UHD / HDR / WCG Calibration

The calibration of UHD displays, with HDR and WCG

The calibration of UHDTVs, specifically those with HDR and WCG (High Dynamic Range and Wide Colour Gamut) capability, is presently what is know as a 'Crap Shoot'... For the real videophile there is presently no truly accurate way to calibrate a UHD/HDR/WCG TV, although we are working with various manufacturers to change this situation.

Most 'professional' UHD/HDR/WCG grading displays can be calibrated, as they have internal 3D LUT calibration, and are integrated with LightSpace. Such displays include the Dolby PRM series, and the Canon HDR displays.

The following outlines the present calibration possibilities for both consumer UHD/HDR/WCG TVs, as well as professional monitors.


Guesswork Calibration

UHDTV (Ultra High Definition TV) has had something of a difficult birth, with different display manufacturers effectively defining their own 'Ultra HD' specifications, based loosely around a set of common specifications.

Having said that. there is nothing to stop a standard gamut display (Rec709), with standard HD or even SD resolution, working with HDR dynamic range for example. A number of professional HDR grading displays are actually HD resolution, not UHD.

The HDR 'standards', such as they exist, are aimed more at consumer viewing, including BluRay playes, as well as HDR TVs.

For the sake of simplicity within this guide, we will assume that UHDTV means UHD 4K resolution, ST2084 HDR gamma (EOTF), and WCG Rec2020 colour gamut, using HDR10, HDR10+, and Dolby Vision. HLG and Philips HDR will be discussed when relevant (when TVs with HLG/Philips HDR are available, and sources mastered in those standards are available).

HDR UHDTV (Home TV) User Calibration

User calibration of home UHDTVs (PQ based HDR and WCG as defined above) is actually something of an oxymoron...

For most videophiles, home cinema calibration means taking control of all aspects of the display's image path, enabling precise control to be attained over all the display's colourimetry. Often, this means using 3D LUT based calibration, as this is the pinnacle of all calibration techniques.

With non-HDR UHDTVs (SDR Rec709 TVs), external LUT boxes, such as the Lumagen Radiance, as well as the software based madVR playback system, can be use to very accurately calibrate connected displays.

Direct internal 3D LUT based calibration of home TVs is not an option as few TV manufacturers offer in-built 3D LUT capability... This lack of in-built 3D LUT capability also extends to new HDR UHDTVs.

However, the issue with HDR UHDTVs at the moment is there are few viable ways to calibrate them with 3D LUTs, as even though LUT boxes, such as the Lumagen Radiance Pro, can work with UHD/HDR/WCG images, most HDR UHDTVs have their factory EOTF presets fixed, and cannot have them disabled, while still maintaining HDR compatibility. This makes secondary calibration impossible at the moment.

We are working on this situation with various TV manufacturers and hardware manufacturers, and hope to have better calibration approaches for home TVs in the near future, including 3D LUT based HDR calibration.

Light Illusion will be delighted to assist any additional TV manufacturers that wish to address correct UHD/HDR/WCG calibration.

Manual HDR UHDTV Calibration - Overview

For home HDR UHDTVs, the lack of viable 3D LUT calibration capability leaves us with Manual Calibration...

The problem with manual display calibration is that few displays (including traditional SDR Rec709 displays) are designed to calibrate accurately through the use of the display's available manual controls. This can be due to poor internal image processing electronics, with poor colour decoupling (RGB cross-colour contamination) for example, which causes the controls to not work as expected, or due to simple poor implementation of the display's CMS (Colour Management System).

This is why true videophiles use 3D LUT boxes for SDR Rec709 display calibration accuracy, as this negates all the display's internal design issues, and produced the best possible final calibration.

And HDR UHDTVs are no exception. They suffer the same issues and limitations with respect to accurate manual calibration.

So, as there are no viable ways to calibrate home HDR UHDTVs via 3D LUTs, we are limited by the level of accuracy in-built into the display by the manufacturer. But, what exactly does this mean for HDR UHDTV calibration?
(As mentioned above, the issue is that at the present time there are no home HDR TVs with 3D LUT calibration capabilities, and most do not allow for their in-built EOTF to be disabled, making external 3D LUT Box calibration unusable...)

With non-HDR/UHD TVs (SDR Rec709 TVs) calibration is based on matching the TV as closely as possible to a given standard, such as Rec709 with a power law or BT1886 gamma. With HDR UHDTVs there is a further issue as the target WCG (Wide Colour Gamut) colour space is Rec2020, which no TV can attain. This means the TV cannot be calibrated directly to the standard, as only a small part of the Rec2020 gamut will be covered by the display. Additionally, the ST2084 HDR Gamma (EOTF - Electro Optical Transfer Function) is not 'relative', as it is a nits based 'absolute' standard, based on a 10000 nit maximum luminance standard. This means different displays with different peak luminance levels will use a different 'portion' of the full ST2084 EOTF, with different Tone Mapping roll-off.

The result is this is less about calibration standards, and more about the process the display manufacturer enables any user to adopt for manual calibration. There is really no 'calibration' being performed, as the process used just verifies display setting, such as maximum luminance, black/white points of the display, the grey scale, and peak RGB gamut values, and then hopes the display's own processing will correctly place the volumetric colours where they should be within the target colour space.

This is the basic reality of HDR UHDTV calibration.

HDR10, Dolby Vision, and HLG
At the moment there are three competing HDR standards - HDR10 (and HDR10+), Dolby Vision, and HLG (Hybrid Log-Gamma).

Dolby Vision is a proprietary format, and requires all elements in the image chain to be Dolby licensed, providing a relative guarantee of final image display.

HRD10 and HDR10+ are open standards, so lack the relative guarantee of image playback, with each display manufacturer defining their own internal HDR processing.

Note: the underlying calibration of HDR10, HDR10+, and Dolby Vision is identical, as all use the same target colour space PQ EOTF and Rec2020 Gamut - the same calibration 'should' work for all standards.

HLG is as yet not widely deployed, but is slowly gaining use, and is simpler to calibrate as it uses a similar approach to SDR calibration, including the ability to easily use external LUT boxes for 3D based calibration.


As an example of the extremely limited HDR calibration offered by TV manufacturers for UHD/HDR/WCG, here is LG's documentation for UHD Alliance 'Greyscale Tracking'.

As can be seen this guide has no 'colour' calibration component at all - it is just setting the greyscale (as the document title states), and setting the white point to D65.

The TV makes the assumption that all UHD material is graded to P3 primaries (which is presently true, although there is no guarantee to that fact, and in reality is an incorrect step, as all UHD consumer sources have the material mapped into Rec2020 gamut as the delivery container!), and then assumes that the internal colour management of the TV is good enough to correctly map all volumetric colour space accurately.

And as all Home Cinema enthusiasts know, no TV manufacturers ever gets that right with SDR TVs. So why should UHD/HDR/WCG be any different?

LG HDR Calibration

Dolby Vision Golden Reference

Another example of the limited HDR calibration offered by TV manufacturers can be seen here with the original Dolby Vision Calibration Guide for Vizio displays.

Click the image to the right to download the PDF guide.

The use of 'Golden Reference' files has since been abandoned, with the introduction of Dolby Vision Calibration 2.0. However the reason for the abandonment is a very good example of the issues facing manual HDR calibration, The unit-to-unit variation of TVs of the same exact model from the same manufacturer vary too much for a single 'golden reference' to be a valid target across the board.

The guide describes nothing more than a 'setup' of the display through greyscale measurements, and a validation that the display's peak colour values matched against the 'Golden Reference' file for the Vizio display.

The 'Golden Reference' file simply contains the expected maximum primary and secondary colour gamut plots, and the 'calibration' process is simply to match the displays (if possible) to these targets, and then assumes that the internal colour management of the TV is good enough to correctly map all volumetric colour space accurately.

Again, as all Home Cinema enthusiasts know, no TV manufacturers ever gets that right with SDR TVs. So why should UHD/HDR/WCG be any different?

As described above, the concept of the 'Golden Reference' file ultimately failed, due to the issues mentioned here. The unit to unit variation in TV manufacturing meant peak colour values of the Golden Reference could not be attained by the TVs they were designed for, with Dolby Vision Calibration 2.0 replacing it.

Dolby Vision Calibration 2.0

Dolby Vision Calibration 2.0 is a better approach, as it disables the display's PQ EOTF, but maintains the display in peak luma mode.

The display can then be manually calibrated as normal for grey scale, targeting the screen's default 2.2 gamma curve, and measurements made for the peak RGBW primaries.

The peak RGBW values are then saved to a Dolby Vision reference file, and uploaded into the TV via USB. The internal Dolby Vision processing with the display then maps the display's HDR capabilities to the target PQ EOTF and Rec2020 colour space.

While still somewhat short of true 3D LUT based calibration, it is a viable manual approach to HDR calibration.

Note: as mentioned previously, the underlying calibration of HDR10, HDR10+, and Dolby Vision is identical, as all use the same target colour space PQ EOTF and Rec2020 Gamut - the same calibration 'should' work for all standards.

HDR10 Profiling & Manual Calibration
HDR10 Calibration

Understanding the above, the first stage for any HDR calibration is about profiling the display to assess its present level of accuracy, and involves using a patch generator connected to the LightSpace CMS laptop to inject the correct metadata into the signal chain, which triggers the display's HDR and WCG UHDTV capabilities, or Dolby Vision's 'Calibration 2.0' mode. Without the correct metadata in the HDMI signal path most home HDR UHDTVs will default to SDR (non-HDR/WCG) operation, so will not profile as expected.

If the TV/display can be manually switched into HDR mode there will be no need for a special HDR signal generator - any patch generator will work as required.

Ideal pattern generators for this include the Murideo SIX-G and Radiance Pro as they will both act as a patch generator, and insert the required HDR10 metadata into the signal path.

An alternative to an actual patch generator is to use HDfury to inject the required metadata into the HDMI signal path.

HDR10 Calibration


An alternative HDR metadata insertion is the HDfury Integral4K60, which can be used within the HDMI signal path from the LightSpace CMS laptop to the HDR UHDTV, injecting the required metadata into the signal path.

Using the HDfury Integral4K60 makes use of the free, in-built Light Space CMS Patch Generator for direct display profiling, using the HDfury's own control program to pre-set the required HDR metadata settings.

With this configuration any HDR display can be profiled with LightSpace CMS.

HDR10 Metadata

Using the HDFury GUI add the required custom HDR10 Metadata.
(Always tick the REC.2020 flag)

  • Custom Metadata (P3 Primaries, 0.005 Black / 1000 nits Peak / 1000 MaxCLL / 400 MaxFALL): 87:01:1a:74:02:00:c2:33:c4:86:4c:1d:b8:0b:d0:84:80 :3e:13:3d:42:40:e8:03:32:00:e8:03:90:01
  • Custom Metadata (P3 Primaries, 0.005 Black / 4000 nits Peak / 1000 MaxCLL / 400 MaxFALL): 87:01:1a:b0:02:00:c2:33:c4:86:4c:1d:b8:0b:d0:84:80 :3e:13:3d:42:40:a0:0f:32:00:e8:03:90:01

And set the required custom AVI InfoFrame.
For For PC/Laptop with HDMI 1.x and PGenerator, set output RGB-Data and patch scale 16-235 while for AccuPel and DVDO AVLab set RGB-Video output with no patch scale.

  • 82:02:0D:AE:00:E8:64:10:00:00:00 for 1080p60 RGB
  • 82:02:0D:AE:00:E8:64:20:00:00:00 for 1080p24 RGB

For DVDO AVLab, as an additional option RGB-Video 2160p24 8bit output can be used

  • 82:02:0D:AE:00:E8:64:5D:00:00:00 for 2160p24 RGB

For PC/laptop with HDMI 2.0 output, RGB-Data and patch scale 16-235 can be used

  • 82:02:0D:AE:00:E8:64:51:00:00:00 for 2160p60 RGB

With the Murideo SIX-G, Radiance Pro or HDFury correctly configured for HDR metadata, and connected both to the LightSpace CMS laptop and the HDR display to be profiled, put up a small size (approx. 10% by area) patch window using the 'Upload' menu or Network manager.

Record the 100% peak white value. This peak white value needs to be used to generate and save a new ST2084_Rec2020 colour space for this specific HDR display, using this maximum peak luminance value as the clip point for the display. Name the new colour space something you will easily remember.

Note: the Nits value in the ST2084 colour space can be set to any value, although matching the peak of the display as measured makes logical sense.

The saved colour space can then be used as the target colour space for the standard 'manual calibration' approach for the given display for grey scale, and gamut, as much as the display allows for.

For Dolby Vision 2.0 Calibration the new ST2084 colour space is not used, as the manual grey scale calibration is aimed at the display's default 2.2 gamma.

After calibration the saved ST2084 colour space can be used to verify the calibration using a Quick Profile as normal.


Note: When doing manual HDR measurements do not leave any patch static of the display for any extended period of time, as most HDR TVs have aggressive ABL (Auto Brightness Limiting, or Average Brightness Limiting, depending on who you talk to) and ASBL (Auto Static Brightness Limiting, or Average Static Brightness Limiting).

ABL will immediately reduce the display's peak luminance output when the average brightness of the displayed image is high, which is why small (10% by area) patches are used for profiling.

ASBL reduces the displays luminance output when a static image is seen for more than a few seconds.

Both these effects are required for HDR displays to reduce power consumption (to required legal levels in many countries), and to protect the display from overheating, as well as screen burn-out.

Further, and more importantly, OLED displays can suffer serious, and irreversible image 'burn-in' at high-brightness. While there is no issue at SDR levels, at HDR brightness levels there are real potential issues, as burn-in happens very, very fast - you have been warned!

HDR UHDTV Calibration via 3D LUT

When calibrating a display that is not a metadata based HDR TV, such as a projector or professional grading display, including the use of an external 3D LUT box, LightSpace CMS provides some specific and unique tools for HDR calibration, such as a 'Multiplier' function to manage the fixed 'nits to bits' relationship defined by the ST2084 PQ HDR standard. This is needed as projection systems cannot match the expected peak luma levels that ST2084 HDR demands.

Profiling & LUT Based HDR Calibration

As no metadata is needed to put the display into HDR mode, direct profiling can be used, either via a patch generator, or via direct HDMI output from the LightSpace CMS laptop.

First, make sure there is no existing EOTF active within the display, as it is impossible to calibrate on-top of an existing HDR EOTF. The display must be set to use a standard power law gamma, ideally 2.6 gamma or lower (a higher value, as the higher the value, the lower the perceived display gamma - the darker the display).

Profile the display using a large volumetric cube based profile (21^3 ideally), to attain the underlying capabilities of the display.

Open the Convert Colour Space menu, and load select the display's profile as 'Destination'.

As 'Source' select the desired HDR colour space - for grading displays this will be ST2084_P3 or ST2084_P3_D65, and for home cinema displays will be ST2084_Rec2020.

If generating a Hard Clip LUT, as for a grading display, set the EOTF Nits to below the peak value of the display being calibrated. This value can be seen in the Limit Luminance 'Maximum' value box. The LUT can then be generated using the desired process option - Peak Chroma, Map Space, etc., using the normal rules for the processing selection. The value to use should be one below the display's peak, and and be a 'standard' value, such as 1000 nits, 1200 nits, etc.

Note: The 'Limit Luminance' option should be used to set the Max Luma for the Destination as being the same as the value used when defining the target colour space, as above.

The 'EOTF Nits' option can be used to add a 'Soft Roll Off' (SR) or BT2390 'Tone Mapping' profile to the peak range of the LUT. This option SHOULD NOT be used for grading displays, as when grading it is imperative that the display 'clips' at its max peak value. Soft Roll Off is only viable for end user Home HDR TVs, and when performing on-set viewing of HDR footage, for example.

Soft Roll Off & Tone Mapping

The 'Target Max Luminance (Nits)' is based on the display being calibrated, and should be the peak luma for the display. Using a lower value will clip information unnecessarily, and using a higher value will generate flat white spots in the viewed image. The 'Target Upper Soft Roll Start (%)' is based on the destination display's peak luma value, and is defined as a percentage point before the max nits clip point of the TV being calibrated, and should be set to generate the required roll-off curve.

The 'Mastering Max Luminance (Nits)' should be the max luma of the mastering display used to generated the footage to be viewed.

The 'Target Min Luminance (Nits)' is also based on the display being calibrated, and the aim is to roll-off the blacks to prevent clipping in the viewed material, due to the 'absolute' nature of PQ based HDR.

The BT2390 Tone Mapping option uses fixed parameters for roll-off, with the option to select R'G'B, ICtCp, Y'C'bC'r, or YRGB colour processing, via the Mapping option.

The 'Load' buttons will load into the BT2390 parameters windows the Target Max and Min Luminance values from any profile loaded into the 'Destination' colour space location, and can be modified as required, or entered manually.

The Mastering Max and Min Luminance values should be set to the values of the mastering display used to generate the source material to be viewed.

Note: The R'G'B' Mapping option is the default, as it will be the closest match the mastering display's colourimetry. The ICtCp, Y'C'bC'r, and YRGB colour processing options perform different saturation reduction in the 'roll-off' area, and will generate significantly different colour results. Obviously, this goes against all 'Calibration' goals, but is as the BT2390 specification.

The process for using Soft Roll Off or Tone Mapping is:

Note: The configuration of 'Soft Roll Off/Tone Mapping' is something that can drastically alter the perception of any HDR display, as different roll off values are required for the different peak luma values used when grading source material. One size will not fit all, due to the issues outlined above.

In the example below the HDR display to be profiled has a max luminance of 1108.4 nits (manually set to be a 1100 nits target) and a 0.0018 min, with a source material (mastering) peak luminance of 2000 nits, and roll off starting at 90% of 1100 nits, and black roll off set to 10%, with no multiplier in use.

Note: The 'Load' buttons can be used to load into the ST2084 Parameters widows the Min and Max Luma values from the 'Destination' colour space in use, and manually adjusted as need, as in this case for Target Max Luminance.

HDR10 Calibration

The 'Multiplier' for projection HDR re-scales the Min/Max nits values, based on the Multiplier value used. For example, a value of x10 will effectively change a peak nits value of 56 nits into a perceived level of 560 nits, as shown below. A value of x1, as shown above will have no effect on the ST2084 scaling.

As the name suggests, this function is used for projection based HDR, as while there is no projection HDR standard as yet, the 'trick' used (including for Dolby Projection HDR) is to use a multiplier (nominally x10) to enable an HDR image to be viewed on a display with a much lower peak luminance output.

This trick works, as the viewing environment for projection based setup is usually much darker than for direct view displays (monitors). But, it is still a 'trick'.

When a Multiplier value is used it will 'multiply' any value loaded into the 'Target Min/Max Luminance' boxes via the 'Load' buttons, enabling a new saved ST2084 Colour Space to work to correctly when used to compare a 'Multiplier' based display profile to it. The Multiplier has to be set first, and the text on the 'Load' boxes will change to show 'Load xXX.X', where xXX.X is the multiplier value - for example x10.0 for a times ten multiplier.

The 'EOTF Nits' value in the main Convert Colour Space menu will always show the underlying peak Nits value, not the 'Multiplied' value, and will be greyed-out when a Multiplier is in use. The 'Multiplier' value in use is also shown on the GUI, next to the Nits value box.

Note: If Min/Max values are entered manually into the 'Target Min/Max Luminance' boxes they must be manually multiplied by the required 'Multiplier' value before entering into the value boxes. Any manually entered values WILL NOT be automatically be affected by the 'Multiplier' value.

HDR Multiplier

When calibrating an HDR ST2084 display you will need to 'save' a new ST2084 Colour Space with the correct 'EOTF Nits' value (and Multiplier if used) for the display being calibrated, otherwise the the Gamma profile will not have the correct target to map to, as the default standard is always referenced to 10000 nits!

Note: When verifying the calibration of a home HDR TV it may be better to not use a Soft Roll Off value when defining the new ST2084 colour space reference, and instead use a hard clip, as this will better define the underlying capabilities of the TV.

HDR Hard Clip


HDR Soft Roll-Off


HDR & Soft Roll Off/Tone Mapping

LightSpace CMS provides two options for Soft Roll-Off/Tone Mapping, with the Soft Roll-Off option being a Light Illusion derived process, with extended user controls, while the BT2390 Tone Mapping option uses fixed parameters, based on the selected colour processing - R'G'B, ICtCp, Y'C'bC'r, or YRGB.

Obviously, for aesthetic viewing any desired options/values can be used, and again show the issue with the HDR concept, as often what are really 'inaccurate' values generate more pleasing results - assuming the EOTF can be user varied!

For true HDR accuracy there is only one option. The display must match, or better, the mastering display's peak luma, and NO Tone Mapping/Soft Roll Off should be used!

The configuration of 'Soft Roll Off/Tone Mapping' is something that can drastically alter the perception of any HDR display, as different roll off values are required for the different peak luma values used when grading source material. One size will not fit all, due to the issues outlined above. This is one of the problems with HDR TVs without the ability to alter the EOTF - they are attempting to use one Roll Off setting for all source material.

If the viewing display matches, or exceeds, the peak luma of the mastering display there is no need for any soft roll-off/tone mapping, and it should not be used.

With the desired new ST2084 Colour Space defined and saved (with a Multiplier value if used), select the desired profiling option from within LightSpace, using the 'Calibration Interface'.

This should be a selection of different Quick Profiles, including Gamut Sweeps and Memory Colours, to enable assessment of different profile reports. While a full Display Characterisation can be used, the fact the HDR10 displays will not be capable of fully matching Rec2020 means the profile results will not accurately depict the display's 'relative' or 'emulated' gamut calibration.

Run the desired profiles, and then assess the results via the 'Colour Space Library', using the various graphs to compare the measured profile data with the new ST2084_Rec2020 colour space previously made and saved (with Multiplier if used) for this specific HDR10 display.

In the example below is a HDR projector, with a 79 nit peak output, matched to a new ST2084 colour space standard pre-set, with a 79 x10 Multiplier peak nits Luma value. This shows the projector default HDR EOTF is not as good as it could be, and has no roll-off, which could enhance the final perception of picture quality, if applied to the EOTF.

HDR Gamma

HLG Calibration

HLG RGB Separation

Calibration of HDR displays to the BBC's HLG standard is a far simpler process, as the approach is basically identical to standard SDR calibration, with no metadata requirement, and as it is a relative standard, rather than PQ's absolute standard, it effectively self scales to any display's luminance capabilities.

The one major difference is the standard has built into it a compensation for variable system gamma.

The standard first calculates the luminance of the source (before system gamma) using a weighted sum of the RGB components, as normal. The destination luminance is calculated by applying a pure mathematical gamma function to the source luminance, with the RGB channels scaled by the ratio of the source to destination luminance.

This introduces colour cross coupling, as will be seen via the RGB Separation graph, so don't be surprised when you see such results post-calibration.

Also, as for PQ HDR, the preset colour space must be adjusted to match the peak luma of the display being calibrated, using the 'System Gamma' button, as well as the desired Surround Luminance value set, and the new System Gamma calculated via the 'Calculate' button.

BBC HLG Calibration

When calibrating an HLG display you will need to 'save' a new HLG Colour Space with the correct 'Nits' value and 'Surround Luminance' for the display being calibrated, otherwise the the Gamma profile will not have the correct target to map to as the default setting is referenced to 1000 nits max luminance and 10 nits surround.

Alternate HDR Calibration

With LightSpace CMS there are no fixed workflows for any form of calibration, and HDR is no different. LightSpace CMS provides a selection of tools that can be applied as required, and the more adventurous and knowledgeable calibrators will quickly understand the possibilities.

The following alternative calibration approaches for HDR will help explain what that means, but are in no way a definitive set of alternative workflows - they are just examples to help with LightSpace CMS understanding.

The following assumes the use of an external LUT box, or a HDR display with in-built 3D LUT capability.

Alternate HDR Gamut Calibration

The calibration of Gamut (saturation/colour space) for HDR is an interesting question, as all HDR footage is delivered in Rec2020, but no displays can actually meet the Rec2020 gamut.

Additionally, all HDR footage is actually mastered (at least presently) to P3, and then mapped into a Rec2020 envelope for delivery.

This means there are a couple of options for calibration when it comes to gamut (saturation).

With LightSpace, as we do not use 'workflows' for calibration (or any other aspect of colour management), instead providing the necessary tools for users to apply as required, there are a number of ways any end result can be achieved. And calibrating a display to a colour space that is a far wider gamut than the display can actually achieve is a good example.

For example, within LightSpace the obvious approach to calibration for a Rec2020 gamut is to simply set the Source as Rec2020 and the Destination as the display profile within the Convert Colour Space menu, and for many calibrations this will just work.

But, an alternative is to set Source to ST2084 P3 (with a D65 white point) and Destination to the Display profile, and then concatenate the resulting LUT with a technical colour space conversion LUT that goes from Rec2020 to P3 (with a D65 white point, and no gamma conversion in this example)... In this way the work the LightSpace 'calibration' algorithms are being asked to perform are less 'stressful' than a direct to Rec2020 calibration, where the difference between the display's actual colour space gamut and the target colour space gamut is great.

What is critical in this approach is the 'concatenation' process, and understanding what work each separate LUT is actually performing, even when concatenated.

For example, from the above description you would probably assume that the first step is the calibration LUT generated with the display profile to P3, with the next step being a Rec2020 to P3 LUT - but that is an inverse of the actual image path process...

In reality is the calibration LUT is generated to expect a P3 input, and maps that to the display's actual gamut. And therefore requires a technical colour space conversion LUT 'before' the calibration LUT that converts the incoming Rec2020 footage gamut to P3.

Source Footage Technical Colour Space Gamut Conversion LUT Gamut Calibration LUT Display
Rec2020 Source UHD Rec2020 to P3
(P3 with D65 white point & 2.4 Gamma - no gamma conversion)
Display calibration to ST2084 P3
(ST2084 P3 with D65 white point & PQ EOTF Gamma)
Rec2020 Calibrated Display
Camera Capture Colour Space Conversion LUT Calibration LUT Working Colour Space

In the above example the first LUT in the image path is a technical LUT just converting the Rec2020 colour space into P3, matching the original graded image before it was mapped into a Rec2020 container, so makes no adjustment to the actual colour 'detail' within the footage, as there is no colour beyond P3 in the source footage.

The second LUT is therefore a standard Gamut calibration LUT, but targeting P3 colour space, rather than the wider Rec2020 space.

Also, from the above flow diagram you should be able to define that the order the LUTs are concatenated is important, as if performed in the wrong order the result will not be as expected!

Concatenating LUTs within LightSpace

As we say often when describing the way LightSpace works, there are no 'set' ways to do anything, and that goes for LUT Concatenation.

LUT Addition

The obvious way, if you have a LightSpace CMS license with the LUT Manipulation tools, is to use the 'Add' function to add together two LUTs, having first 'Saved' the second LUT (the LUT to be added to the first LUT) via the 'Export' function. With the first LUT held within LightSpace simply select the Add function and navigate to the previously saved LUT.

LUT Image

A less obvious, but actually extremely useful method, is to use the 'LUT Image' function, and 'Save' the second LUT as a tif or dpx image. You can then use the 'LUT Preview function' to open the saved LUT Image, and apply the first LUT to it, saving the resulting LUT Image as a new LUT Image. To save the new LUT Image right click on the image and select 'Save As'.

When the new LUT Image is 'Opened' within LightSpace you will have a the concatenated result!

Again, what is key to concatenating LUTs is the order of addition. Swapping the order will generated different results!

For additional information see the LUT Image page of the website.

Convert Colour Space

Yet another alternative option is to combine the LUTs via the 'Convert Colour Space' menu. If there is a LUT already within LightSpace you can use the 'Use existing' to combine the LUT you are about to make with the LUT already within LightSpace.

Combining LUTs

The 'Apply to the Image' and 'Apply to the Data' both do the same function as far as the resulting LUT is concerned. The 'Image/Data' concept refers to any 'Reference Image' that may or may not be held within the LUT Image. See: LUT Image for more info.

Note: depending on the profile data set being used within Convert Colour Space there may be some variations in the final result compared to a pure 'Add' process. That is to be expected. But, there is always another way...

If using a LUT box with multiple LUT capabilities the different LUTs can be applied separately within the LUT box, rather than being concatenated.

Alternate HDR Gamma Calibration

What should also be fairly obvious from the above is that the same concept can be applied to Gamma. For example, calibrate the display to a standard power law 2.4 gamma, and then concatenate with a ST2084 PQ EOTF gamma, with Tone Mapping/Roll-off...

HDR Source Footage Technical HDR EOTF LUT Calibration LUT Display
ST2084 PQ Source Tone Mapped PQ EOFT to 2.4 Gamma 2.4 Gamma Calibration LUT ST2084 PQ EOTF Calibrated Display
Camera Capture PQ EOTF Gamma Working Colour Space

In the above example the first LUT in the image path is just applying a PQ EOTF with tone mapping, with no adjustment to colour space. In this example targeting a 2.4 power law gamma, but could be any gamma value desired to match the display's underlying gamma.

The second LUT is therefore a simple gamma calibration LUT, calibrating the display to 2.4 gamma (or any other gamma value desired to match the display's underlying gamma).

Multi-step Alternate HDR LUT Calibration

And then equally as obviously, you can combine Gamut and Gamma. So for HDR calibration you could generate a LUT that converts Rec2020 ST2084 PQ to a power law 2.4 Gamma with P3 Gamut, and combine it with a LUT that calibrates the display to P3 Gamut and 2.4 Gamma.

The combination of Gamma and Gamut for HDR calibration using more than one LUT should therefore be fairly simple to understand, as should the fact that there are any number of different 'steps' that could be used - with gamma and/or gamut being manipulated different within each of the 'concatenated' LUTs.

HDR Source Footage HDR EOTF Calibration Display
ST2084 PQ Source Tone Mapped PQ EOFT to 2.4 Gamma & Rec2020 to P3 Gamut
(P3 with D65 white point)
2.4 Gamma & P3 Gamut Calibration LUT
(P3 with D65 white point)
ST2084 PQ Calibrated Display
Camera Capture PQ EOTF Gamma Working Colour Space

In the above example the first LUT in the image path is a technical LUT, applying a PQ EOTF with tone mapping, plus a Rec2020 to P3 colour space conversion. In this example we are again targeting a 2.4 power law gamma, but could be any gamma value desired to match the display's underlying gamma.

The second LUT is therefore a standard display calibration LUT, targeting P3 colour space, and 2.4 power law gamma.

LightSpace CMS - True Calibration Flexibility

With LightSpace CMS there are no fixed workflows for any form of calibration, and the above examples for Alternative Calibration are just a set of examples to help show the possibilities. For example, the more adventurous and knowledgeable calibrators will quickly understand that although we have used P3 as the intermediate colour space in the above examples an interesting alternative would be to extract the display's own native colour space, and use that as a 'Custom' colour space for the calibration workflow.

It is such alternative thinking that separates good calibrators from the rest...

As we say, there are no fixed workflows with LightSpace CMS - just tools that can be used and combined as needed!