With the desire to have fast profiling, low cost, and colour accuracy being a bit of an oxymoron, an alternative method is needed to increase the colour accuracy of faster, and cheaper, colourimeter probes.
This is performed by calibrating, or matching, a colourimeter based probe to a spectroradiometer on the same display.
In this way the colourimeter is accurately matched to the spectroradiometer for the specific display to be profiled, producing more accurate results.
There are three different methods available for probe matching - ColourSpace's unique 4 Colour Volumetric Matching (FCVM), and Multi-point Volumetric Matching (MPVM), with the option to force the use of the traditional simple matrix method (FCMM).
Probe Matching Benefits
Using probe matching, the speed benefits of a colourimeter, and more importantly the lower-light reading capabilities, can be employed while simultaneously enjoying the greater accuracy of a spectroradiometer.
How To Probe Match
While probe matching can often be performed, and used, within a given tristimulus probe, all such matching will be based on the standard Four Colour Matrix Method (FCMM), which has accuracy limitations with modern display technologies, and additionally applies the probe matching to all measurements, preventing different probe match configurations to be compared.
Consequently, all Probe Matching should be performed within ColourSpace, as the standard ColourSpace probe match algorithm is the far better FCVM approach, with the even more advanced MPVM another alternative. The simple FCMM matching can optionally be applied.
More importantly, all ColourSpace Probe Matching is uniquely a Post-Profiling application, meaning any saved profile data can have different probe matching configurations applied as desired, at any time after any measurements have been performed.
This post-profiling approach to Probe Matching is totally unique to ColourSpace, and provided a level of control unmatched by any alternative calibration system.
Before performing Probe Matching, any Tristimulus probe should have any internal matching matrices removed/disabled, including Matrices/Correlation Files/EDRs, etc. The Tristimulus needs to be un-corrected, using the probe's default filter response.
The most accurate way to align a tristimulus with a spectro is to measure the required 4 colour patches - RGBW for FCVM/FCMM probe matching - on the exact same spot on the screen, one probe after the other, aligning the two probes to have the same field of view (measurement area), as well as the same inclination to the screen.
Attempting to read the same patch with two probe simultaneously will not work as well, due to measuring different areas of the screen, or different angles to the screen if attempting to read the same screen area, which will almost certainly introduce polarization errors. Additionally, tristimulus probes will always read far faster than spectros, causing potential timing issues.
With ColourSpace two (or more) probes can be connected simultaneously, through the use of opening multiple Profiling Windows, enabling different probes to be warming-up, and for them to be quickly switched between as needed.
Also uniquely, ColourSpace takes three readings for each of the 4 colours, averaging the readings to gain the best possible probe match accuracy.
Probe Match Verification
With ColourSpace's unique Post-Profiling application of Probe Matching, there is no need for any separate verification of the probe match performed, as every measurement you take with ColourSpace can be viewed within all the available graphs with/without probe matching applied, instantly providing a visual reference to the Probe Match application.
This very uniquely means possible areas of probe match error can be instantly seen after performing a volumetric profile, and comparing the without Probe Match to the Probe Match being applied.
It obviously also means different probe match configurations can be compared to each other.
Tristimulus to Spectro Verification
An alternative verification is to compare the Tristimulus readings, with Probe Match active, directly to the Spectro's readings.
With ColourSpace this is very simple, as you can make (Extract) a User Colour Space from the spectro readings, and directly compare the tristimulus readings using the ColourSpace graphs.
- Using the RGBW patches used for Probe Matching (240 8-bit for Full Range; 222 8bit for Legal) measure all 4 patches with the spectro
- Save the profile with a suitable name
- In the Library use Modify to Extract a User ColourSpace from the saved profile
- Set the new User Colour Space as the Target Gamut/EOTF in the Settings window
- Measure the same RGBW patches with the tristimulus
- Use Probe Matching Active to compare the tristimulus readings with the spectro generated Target colour space
Any user should also realise the Probe Match values shown in the Configure Matching window can also be directly copied into a User ColourSpace, without the need to re-measure the 4 RGBW patches. as always with ColourSpace there are many different solutions to any given requirement.
Only the 4 RGBW patches used for the probe matching, with the same exact RGB values, will provide a true verification of the Probe Match, as the display being used may suffer non-linear volumetric errors.
Alternative Probe Match Verification
An alternative Probe Match Verification is to use the Remote Control capabilities of ColourSpace, and use two Profiling Windows to simultaneously measure the same patches with the Spectro and the Tristimulus, and directly compare the two profile graph sets.
4 Colour Volumetric Matching
4 Colour Volumetric Matching (FCVM) is based on the older, traditional, 4 Colour Matrix Method (FCMM), but provides a greater level of accuracy, and is unique to ColourSpace. For the majority of displays it is all that is required for accurate probe matching, including non-additive displays, such as WOLEDs. The use of FCMM matching can be enabled if required.
- Attach the first (colourimeter) probe, selecting standard probe parameters as required from Probe Options
(A None/Bypass/Default pre-set within the colourimeter should be used, not a display specific matrix pre-set)
- Select the Probe Matching option, and press New
- Enter a unique descriptive name and press OK
(Use a name that includes the display and probe details)
- Place the probe on the patch window and press Measure All
(The patch window will cycle R, G, B and W patches, and save the probe/display measurement data)
- Change the probe to the second (spectro) probe, and repeat the process
(Spectro's do not use matrix pre-sets)
The order the probes are measured in is not important, and you can measure the spectro first, with the colourimeter second.
- If using a separate patch window, not controlled via ColourSpace, use the RGBW << Measure buttons in-turn
(Each patch colour should be based on a value of 240 to match the ColourSpace generated patches on 0-255 range (8-bit), and 222 for 16-235 (8-bit), although this is not critical so long as the same exact patch values are used for both probes)
- With the probe placed on a patch of the correct matching colour press the relevant << Measure button to take a measurement
(When White is measured the Luma value will be updated too)
- Alternatively, manually enter the xy values, remembering to record the Luma value when reading white
Regardless of the procedure used to take the probe measurements, the profile files are used in the same way.
- Select the spectro Reference Probe/Display Preset file from the Probe Matching lower drop-down menu
- Select the colourimeter Active Probe/Display Data file from the upper drop-down menu
- The colourimeter measurements can now be corrected to match the spectro using the Probe Matching Active tick box with Graph Options, Processing
Multi-point Volumetric Matching
Multi-point Volumetric Matching (MPVM) is also unique to ColourSpace, and as the name suggests uses more than the 4 colour measurements of the above 4 Colour Volumetric Matching method. This is aimed at displays that have a serious non-linear volumetric colour response, with spectral colour variations throughout the display's brightness and colour range.
The fundamental difference with Multi-point Volumetric Matching is that the entire display's colour volume is profiled to perform the match, not just 4 bright patches.
- For Multi-point Volumetric Matching a profile really needs to be cube based
(Multi-point Volumetric Matching will work best with uniform profiles, not random ones)
- Best results will be attained when the display peak white is set to be close to the target peak white
(For example, 100 nits for SDR calibration)
- Profile the display with each probe in-turn as normal, with no probes selected with the Probe Matching, Configure option
(Use a None/Bypass/Default pre-set within the colourimeter, not a display specific matrix pre-set)
- After profiling both probes, open the Space Management library, and use Modify, Extract BPD to convert the profiles into Probe Matching files
(Use a descriptive name for the extracted BPD file)
The probe matching files can now be selected within the Probe Matching option, exactly as for 4 Colour Volumetric Matching.
Application of Probe Matching
With the unique Post-Profiling application of probe matching within ColourSpace, any saved profile can have any probe matching applied as desired, at any time after profiling has been performed.
When performing Manual Measure adjustments probe matching needs to be performed on the live measurements, not post-profiling.
With ColourSpace, probe matching for both pre-and-post profiling is equally applicable.
For any probe matching process - Manual Measure, or Post-Profiling - simply set the Probe Matching options within the Probe Options menu, and enable matching when required via Probe Matching Active within Graph Options. Any probe readings, both live measurements and saved profiles, can therefore be displayed with or without probe matching applied, as desired.
When a profile is saved into the library, the status of Probe Matching Active is applied to the saved profile. It is therefore recommended to use a profile name that states if Probe Matching was active, or not, when the profile was saved, to prevent double application of Probe Matching.
If Probe Matching is being used, the profile MUST be saved with Probe Matching Active for use with LUT Generation.