3D LUT Display Calibration

LightSpace CMS Guide to 3D LUT based Display Calibration

LightSpace CMS is very unique when used for 3D LUT Calibration as it uses no pre-set workflows or fixed structure step-by-step walk-throughs, as these are very restrictive approaches to calibration, and often mean the results are far from optimal for any given display.

Using this guide it should be possible to get accurate results from LightSpace CMS without having to fully understand the software, although it is highly recommended to learn the full software as there are many, many additional capabilities not covered by this guide.


An Overview Guide to 3D LUT Display Calibration

Display Calibration

LightSpace CMS takes a very open 'free-flow- approach to calibration, without the need to adhere to rigid pre-set structures or workflows.

Instead, LightSpace CMS provides a tool-box of features that can be used in different ways to best perform accurate display calibration, end-to-end colour workflow management, on-set Look Management, LUT manipulations, and more.

For calibration with LightSpace CMS there are four basic stages - Display pre-setup - Display Profiling - LUT Generation - LUT Application - Calibration Verification.


Why Calibrate?

The real requirement for display calibration is actually very obvious, as without it you will never be seeing images as the production team, specifically the DoP, intends. This is true for both displays used on film and TV productions, as well as for the home consumer.

Therefore, display calibration is required so that the viewed images match as closely as possible the colour standards expected by the images being viewed, as such video standards define how an image should look on any given screen when accurately matched to the given standard.

Unfortunately, nearly all displays are provided with very poor 'factory' calibration out of the box, especially home TVs which tend to come with over saturated colours, widely inaccurate gamma and colour temperature, and incorrect black and white levels, with such settings aimed at 'looking pretty' in the TV show-room.

'Showroom' settings are anything but 'accurate', and demand the TV be accurately calibrated before any images will look as intended.

Beyond this, many TVs have what are claimed as 'professional' calibration pre-sets, such as ISF or THX. Unfortunately such settings should be considered as nothing more than 'marketing gimmicks', as rarely are they accurate at all. At best, they are just 'less wrong'.

Regardless of the initial state of any factory calibration, all displays drift over time, and should be recalibrated on a regular basis. We recommend at lest once a month, and manufacturers such as Eizo concur with this:

"An LCD monitor used for graphics should be calibrated at least once every 200–300 hours (in ordinary use, once per month)."

If you are serious about calibration the only truly viable option is to turn off all the display's internal Colour Management Systems, and use a 3D LUT for all calibration. No display's manual calibration capabilities come close to this level of accuracy.

LightSpace CMS 3D LUT Calibration Process

LightSpace CMS uses a very simple 4-step approach to 3D LUT calibration, without the need to have pre-defined or fixed workflows with the tools used in each step able to be applied totally independently, enabling each process to be used as required within any calibration operation without restriction.

Display Pre-configuration Profiling LUT Application Verification
Set-up the display to have minimal internal Colour Management control, with basic Black level & White level settings Profile the display with a suitable Patch Sequence, which could be a Quick Profile, a Cube Based Profile, or a User Generated Patch Sequence Generate and apply the Calibration LUT, using the Profile Data and the Target Colour Space Re-profile the display with the Calibration LUT active to verify the final calibration

It is this unrestricted approach to calibration that helps makes LightSpace CMS so powerful, combined with the additional LUT Manipulation tools than enable issues such as poor probe response to be overcome.


Initial Display Setup

The 3D LUT calibration of Home TV displays is often very different to the calibration of Professional displays, as home TVs have many more (useless) colour controls that just don't exist in professional displays, as they are not necessary when the underlying display electronics has been designed correctly. Unfortunately, as home TVs often have very poor internal electronics the manufacturers attempt to cover this up through the provision of totally unnecessary user controls.

A classic example of such 'unnecessary' colour controls are the use of 6 colour calibration - RGBCMY - as all secondary colours (CMY) should simply be a direct calculation of the Primary (RGB) colours, with no need for independent control.

This 3D LUT calibration guide will define the different approaches required for each type of display - Professional and Home TV.

Before commencing with 3D LUT Calibration it is imperative that the majority of inbuilt display calibration and management options are deactivated prior to accurate profiling and calibration being performed, with just the minimum of controls used to set basic Black & White clipping levels, and possibly the Gamma response and Grey Scale. The goal is to maximise the available display gamut, while placing gamma relatively close to (if necessary, slightly below) the final target value, with the same for Grey Scale (Colour Temperature).

With this understanding in mind, there are six manual pre-calibration set-up steps to be checked before commencing 3D LUT Calibration, depending on the display being calibrated. For example, professional displays will need far less pre-calibration adjustments compared to home TVs:

Black & White levels
Peak White
(Optional)
Best 'Picture Mode' (Home TVs only)
Gamma (Optional)
Grey Scale/White Balance (Optional)
Gamut/Colour (Widest Mode)

While the above list is fairly simple, it will be necessary to re-visit most entries for a second, or even third time, after checking the other entries. For example, for home TVs the Black & White levels will need to be re-verified after the best Picture Mode' has been found - but you really need to set the black and white levels before evaluating the display's different Picture Modes, as incorrect black & white levels will make a Picture Modes look wrong...

It is imperative to understand that such a 'repetitive' and 'circular' approach to manual calibration is a requirement due to to the way just about all manual TV calibration controls work.


Manual Display Controls

The actual controls that will be used during manual pre-calibration setup will vary depending on what is provided by the specific display being calibrated. Knowing what controls are available, and what they are used for is a critical step in gaining accurate final calibration.

It is also important to note that many home TV displays have very poor controls, to the point that some controls do not work as expected (including wrong control labeling, incorrect functionality, and just plain poor design!). It really is key to know if the display in question has controls that operate as expected, or not.

Picture Mode
On Home TVs Picture Mode selects different display-wide settings, on-top of which the other display controls work (often with different controls active or not, depending on the Picture Mode selected). For accurate display calibration the need is to find the mode that is the least inaccurate. This is usually 'Movie' or 'Cinema' mode, which contrary to their names is actually closer to a the correct colour and gamma calibration for TV's, and has nothing to do with trying to emulate Cinema imagery. Additionally, when selecting these modes many home TVs provide additional manual controls, as described below. If a User Mode is available, that is often the best to select, as it provides the best 'blank canvas' with which to work, with the best selection of additional manual controls.

Professional displays will not have such controls.

Colour Profile
Colour Profile options (or Picture Options, or similar names) on some displays are usually a sub-set of Picture Mode, and provides yet another level of 'options'. As with Picture Mode, the best setting to chose is often 'User' or 'Custom'. But, it is always best to verify each mode with direct profiling.

Professional displays will not have such controls.

Backlight
Backlight, where available (usually on LCD displays, not Plasmas or OLED) controls the overall illumination level used for the screen, and can be used to set the overall 'brightness' of the screen. The control will affect both peak white and minimum black, with more effect on white. Unlike Brightness and Contrast controls there is little chance of 'clipping'. The primary function of the Backlight control is to enable Peak White to be set, in conjunction with the Contrast control.

Backlight controls are found on most LCD displays, although some displays combine this with their Brightness control. Displays with self illuminating pixels (Plasmas and OLEDs for example) will not have such a control, and projectors may use a 'Bulb Power', or 'Iris' control.

Wile the Backlight control can be used to move between 'Night' and 'Daylight' settings, with 3D LUT calibration is is often better to use different 3D LUTs for such changes.

Brightness
Brightness controls the point at which black detail on the screen becomes clipped or crushed if set low, or if set too high will cause blacks to look grey and washed-out.

Contrast
Contrast controls the point at which white detail on the screen clips or crushes if set too high, and will make whites appear dim, grey and washed-out if set too low. Contrast needs to be set in conjunction with Backlight where available, and on displays without a Backlight is used to set the peak white value directly.

Sharpness
While Sharpness has no direct effect on calibration, it does have a perceived effect if set incorrectly. Usually, incorrect means set to a too high value, causing 'ringing' artefacts around image edge transitions.

Professional displays will not have such controls.

Colour
Colour usually controls the colour saturation within the display's fixed gamut. That means it will increase the saturation colours that are within the display's gamut, but will have little or even no effect on colours at the gamut edge. For 3D LUT calibration Colour should be set to its Null setting - disabled, allowing the maximum available display gamut.

Professional displays will not have such controls.

Tint
Tint is often a very simple colour ratio balance, usually altering the ratio of green to red, making one colour more prevalent compared to the other. For 3D LUT calibration Tint should be set to its Null, or disabled setting.

Professional displays will not have such controls.

Hue
Hue can be present as an alternative to Tint, and changes the overall screen colour based on a vector rotation. As with Tint, for 3D LUT calibration Hue should be set to its Null, or disabled setting.

Professional displays will not have such controls.

Tone
Tone, or Colour Temperature, is usually a simple set of presets ranging from Cool, to Warm 1, and Warm 2. The correct setting is the one that sets the white point colour temperature closest to the desired standard. Warm 2 is usually closest.

Professional displays will not have such controls, although they may offer 'Colour Temperature' pre-sets.

Gamma
Gamma controls are often a simple selection of presets, meaning the closest value to the desired target should be used, or a slider with relative values. The offered presets are often not the same as the suggested value they are labeled with, so the result must be profiled (measured) for verification.

Many Home TVs have additional 'Advanced Settings'. Theses controls usually include more options for further CMS (Colour Management System) adjustments, but may also included some of the controls listed above.

White Balance
White Balance sets the grey scale neutrality (colour temperature), often via 2 point or 10 point controls, depending on the display. 2 point allows for the colour temperature to be set individually for low brightness levels and higher ones. 10 point provides for control in 10% steps throughout the brightness range.

Professional displays will not have multi-point controls, offering just 2-point via RGB Offset/Bias/Gain/Cuts/Drive, as listed below.

With multiple point White Balance you can often use the controls as 'fine' adjustments for Gamma, as changing the RGB values for each 'point' by the same amount, positive or negative, will change the relative luminance of the selected point, so altering gamma at that point. Multi-point controls should not be used for 3D LUT calibration.

RGB Bias & Gain
On many displays White Balance is controlled by settings called RGB Bias, Offset, Cuts, or similar for the low-end, and RGB Gain, Drive, or similar for highlight control. Such controls work as for 2 Point White Balance.

Gamma
Some displays - very few - offer an advanced Multi-Point Gamma control, which can be used to finely tune the overall gamma response of the display. Such multi-point controls should not be used for 3D LUT calibration.

Colour Space
Colour Space provides control of the display's gamut, within the limits of the screen's capabilities. The only option we are interested in is 'Custom', as this allows the user to set the colour space as accurately as possible to the desired colour space standard. This is often via controls for RGB primary as well CMY secondary colours. Such controls should be set to Null/Disabled for 3D LUT calibration.

Having secondary CMY controls goes against standard display calibration colour science, as secondary CMY colours should be a simple and direct calculation from the primary colours, and shows the poor colour management inherent in most home TVs.

Most home TVs also have a plethora of additional modes/controls that must be disabled/set to null for accurate display calibration. Such modes include:

Dynamic Contrast
Advanced Contrast
Black Tone
Black Correction
Advanced Contrast Enhancement
Auto Light Limiter
Live Colour
Dynamic Colour
And many, many more...

Such modes, if active, will defeat any and all attempts at accurate display calibration.


The Toolbox

The tools required for 3D LUT Display Calibration vary a bit depending on the display being calibrated, and the image workflow to be used for viewing after calibration.

LightSpace CMS
The specific version of LightSpace CMS to be used will depend on the requirements of the display, the probe in use, and the workflow the user requires, as well as the need for professional or Home Cinema calibration.

Probe
A probe is required to measure values from the display, enabling the correct manual settings to be made. The cheapest probe we recommend is the i1Display Pro OEM RevB tristimulus for the lower end of the calibration market, often combined with the i1Pro 2 spectro. At the higher end of the market the Klein K10-A and CR-100 tristimulus probes are preferred, often partnered with the CR-250RH or Jeti-1211L spectro's.

Patch Generator
Patch Generation is required to enable known stimulus colours to be sent to the display being calibrated, enabling the probe to take readings and allow LightSpace CMS to compare the measured values with the actual target vales for the required colour space during D LUT generation.

Many professional displays include in-built patch generation, directly controlled via LightSpace CMS. For displays without in-built patch generation, and external patch generator will be required.

For most calibration work the HDMI output from the LightSpace CMS laptop is perfect from this, as it enables Closed-Loop (the probe and patch generator are both under LightSpace CMS control) measurements. More information can be read on the Direct HDMI for Display Profiling page.

A very useful alternative Patch Generator is LightSpace Connect, an Android and iOS App that can be run on any Android or Apple mobile phone or tablet, as well as any TV or other display/monitor through the use of Screen Mirroring, MHL Cable, AirPlay, FireTV device, Apple Digital AV Adaptor, Chromecast, etc.

If it is desired to use an external Patch Generator, LightSpace CMS is compatible with many different systems, from the IS-mini, to Resolve, Scratch, and Mistika, as well as the Lumagen and Prisma LUT Boxes, DVDO Test Patch Generator, and madVR HTPC system, plus many others. See the Tips & Tricks pages for more info.

Calibration Test Patterns
Test Patterns are used for two distinct applications. The primary use is to enable the manual setting of display controls, for black and white levels for example. The second application is to enable controls that are not directly colour related to be set - such as 'Sharpness'.

Calibration Discs
Calibration Discs can be used as an alternative to patch generator, specifically with the DIP mode (Display Independent Profiling) capability of LightSpace CMS, which enables the calibration disc to automatically play the correct colour patches as required for LightSpace CMS profiling, as well as providing a plethora of alternative Test Pattern images. We recommend the use of Ted's LightSpace CMS Calibration Disc.


Pre-Calibration Display Setup

With the required tools at the ready it's time to start the Pre-Calibration process, to set up the display in readiness for final 3D LUT calibration.

This guide will focus on the basic requirements, but any competent individual will quickly understand that there are additional possibilities that can be utilised to enhance the whole calibration procedure. There are additional User Guides that can be reviewed to advance user knowledge.


Pre-Calibration Profile

Before embarking on any calibration workflow it is a good procedure to first profile the display to assess its present calibration status. With LightSpace CMS this is simple to perform using the system's Quick Profiling capabilities, and will help define the areas of manual display control that need particular focus.

Probe Matching

Probe Matching is a technique used to 'calibrate' one probe and display combination to another probe, on the same display. This means faster and cheaper Tristimulus filter based probes can be matched to slower and more expensive Spectroradiometer, increasing the profile accuracy, while benefiting from the filter probe's speed and better black level reading.

There are two different methods available for probe matching - 4 Colour Matrix, and multi-point Volumetric Probe Matching.

4 Colour Matrix

The 4 Colour Matrix probe matching method is the simplest, and most commonly used, process, but is limited in accuracy for displays that have a non-additive colour response, such as WOLEDs. For the majority of displays it is all that is required for probe matching.

  • Attach the first probe, selecting standard probe parameters as required
    (You should use a None/Bypass/Default pre-set within the Tristimulus, not a display specific matrix pre-set)
  • If using the LightSpace inbuilt patch generator, enter a matrix name and press 'OK'
    (Use a name that includes the display and probe details)
  • Place the probe on the patch window and press 'Measure'
  • The patch window will cycle R, G, B and W patches, and save the probe/display matrix data
  • Change the probe to the second (Spectro) probe, and repeat the process
    (Be aware Spectro's do not use matrix pre-sets)

Note: the order the probes are 'measured' in is not important, and you can measure the Spectro first, with the Tri-stimulus second.

  • If using a separate patch window, not controlled via LightSpace, use the RGBW 'Update' buttons in-turn
    (Each patch colour needs to be a value of 240 to match the LightSpace patches)
  • Enter a name for matrix when prompted
    (Use a name that includes the display and probe details)
  • With the probe placed on a patch of the matching colour press 'Measure' to take a measurement
  • When White is measured the Luma value will be updated too
  • Alternatively, manually enter the xy values, remembering the Luma value too when reading White
    (As with direct LightSpace matching, the colour patches MUST be based on 240 data, NOT 255)
  • Select the Reference Probe/Display matrix from the lower drop-down menu
  • Select the Active (in-use) Probe/Display matrix from the upper drop-down menu
  • All measurements will now be 'corrected' using the Probe/Display matching function
    (You must always use the same Tri-stimulus probe's preset matrix as used when performing the probe matching!)
Volumetric

Multi-point Volumetric Probe Matching is unique to LightSpace, and as the name suggests uses more than the 4 colour measurements of the above 4 Colour Matrix method. This is aimed at displays that have a non-additive colour response, such as WOLEDs, where the addition of the white pixel breaks the expected additive of the display.

The fundamental difference with Volumetric Probe Matching is that the entire displays colour volume is profiled to perform the match, not just 4 'bright' patches.

  • For Volumetric Probe Matching a profile will need to be cube based, or valid Quick Profile such as Primary Only, or Edge
    (Volumetric Probe Matching will always 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
    (Say, 100 nits for SDR calibration)
  • The Black Level of the display can also be raised slightly to a level that both probes can read black consistently, if using less sensitive probes
    (Most probes can read around 0.1 nit accurately - but results are not guaranteed with this method)
  • Profile the display with each probe in-turn as normal, with 'None' selected within the 'Active Probe/Display Data' options
    (You should use a None/Bypass/Default pre-set within the Tristimulus, not a display specific matrix pre-set)
  • The profiles can be 'Characterisation' cube based profiles, or 'Quick Profiles'
  • After profiling both probes, open the 'Manage Colour Space' library, and use 'Generate BPD' to convert the profiles into Probe Matching files
  • The probe matching files can now be selected within the 'Active Probe/Display Data' drop-down menus within 'Options', exactly as for 4 colour matrix probe matching
  • The display black level can be returned to its correct setting after Volumetric Probe Matching has been completed

See the Probes User Guide for further information.

Direct HDMI

Extra Delay

Probe Options

Quick Profiles

The different Quick Profiles can be used in different ways, and can plot different data, as defined here:

  • Grey Scale Only
     Can be used for for any LUT generation, but will not plot RGB Separation graph data
  • Primary Only
     Can be used for any LUT generation, and will plot all graphs
  • Primary & Secondary
     Can be used for any LUT generation, and will plot all graphs
  • Memory Colours
     Cannot be used for any LUT generation, and will not plot RGB Separation graph data
  • Memory Colours with Secondaries
     Cannot be used for any LUT generation, and will not plot RGB Separation graph data
  • Gamut Sweep 75%
     Cannot be used for any LUT generation, and will not plot RGB Separation graph data
  • Gamut Sweep 75% with Secondaries
     Cannot be used for any LUT generation, and will not plot RGB Separation graph data
  • Gamut Sweep 100%
     Cannot be used for any LUT generation, and will not plot RGB Separation graph data
  • Gamut Sweep 100% with Secondaries
     Cannot be used for any LUT generation, and will not plot RGB Separation graph data
  • Grey Only Large (33 step Augment)
     Can be used for any LUT generation, but will not plot RGB Separation graph data

Note: When a Quick Profile has been performed, accessing the profiles via the 'Manage Colour Space' option shows additional data through additional capabilities not found in the graphs displayed immediately after running a profile.

Calibration Quick profile Menu

Having performed Quick Profiles, the results can be assessed by comparing the plotted data against the ideal target points. In most cases incorrect calibration will be easy to see as the plotted points will vary from the colour space targets. The Delta-E graphs are really of little use at this stage, as unless the display is close to accurate the errors will just be huge!


Possible Display Issues
Bad Gamut

Bad Gamut can be inaccurate colours, smaller than target Gamma, or as above, obvious 'Gamut roll-back'. Unfortunately, such 'roll-back' issues show very poor display internal colour management, and would take 3D LUT calibration to fix.

Bad Gamma

Bad Gamma can be just the wrong value compared to the target, or more obvious 'Dynamic Gamma' controls being turned on, as in the above graph, which need to be turn off if at all possible. If the effect is the result of ABL (Automated Brightness Limiting), which can't be turned off, a smaller patch size will be required.

Bad RGB Separation

Bad RGB Separation shows issues with the independence of the display's colour channels, with cross-coupling of the red, green, and blue colour channels such that when an input colour change that should affect only a single colour channel also causes changes within the other colour channels. This is a very useful graph, as it shows how a given change in input stimulus will affect the Red, Green and Blue channels with respect to Grey.

Bad RGB Balance

Bad RGB Balance will show that the Grey Scale has colour contamination, and/or that the colour temperature is not accurate to the target colour space.

Any errors within the profile graphs shows inaccurate calibration, and helps define the areas of the display that need looking at, including turning off Display Modes that cause serious issues.

Note: Ideally, any un-calibrated displays should show no 'roll-back' type issues within the CIE graphs; should show a near standard power law gamma; and zero RGB Separation issues. If any such issues exist in an un-calibrated display profile the display should be considered suspect if the issues cannot be corrected.


Correcting Display Pre-Calibration Errors

With the Pre-Calibration Profile data available and saved as a PDF report, the inaccuracies of the display can be assessed and reconfigured to correct for the errors.

Remember, that as final calibration will be via a 3D LUT, just the basics need to be pre-set, using the minimum of display controls, with all unnecessary control set to default/bypass/turned off...


Set Black & White levels

The first Pre-Calibration settings to be set and verified are the relative Black & White clipping levels of the display. This can be performed using the downloadable Light Illusion CalImages from the Light Illusion website, or via Ted's fantastic LightSpace CMS Calibration Disc.

Whichever source you use, the process is much the same. Display the BrightnessCal and ContrastCal (or similar) images on the display to be calibrated, and adjust the manual controls as required to limit clipping/crushing.

If possible, show the CalImages on the display being calibrated using the same workflow as the display is intended to be used with, so via the DI grading system for example.

When displaying the CalImages on the display being calibrated it is imperative you understand the difference between TV Legal range - 16-235 (64-940) - and Data range - 0-255 (0-1024), and if the system displaying the images is rescaling or not. You need to understand this to know if there will be 'below black' or 'above white' image detail, or not, and set the clipping levels accordingly.

The following assumes the display is either Data Range, or if TV legal the system displaying the CalImages is rescaling from Data to TV Legal.

BrightnessCal
Brightness
ContrastCal
Contrast

For more information on understanding Black and White level issues see Data vs. Legal TV Levels.

Displays with 'Strange' Brightness/Contrast Controls

Some displays have Brightness/Contrast (and possibly Backlight) controls that do not function as expected, with Brightness acting more like Backlight, and Contrast just setting the peak clipping levels at any white level, with no direct 'Black Level' control.

With such display the aim is still the same - to get the best Blacks Level, with the correct White Level, with no clipping at peak white (Black clipping being uncontrollable).

Set Peak White

While not strictly necessary with 3D LUT based calibration, it can be helpful from a user perspective to pre-set the Peak White level of 100% white to the desired target value. For displays within an ideal viewing environment, one that is close to that encountered within a grading environment, and with Rec709 (or BT1886) as the Colour Space target that would be 80 to 120 Nits. For displays within a less than perfect viewing environment some compromises will have to be made - and that is a 'personal' issue, based on what 'feels' and 'looks' perceptually right...

With LightSpace CMS there is a very powerful Peak White setting capability that can be used to generate 3D LUTs with different Peak White values - for example, one for Night Time viewing, and one for day Time. This is via the 'Limit Max Luminance' option provided when generating the final 3D Calibration LUT. This is a potentially a much better way to set Peak White, See later for more information.

As stated above, within LightSpace CMS there is a very powerful Peak White setting capability that can be used to generate 3D LUTs with different Peak White values using the 'Limit Max Luminance' option provided when generating the final 3D Calibration LUT, and is a potentially a much better way to set Peak White. If using this approach leave Peak White at the display's maximum before clipping occurs, and set Grey Scale/White Balance, and then set the Peak White value when generating the calibration LUTs. When using 'Limit Max Luminance' Peak Luma should be used for the LUT Generation process, not the default Peak Chroma option.


Select The Best Picture Mode

On Home TV's, with The Black & White levels set, the next step is to select the best 'Picture Mode' to obtain the best initial start point for final 3D LUT calibration.

Professional display have no such control, although they may have a 'Colour Temperature' control, which should treated as for Grey Scale Balance, outlined below.

Note, that after the best Picture Mode has been defined and selected, the Black & White levels, and the Peak White Luminance value will need to be re-set.

Pre-Calibration Display Setup using Manual Controls

For more in-depth information on Manual Display Calibration see the Manual Display Calibration Guide, but remember that with Pre-Calibration Display Set-up for final 3D LUT calibration the goal is always to use as few of the display's internal controls as possible, with the majority set to Null/Bypass, or turned off.

Set Gamma

With the best Picture Mode selected on any Home TV, and all unnecessary modes turned off, the next stage of calibration is to verify the display Gamma and set to the desired colour space standard.

Professional displays rarely need this setting to be pre-configured, with the display just left at it's default Gamma value after a Null Cube has been applied, and/or all other settings bypassed/disabled.

For most home TVs the target colour space will be Rec709, with a gamma within the range 2.2 to 2.4, or possibly BT1886, with a variable shadow range EOTF (Electro Optical Transfer Function). The best target will depend on the Gamma controls available. With many display that will be just different pre-sets, or a slider to set different power-law values.

New Gamma Value

Grey Only

Avoid using multi-point Gamma capability as such settings will potentially 'fight' with the later 3D LUT calibration. It is best to have a 'close' and 'smooth' pre-calibration gamma, rater than totally 'accurate', with the gamma being slightly 'low' (darker) if a more accurate result cannot be selected.

Calibrated Gamma Gamma
Calibrated Differential Gamma Gamma

Set Grey Scale/White Balance

With the Gamma set correctly, it is possible to use the same basic approach to set Grey Scale/White Balance, if desired.

However, with many displays it is not necessary to set the Grey Scale/White Balance at all, as the later 3D LUT will take care of this very accurately.

But, one reason to pre-configure the Grey Scale manually is to enable the Peak White to be set accurately during the pre-configuration stage, as if the Colour Temperature of the display is not set to the desired target prior to 3D LUT calibration the Peak value will be reduced by the 3D LUT. This makes a lot of sense, especially if using the 'Set Maximum Luminance' option within 'Convert Colour Space', when making the final 3D Calibration LUT.

Depending on the display, Grey Scale Balance may be called White Balance, or RGB Gain, Bias, Offset, Cuts, Drive, or similar for 2-point control, and may also offer multi-point control, from 10-point, or more. If multi-point control is offered is should be avoided for later 3D LUT based calibration, as the settings can 'fight' with the 3D LUT calibration.

As already stated, for most home TVs the target colour space will be Rec709, or possibly BT1886, which sets the expected Grey Scale Balance at D65, which is x:0.3127, y:0.3290. In very simple terms the target for Grey Scale Balance is to set the entire grey scale range to these exact coordinates, using the probe readings as displayed in the Zoom Widgets, and Active Measurement 'Actual' values.

For 3D LUT Calibration, especially if using the 'Set Maximum Luminance' option within 'Convert Colour Space', the key focus is just to set the peak white Colour Temperature accurately, without worrying too much about the rest of the grey scale.

Using just 2-point Grey Scale Balance controls use approximately 20% and 80% grey to set the two points, starting with 80%, and cycle between the two checking and re-checking the resulting accuracy.

Initial Grey Measurement

Initial Grey

In this example the initial grey measurement is too Green (low Red), as is shown by the fact the cross is offset towards the Green/Blue edge colours surrounding the white central part of the Zoom Widget, and as defined by the RGB bars. The White Zoom Widget bar shows the Delta-E error.

Corrected Grey Measurement

Corrected Grey

Here the Grey has been corrected by using the display's RGB High controls to increase Red and reduce Green, pulling the cross back to the centre of the Zoom Widget and balancing the RGB bars, so reducing the Delta-E error, making the the 'Actual' measured xy values as close as possible to the Target values.

When adjusting RGB High values it is a common rule of thumb to leave Green alone, and just adjust Red and Blue. While this is a good basic rule, there is a potential problem if raising Red or Blue causes the colour channels to 'clip' at 100% white, making the white balance at 100% incorrect, even though 80% is correct. So, 100% white needs to be checked after 80% Grey has been corrected, including using the ContrastCal image to check for possible clipping.

There is a second rule of thumb for RGB High adjustments that says RGB values should be reduced, never increased, to prevent potential clipping issues at 100%. This may, or may not, be a better approach than simply leaving Green alone... in reality a combination of both approaches is usually better. But always double and triple check with the ContrastCal image to check for possible clipping after any RGB High adjustments.

There is a real potential issue on some displays with using 20% Grey for RGB Low balance, as the backlight on many displays - White LED illuminated LCD displays specifically - can be overly 'Blue', and the backlight 'colour' can't be corrected for through calibration (it can only be 'changed' by using a different backlight with a less blue spectral response, such as changing to a display with a full array RGB LED backligh). With LCD displays the backlight has an ever increasing influence on the display colourimetry as the brightness of the display gets lower. If a problem is encountered with 20%, move up to 30%, and don't worry about 20% and below.

Initial Grey Scale Balance

Initial Grey Scale Balance

The initial RGB (Grey Scale) Balance, with obvious blue bias from the display's backlight from around 20% and below.

Corrected Grey Measurement

Corrected Grey Scale Balance

The final calibrated RGB Balance, showing that the backlight bias is still prevalent in the shadows, but with some improvement.

When adjusting RGB Low values the rule of thumb is to only raise the necessary RGB values to attain Grey Scale Balance, and not reduce values. This is to prevent 0% blacks crushing. As with the RGB High rules of thumb, this is not always the best approach as lifting values may cause black to lift too much... Just always double and triple check with the BrightnessCal image to check for possible crushing after any RGB High adjustments.


Set Gamut & Colour

If the display has 'Colour' or similar controls they should be set to Default/Null/Bypass, or off, as such controls can only 'reduce' the display's gamut, which needs to be left at the maximum Gamut possible.

Professional displays do not have such controls.


Good Pre-Calibration Display Configurations
Gamut

Good Pre-Calibration Gamut configuration for final 3D LUT calibration is a Gamut that is wider than the desired target.

Gamma

Good Pre-Calibration Gamma configuration is close to target, and slightly 'low' (darker) if a more accurate setting is not available.

RGB Separation

Good RGB Separation shows all measurements track closely to the idea target, with no cross-coupling issue. Any display that is 'un-calibrated' should show good RGB Separation. Any display that shows poor RGB Separation when in its 'native/raw' configuration should be considered suspect.

RGB Balance

Good RGB Balance will show the Grey Scale tracking well from around 20% upwards, with 100% accurate. Remember good RGB Balance is not required for final 3D LUT calibration accuracy, but can help with Peak White level setting.

Black Clipping
Brightness
White Clipping
Contrast

Note: The only real Pre-Calibration requirements for final 3D LUT Calibration are correct Black & White clipping levels, and a Gamut wider than target (or as wide as possible if Gamut is smaller).


Sharpness

Sharpness can be set just about any time during Calibration, and the goal is to use the SharpnessCal image and set the display to have no 'Ringing' or 'Edge Artefacts' around the single pixel lines on the image.

Sharpness Ringing
Sharpness Ringing
Correct Sharpness
Sharpness

Professional displays do not have Sharpness controls.


3D LUT Calibration

With the basic pre-calibration settings configured the next stage is to fully profile the display using a Cube profile to measure all Volumetric Colours.

LightSpace CMS 3D LUT Calibration is relatively simple when compared to Manual Display Calibration, as most of the process is handled by the advanced Colour Engine Mathematics within LightSpace CMS. Unlike alternative calibration systems, and due to the advanced Colour Engine, LightSpace CMS also separates the two main processes within the 3D LUT Calibration process - Display Profiling, and LUT Generation. This means there are five stages to the 3D LUT Calibration process, with the last three just different ways to verify the generated LUT and the final Calibration:

Volumetric Cube Profile
Calibration LUT Generation
LUT Checking
LUT Verification
Uploaded LUT Verification

This is a very simple list, as with LightSpace CMS 3D LUT Calibration really is very simple.


Volumetric Profiling

Cube Profiling

Using the 'CSV File' import capability of LightSpace CMS any User Generated Patch Sequence can be used. Drift Compensation can also be used to automatically insert a user selected colour patch every 'nnn' frames, providing information on display and probe drift, as above.


3D LUT Generation

The Volumetric Profile is a record of the entire display's colour response, including full Gamut, Gamma, Black & White levels, from which 3D Calibration LUTs can be generated for any desired target colour space.

LUT Generation

Due to the extremely flexible way LightSpace CMS works there no restrictions the the way 'Convert Colour Space' can be used, so it is possible to generate LUTs that match Profiles to Colour Spaces, Colour Spaces to Colour Spaces, or Profiles to Profiles.

Colour Spaces can also be user defined, including the use of Parametric Gamma curves.


LUT Checking

After LUT Generation, the LUT is held within LightSpace CMS, and can immediately be viewed using three different methods, starting with the LUT Image, which is an image with the LUT data contained in the coloured stripes, a 3D Cube view, and a 1D LUT graph.

LUT Image
3D Cube
1D Graph

The LUT Image is a very powerful capability of LightSpace CMS as it enables external graphics systems to perform LUT Manipulations, which can be used to manage issues with the profile (when using low-quality probes for example), as well as creative manipulations, such as slightly lifting the shadow detail on displays that have such issues.

The 3D Cube view shows the corrections the LUT is going to make to the display in volumetric space. A Cube that is 'compressing' the colours, as above, shows that the display's profiled Gamut is larger than the target Colour Space, which is good for accurate final calibration. If the Cube has colour points that are 'squashed' against the edge of the cube, that shows the display's Gamut is smaller than the target, which is bad for accurate calibration.

The 1D LUT Graph shows the changes to the Grey Scale/White Balance and Gamma the 3D LUT will make. If the display Grey Scale/White Balance and Gamma have been corrected during the Manual display set-up, as defined previously, this graph will show a single straight line. As above, if the graph shows three RGB lines with separation, that shows the LUT is correcting Grey Scale/White Balance. Also as above, if the graph shows 'humps', or 'curves' in the lines it is showing changes will be made to the display Gamma.

There should not be any visible 'clipping' in the 1D LUT Graph. If any is seen, the cause must be identified and fixed, which may include the use of the wrong 'Peak Chroma, Peak Luma, Fit, or Fix Chroma' function when the LUT was generated.

The 'LUT Preview' function can also be used to view the LUT on any image within LightSpace CMS, which will show any possible issues with the calibration LUT, again potentially caused by the use of a poor probe.

Original
LUT Preview
LUT applied
LUT Preview

If issues are found during LUT Checking, such as poor black/shadow correction due to the use of a poor probe, there are tools built into LightSpace CMS to help rectify them.

The following images show as issue with the shadows, caused by poor probe readings, which have been corrected using the 'Relax' LUT Manipulation Filter.

Original
Shadow Error
Corrected
Shadow Corrected

See the LUT Management User Manual, under 'Edit/LUT Manipulation/Filters' for further information.

Note: when generating a LUT LightSpace CMS actually uses some very powerful algorithms to deal with probe reading errors, and the need to use the LUT Manipulation Tools for LUT Correction are actually very rare.


LUT Verification

With the LUT still held within LightSpace CMS the actual calibration result can be verified using the 'Active LUT' function built into LightSpace.

Active LUT Verification

With the 'Active LUT' selected, all the calibration patches sent to the display via LightSpace CMS or the external TPG are pre-processed via the LUT, so verifying the 3D LUT Calibration.

Rec709 Gamut Sweep

Rec709 Gamut Sweep

Gamut Sweeps maintain the same 'colour' level (100%, or 75%), and change the saturation level in progressive steps.

Rec709 Memory Colours

Rec709 Memory Colours

Memory Colours are a selection of colours that are common in every day life, and which the human eye is accustomed to seeing, so quickly spots unexpected variations in such colours.

The closer the 'crosses' are to the target circles, the more accurate the display.


LUT Based Verification

For more advanced verification of the LUT, it is possible to use the same 'Active LUT' approach, but this time re-run a Cube based 'Display Characterisation.

Active LUT Verification

The new 'Verification Profile' can now be used to 'make' a new LUT, using the same procedure as before to make the 'Calibration 3D LUT', but this time making a 'Verification LUT'.

The closer the 3 Cube view, and 1D LUT Graph are to being 'perfect'. the better the calibration is.

3D Cube Verification LUT Verify
1D LUT View Verification LUT Verify

Uploaded LUT Verification

The final phase in Calibration Verification is to Upload the LUT into its final location for use, and to re-run the the previous Verification procedures.

The actual LUT Upload procedure will be different for different LUT devices, and may use the in-built 'Upload' feature of LightSpace CMS, or may require the LUT to be exported and loaded via the LUT device's own software. Refer to the associated Hardware Integration Guides for more information.

If any differences are seen between this final verification and the previous 'Active LUT within LightSpace CMS' verification there is something wrong with the device being used to apply the LUT.

One possible error is that the LUT device needs the 3D LUT data to be re-scaled to Legal Video range, within a data range LUT before being used, if the signal path is TV Legal. See VideoScale for more information.


Calibration Report

After calibration has been completed and verified, a PDF Report can be generated of the results.

Post Calibration Report