displays 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.

Dual Displays White
Dual Displays White

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 ColourSpace 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 channel 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.

Display Gain Controls
Display Gain Control

Perceptual White Point Matching

When perceptually matching white points, any other display capabilities are not relevant - gamut, gamma, etc. Interest is only in the white point. This actually means any display that has an accurate white point (can be calibrated to be accurate using normal probe based calibration.) can be used as the perceptual reference.

For example, just about all standard gamut LCD displays provide accurate white points based on probe measured data, so it is possible to white point calibrate any LCD display and use that as the reference to match any other display to when performing calibration. This means a simple Laptop screen can be used, regardless of its gamut and/or gamma capabilities.

Note: this does not mean LCD displays are accurate out of the box! Far from it usually - they must be calibrated to have an accurate white point. It also doesn't mean that all LCD displays will not suffer metameric failure themselves, as wide gamut RGB LED backlight LCDs can indeed suffer too.

Place a 85% to 95% white patch on the Laptop screen and measure with a probe and ColourSpace, adjusting the RGB Gain to get an accurate white point (colour temperature), to which the alternate display can be perceptually matched.

In this image the laptop has been accurately calibrated to display a perfect white point, and while brighter than the larger display the white point is perceptually accurate. In the real world reducing the laptop brightness to match that of the other screens would further improve the perceptual matching.

The OLED screen's white point can then be manually adjusted (via RGB Gain for example) to perceptually match the laptop screen (and the larger screen).

Laptop White

6) When a perceptual match is attained measure the new colour temperature xy values with ColourSpace in live measure mode, and note the xy values for later use.

Measure Matched White
Measure Matched White

7) Make and save a new Colour Space Target using the previously recorded xy values, and save with a specific name.

OLED Colour Space Target
OLED Colour Space Target

8) Reset the adjustments made to the second display to attain the perceptual match, putting the display back to its default colour temp.

9) Profile the second display as normal.

10) Create a 3D calibration LUT as normal with ColourSpace, selecting the new Colour Space Target as the Source. This will generate a custom Perceptually Matching 3D LUT for use on the second display, matching it to the master display as accurately as possible.

Overcoming Judd Modification Limitations

The above approach will overcome the limitations of the simplified offset approach of Judd Modification, providing the best possible match for any display technology, while still using the actual measured profile data to perform the display calibration, with a weighted target white point offset that is used throughout the entire 3D LUT generation procedure.

This approach is far more effective than a post-calibration white point offset, or fixed probe offset, as is suggested and used with alternative calibration procedures that attempt to overcome metameric failure.

Using the ColourSpace Laptop Reference White

As is discussed above, most laptop screen can easily be calibrated to display a perfect reference white using standard ColourSpace calibration techniques, as the LCD screens used perform as expected with regard to spectral colour distribution.

A very simple way to calibrate a ColourSpace laptop screen is to use the DCM option of ColourSpace, as that will perfectly align the screen's greyscale, and hence white point.

With the laptop screen accurately calibrated for the correct white point (colour temperature) it can then be used as a visual reference for perceptual colour matching.

With the DCM option the user can quickly and easily turn on/off the applied correction, as you will NOT want it active when using the laptop's HDMI output for closed-loop display calibration of an external display.

Note: Until the DCM fuunctionality has been integrated into ColourSpace the original stand-alone SpaceMatch program is being supplied.

OLEDs & Low Light Saturation

OLED is one of the few display technologies that maintains full gamut (saturation) all the way down to near black. This has a real impact on the perception of the viewed image, and it is very interesting to see how matching this to the more normal response of CRT & LCD displays helps with the viewer's perception of the displayed image.

Therefore one of the tricks often employed when calibrating OLED displays for professional grading use is to artificially reduce the saturation in the low-light region, to better match the look & feel of more traditional display.

There is some real logic to this, as human perception of colour drops rapidly with a drop in scene brightness.

For ColourSpace users with the Adjust LUT tools, it is easy to perform such a desaturation directly on the LUT using the Mono Blend tool.

For those without the Adjust LUT tools, the LUT Image approach can be used, via Photoshop or similar.