3D LUT Display Calibration

ColourSpace is 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 ColourSpace without having to fully understand the software, although it is highly recommended to learn and fully understand the tools within ColourSpace as there are many, many additional capabilities not covered by this guide.


ColourSpace 3D LUT Display Calibration

The real requirement for display calibration is actually very obvious, as without it you will never see images as the production team, specifically the DoP & Colourist, intends. This is true for displays used within film and TV productions, as well as for the home consumer, be it Gaming or Home Cinema use. It is equally true for Medical applications, where medical image assessment requires critical display calibration.

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 display.

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/EOTF and colour temperature, and incorrect black and white levels, with such settings aimed at 'looking pretty' in the TV show-room.

Beyond this, many TVs have what are claimed as 'professional' calibration pre-sets, such as ISF or THX. Unfortunately such settings are often nothing more than 'marketing gimmicks', as rarely are they accurate. 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:

When using a monitor for graphics, stable color can be maintained at all times by conducting calibration at a frequency of once every 200–300 hours. An LCD monitor used for graphics should be calibrated at least once every 200–300 hours (in ordinary use, once per month).

Colour Management Resources Eizo

ColourSpace 3D LUT Calibration Process

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

  1. 1
    Display Pre-configuration

    Set-up the display to have minimal internal Colour Management control, with basic Black level & White level settings

  2. 2
    Profiling

    Profile the display with a suitable Patch Sequence, which could be a preset ColourSpace Profile, or a User Generated Patch Sequence

  3. 3
    LUT Application

    Generate and apply the Calibration LUT, using the Profile Data and the Target Colour Space

  4. 4
    Verification

    Re-profile the display with the Calibration LUT active to verify the final calibration

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

Initial Display Setup

3D LUT calibration of Home TV displays is often very different to the calibration of Professional displays, as home TVs have many more (often 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 signal processing, which manufacturers attempt to overcome through the provision of totally unnecessary user controls.

A classic example of such 'unnecessary' colour controls is the use of 6 axis 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 - both Professional and Home TV - using ColourSpace.

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 set the Gamma/EOTF response and Grey Scale. The goal is to maximise the available display gamut, while placing gamma/EOTF 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. Professional displays will need far less pre-calibration adjustments compared to home TVs.

  1. 1
    Black & White clipping

    Ensure no clipping of shadows or highlights

  2. 2
    Peak White

    Set desired peak white level

  3. 3
    Best Picture Mode (Home TVs only)

    Select mode that provides best potential for LUT based calibration

  4. 4
    Gamma/EOTF (Optional)

    If desired, set EOTF to closest to target

  5. 5
    Grey Scale/White Balance (Optional)

    If desired, set white point/colour temperature to target

  6. 6
    Gamut/Colour

    Set display to widest gamut, which may be set via Pictue 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 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' iterative approach to manual setup 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/EOTF setup 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 some similar name) 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 Plasma or OLED/WOLED) controls the overall illumination level used for the screen, and can be used to set the overall brightness. 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 WOLEDs for example) will not have such a control, and projectors may use a Bulb Power, or Iris control.

While 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, in combination with different peak luma settings.

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/EOTF

Gamma/EOTF 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/EOTF, 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/EOTF 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.

Multi-Point Gamma/EOTF

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

Colour Space

Different Colour Space selection 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, or 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 will vary depending on the display being calibrated, and the image workflow to be used for viewing after calibration.

ColourSpace

The specific version of ColourSpace 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 LUT based 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 for the lower end of the calibration market, often combined with the i1Pro 3 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-1501 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 ColourSpace to compare the measured values with the actual target vales for the required colour space during 3D LUT generation.

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

For most calibration work the HDMI output from the ColourSpace laptop is perfect for this, as it enables Closed-Loop (the probe and patch generator are both under ColourSpace control) measurements.

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, ColourSpace is compatible with many different systems, from the BoxIO and IS-mini LUT Boxes, to Resolve, Scratch, and Mistika grading systems, as well as the Lumagen Boxes, DVDO Test Patch Generator, and madVR HTPC system, plus many others.

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 generators, specifically with the DIP mode (Display Independent Profiling) capability of ColourSpace, which enables the calibration disc to automatically play the correct colour patches as required for ColourSpace 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. This website contains 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 ColourSpace this is simple to perform using the system's Profiling capabilities, and will help define the areas of manual display control that need particular focus.

  • Start ColourSpace, and navigate to Probe Options and select the correct probe
  • Connect the probe
  • Allow 20 to 30 minutes probe warm-up time
  • Start the display, and again leave for 20 to 30 minutes warm-up time
    (To shorten warm-up times run dummy profile sequences, or use the Pre-roll option)
  • Select Profiling from the main ColourSpace window
Main Menu - Profiling
Profiling

Probe Matching

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

Probe Matching

  • Select Probe Options and set the required parameters for the probe and display combination
Profiling - Probe Options
Probe Options
  • Set Extra Delay Time if the display requires 'settling' time between patch changes - WOLEDs for example - or if there is a noticeable delay in the patch changes on the display with respect to the patch window within ColourSpace
  • From within the Settings menu, within the Luma Target section, the target Min and Max Y values for the display can be set manually to the expected targte values

The Min & Max Y values do not 'need' to be set at this point - as they can be changed at any time - but they set the Target Values, and are the basis for the reported Y accuracy during measurements, and so will affect the reported Delta-E values when performing Manual Measurements.
For initial profiling these can be set manually to the ideal values for the display to be profiled - such as 0.05 Nits Min, and 100 Nits Max, as is defined ideal for a Grade-1 professional Rec709 display.

Profiling - Settings
Settings
  • Set the Target Colour Space from within the Settings menu
    (The target colour space is the colour space the display is supposed to be matched to - Rec709, for example)

The Target Colour Space does not 'need' to be set at this point - as it can be changed at any time, during profiling, or after profiling is complete - but it sets the Target Values, and in conjunction with Max & Min Y, is the basis for reported accuracy during measurements.

  • Decide on the preferred Patch Generation workflow - Direct HDMI, internal display patch generator, external hardware TPG (Test Patch Generator), software based TPG, etc
  • Set Patch Size, Background, etc., from the Settings menu
    (Patch size and probe location - contact or non-contact - depends on the display technology being profiled, usually contact for LCDs, non-contact for displays that heat up substantially, such as WOLEDs, and also non-contact for projectors
Profiling - Hardware
Hardware
  • If using a professional display with in-built patch generator do not, at this stage, Upload a Null Cube. Leave the display as is
    (The intent is to assess the display's present calibration accuracy)
  • If using an external TPG that also has LUT capability it WILL need to be set to Null Cube, to ensure just the display is being profiled, so any active LUT within the TPG is not profiled along with the display
  • Using one of the integrated TPG's enables ColourSpace to run on its laptop, and the Test Patch Sequence to run on the display to be calibrated, under ColourSpace control, including patch size, ABL settings, etc
Profiling - Characterisation
Characterisation
  • From within the Characterisation menu select Primary & Secondary profile Mode
  • Press Start, and profiling will start
    Should a 'delay' be needed on the start of profiling use the Pre Roll option
  • The ColourSpace patch window and connected TPG will run through the selected Patch Sequence, plotting the results on the various graphs
    While profiling it is possible to change the selected menu to Manual Measure, to see the patch values being displayed, as well as change the selected graphs to view the profile build-up while profiling
  • After profiling is complete the various graphs can be inspected in further detail, zooming into them, quickly and easily displaying the accuracy/errors of the display, including a double click on any CIE graph plot-point to see colour details in the Zoom and RGB Bars widgets, pop-up point info box, etc
    (If desired, a second or even third profile can be performed using different Profile options, such as a Gamut Sweep or Memory Colours selected from the drop-down list, enabling further display evaluation)

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 can cause serious accuracy issues.

CIE Gamut Error
Gamut Error

Bad Gamut can be inaccurate colours, smaller than target gamut, or as above, gamut roll-back & low gamut, which show very poor display internal colour management.

Gamma Error
EOTF Error

Bad Gamma/EOTF plots can show other issues, such as channel clipping.

DifGamma Error
Dif EOTF Error

Dif EOTF is an alternative plot of Gamma/EOTF, with the graph normalised into a horizontal line, making errors easier to spot.

RGB Balance Error
RGB Balance Error

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.

RGB Separation Error
RGB Separation Error

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.

Clip Error
Clipping Error

Bad Clipping show as errors usually within the blacks, or whites.

The Graph Options menu within ColourSpace has additional tools that help evaluate the display profile data, including point isolation via various filters; tangent lines; access to the 3D Volumetric graphs, and a pop-up info box for detailed info on any given point.

Profiling - Graph Options
Graph Options

The pop-up Points Info window can be activated by double clicking on any graph point, with associated point information being provided in the Manual Measure menu, including the Target/Actual point values, and the Zoom and RGB Bars Widgets.

Any graph can also be zoomed and positioned, and the 3D Volumetric graphs can be rotated.

Self Verification

The above display profile assessment is based on comparing the display to a given colour space standard, such as Rec709, based on the display's existing factory calibration.

However, a good way to really understand the underlying capabilities of any display is to clear/disable any/all calibration, profiling the display in its native state, and assess the profile matched to itself, making any volumetric issues simple to see.

Such Self Verification is performed by extracting the display's native colour space from the profile using 'Extract Space'.

Extract Colour Space

Correcting Pre-Calibration Errors

With the Pre-Calibration profile data available the inaccuracies of the display can be assessed and the display reconfigured to correct for the errors.

For professional displays that really means disabling any internal calibration, including uploading a Null LUT to re-set any/all displays parameters to null, including any existing 3D LUT calibration. Refer to the specific User Guides for information on professional displays that are integrated with ColourSpace.

For consumer TVs it means selecting the best TV settings for the widest possible gamut before LUT based calibration.

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 Test Patterns built into ColourSpace, via the downloadable Light Illusion CalImages from the website, or using Ted's fantastic LightSpace CMS Calibration Disc.

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

When displaying the Test Patterns 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 (0-1023), 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 Test Patterns is rescaling from Data to TV Legal, meaning black and white levels will be the same on the display regardless.

Image rescaling is the normal image path workflow as it means black ands white (and all points in-between) look the same on a TV Legal display, and a Data level display, and any variation on this points to an image path issue.

Brightness Level Setting
Brightness
Contrast Level Setting
Contrast

The ColourSpace internal Test Patterns flash, to help spot the point at which clipping occurs.

  • With the Brightness Test Pattern displayed as required, use the display's Brightness control to set the image such that all patches are JUST visible, with the darkest top left black patch more a feeling of being visible, than really visible
  • With the Contrast Test Pattern displayed, use the display's Contrast control to set the image such that all patches are again JUST visible, with the brightest white patch more a feeling of being visible, than really visible
Clipping
Clipping

Brightness and Contrast Clipping can also be assessed via the Clipping graph within ColourSpace.

Run a Grey Only profile, and assess the graph for issues. Variation from the horizontal at either end of the plot indicates clipping.

Alternative 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 probably being uncontrollable).

Set Peak White

Setting the peak white level of 100% white to the desired target value manually is preferable to using the 3D LUT to manage the peak white, as it also minimises the black level for displays with independent back lights, such as LCD displays. For displays within an ideal viewing environment - one that is close to that encountered within a grading environment - and with Rec709 as the colour space target, that is 100 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.

  • To set Peak White manually, display a 100% white patch on the display, from any source - via the ColourSpace patch window, Ted's Disc, or any other method
  • Place the probe on the patch (or aim at the patch if in non-contact mode), and from the ColourSpace Manual Measure menu select Repeat & Measure, and continuous measurement readings will be displayed within the Actual Measurement area of the menu
Profiling - Manual Measure
Manual Measure
  • While taking continuous measurements adjust the Backlight control on the display to get the target Nits reading
  • If the display has no Backlight control, the Contrast control will need to be used to set peak white. Obviously, this could cause clipping if the Contrast value is set higher than set previously with the Contrast Test Pattern, so this defines the maximum Peak White the display can be set to
  • Regardless, after setting the peak white value it is imperative that the Black and White levels are re-checked, as on many displays (especially home TVs) it is very likely they will be interactive, and clipping/crushing can be caused
    (After the Black/White levels have been re-checked, the Peak White value will need re-checking too...)
Set Best Picture Mode

On Home TVs, 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.

(After the best Picture Mode has been defined and selected, the black & white levels, and the Peak white Luminance value will need to be checked and potentially re-set.)

  • Using the above defined Profiling process, select each available Picture Mode in turn, and run a Primary & Secondary profile, as well as Gamut Sweeps, and Memory Colours, and compare each of the measured results.
    (The aim is to find the mode that provides the widest Gamut, NOT a Gamut that best matches the target colour space, and Gamma/EOTF that does best match the colour space target)

Remember to turn off all the unnecessary additional TV Modes that will prevent accurate calibration, and which will again require running a selection of Profiles after each change in Mode to assess the benefits for turning each mode on/off

Set Gamma/EOTF

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/EOTF 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/EOTF value after a Null Cube has been applied, and/or all other settings bypassed/disabled.

For non-HDR home TVs the target colour space will be Rec709, with a power-law gamma of 2.4, or possibly BT1886 (although we do not recommend BT1886). The best setting will depend on the gamma/EOTF controls available, and with many display that will be just different pre-sets, or a slider to set different power-law values.

  • To test the Gamma/EOTF within a Home TV, from within ColourSpace select the target colour space from with Settings, and from the Characterisation menu run a Grey Only profile to assess the TV's gamma/EOTF for each available pre-set

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

Set Grey Scale/White Point

With the Gamma/EOTF 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.

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 White value will be reduced by the 3D 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.

(or many TVs, 2-point Grey Scale control uses 20% and 80% patch values. As such, these values are used within this setup guide.)

As stated, for most home TVs the target colour space will be Rec709, 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, CIE charts, and the Actual Measurement values.

(For 3D LUT Calibration, the key focus is just to set the peak white Colour Temperature accurately, without worrying too much about the rest of the grey scale.)

  • From within ColourSpace select the Settings menu, and select the desired colour space
  • From the Manual Measure window open the Patch Colour window, and set the required grey scale value to match the display's Colour Temperature controls
    (Patch values can be set via the sliders, or using Presets, loaded via .csv files)
  • Place the probe on the patch (or aim at the patch if in non-contact mode), and from the ColourSpace Manual Measure menu select Repeat & Measure, and continuous measurement readings will be displayed within the Actual Measurement area of the menu
  • Adjust the display RGB High balance controls to hit the required xy values, which places the 'cross' directly in the centre of the Zoom Widget, and balances the RGB and Delta-E Bars, repeating for each grey scale patch/white that matches the TV's adjustment controls
Manual Measure - Grey Scale
Grey Scale

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 Contrast Test Pattern 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 Contrast Test Pattern 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 backlight). 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.)

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 Brightness Test Pattern to check for possible crushing after any RGB High adjustments.

  • Swap between the 100%, 80% and 20% patches and double and triple check the results, as altering one patch will effect the others
    (100% white is the critical value)
  • If the display has multi-point Grey Scale Balance the controls should be set to default, as such controls can 'fight' with later 3D LUT based calibration
  • Run another Grey Only Profile, and verify the results, with attention being given to 80%-100%
  • After Grey Scale/White Balance has been set, go back and re-verify Gamma/EOTF as it is likely it will have changed. This is because if all three RGB channels are raised or lowered in relative unison, the 'brightness' at that point in the grey scale will be altered, as for a Gamma/EOTF correction
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. If necessary select each option in turn, and run profiles to assess the best setting.

Professional displays do not have such controls.

Good Pre-Calibration Setups

The following show examples of good/acceptable display pre-calibration setup.

CIE Gamut plot
Gamut

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

Gamma plot
EOTF

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

DifGamma plot
Dif EOTF

Dif EOTF is an alternative plot of Gamma/EOTF, with the graph normalised into a horizontal line.

RGB Balance plot
RGB Balance

Good RGB Balance will show the Grey Scale tracking reasonably from around 20% upwards, with 100% (white) accurate.

RGB Separation plot
RGB Separation

Good RGB Separation shows all measurements track closely to the idea target, with no cross-coupling issues.

Clip plot
Clipping

Clipping should show no major deviations either within the whites, or blacks.

Volumetric Gamut

A very powerful way to assess the underlying capabilities for calibration accuracy of any display using ColourSpace is to plot a volumetric pre-calibration profile against the display's own native gamut. A good display will show a near perfect volumetric plot, as the display is being compared to itself.

Volumetric CIE plot
Volumetric CIE Gamut
Volumetric Cube plot
Volumetric Normalised Cube Gamut

Using volumetric plotting, mapping the profile to the display's own native gamut shows the underlying Volumetric linearity of the display, which in turn defines the level of calibration accuracy the display is likely to be able to attain. A good display will have a near perfect self-mapped profile. Any Red points show a dE above 2.3, which shows underlying non-linear volumetric issues. Orange points show a dE of between 1 and 2.3, and Green a dE below 1.

The Cube view further assists in display assessment, by plotting the profile data in a Normalise form, enabling an easy visual understanding of non-linear volumetric errors, as any deviation from a pure cube shape denotes an error.

3D LUT Calibration

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

ColourSpace 3D LUT Calibration is very simple when compared to Manual Display Calibration, as most of the process is handled by the advanced Colour Engine mathematics within ColourSpace. Unlike alternative calibration systems, and due to the advanced Colour Engine, ColourSpace 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.

  1. 1
    Display Volumetric Characterisation

    Profiling the display with a suitable Volumetric Patch Sequence, to accurately map the display

  2. 2
    Generation of Calibration LUT

    Using the profile to generate an accurate calibration LUT, to correct the display's underlying inaccuracies

  3. 2
    LUT Checking

    Initial assessment of the generated LUT to check for obvious errors that show potential issues with the initial profiling

  4. 3
    Active LUT Verification

    Verification of the LUT within ColourSpace, prior to uploading into the display/LUT Box/Grading system

  5. 4
    LUT Upload Verification

    Verification of the LUT after uploading into the display/LUT Box/Grading system, for final assessment

This is a very simple list, as with ColourSpace 3D LUT Calibration really is that simple.

Volumetric Characterisation

Volumetric Characterisation is the process of accurately measuring the display throughout its entire colour and luminance range, to fully understand the display's raw, pre-calibration parameters.

  • Select Characterisation, and the profile size to use, with 10^3 being the minimum suggested for final accuracy, although 17^3, or 21^3 is preferable if time allows
    (With ColourSpace, advanced options such as Augment Data, Hint, etc., can be used to increase LUT Generation accuracy, while using smaller initial patch sets.)
Volumetric Characterisation
Volumetric Characterisation
  • Select Anisometric or Sequential, with Anisometric better for displays with ABL type issues, or HDR displays with heat related drift issues
    (In general, Anisometric sequences work well for all display types)
  • Drift Compensation can be set to automatically insert a user selected colour patch every 'nn' frames, providing information on display and probe drift that can later be used for corrections when generating the calibration LUT
  • Stabilisation patches can also be defined within the Settings menu
  • The default Closed Loop mode is used for all integrated TPG's, while DIP Mode should be selected for patch sequences generated via external sources
    External sources include Ted's Calibration Disc, or timelines built on grading/edit systems via the downloadable edls
  • Within Characterisation press Start to initiate the volumetric profile
    If a delay to the start of profiling is required, set a Pre-roll sequence of the desired duration
  • The ColourSpace patch window and connected TPG will run through the selected Patch Sequence, displaying the results on the various ColourSpace graphs
  • When profiling is complete, the full Volumetric Profile can be saved into the library via the Graph Options menu
3D LUT Generation

The Volumetric profile is a record of the entire display's colour response, including full Gamut, Gamma/EOTF, & Luminance levels, from which 3D Calibration LUTs can be generated for any desired target colour space, using the LUT Tools menu within ColourSpace.

Main Menu - LUT Tools
LUT Tools
  • Access the LUT Tools menu, and within LUT Generation select the target colour space the display should be calibrated to within Source
    (The Source colour space is the colour space the source images expect to be seen within; Rec709 for example)
LUT Tools - Cube
LUT Cube
  • Within Destination select the previously generated Volumetric Profile for the display being calibrated
  • Drift Compensation, if available, shows that the profile has Drift data, and can be used to compensate for any display/probe drift while building the calibration LUT

Due to the extremely flexible way ColourSpace works there are no restrictions the the way LUT Generation can be used; it is possible to generate LUTs that match Profiles to Colour Spaces, Colour Spaces to Colour Spaces, or Profiles to Profiles, colour spaces able to be user defined, including the use of Parametric Gamma curves.

  • From within Output select the preferred method for LUT Generation. The default is Peak Chroma, and is the most likely correct option
  • Enter a suitable LUT Name, and select Create to generate a 3D Calibration LUT using the selected options
    (New LUT will generate a new, independent LUT, while the Add options can be used to concatenate the generated LUT with any existing LUT already held within ColourSpace)
LUT Checking

After generation, the LUT is held within ColourSpace, and can immediately be viewed as a 3D Cube showing the LUT volumetrically, a 1D Graph showing the LUT's grey scale, and as a LUT Image, which shows the LUT applied to a visual image - pre-set image, or user selected.

LUT Cube Viewer
LUT Cube Viewer

The LUT Cube viewer shows the correction the LUT will apply.

LUT 1D Viewer
LUT 1D Viewer

The 1D LUT viewer isolates the LUT's grey scale for independent assessment.

LUT Image Viewer
LUT Image Viewer

The LUT Image viewer enables te LUT to be applied to any test image to assess its effect.

The Cube viewer shows the corrections the LUT is going to make to the display in volumetric space. A Cube that is 'compressing' the colours 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 Viewer shows the changes to the grey scale/white balance and Gamma/EOTF the 3D LUT will make. If the display's grey scale/white balance and gamma/EOTF have been corrected during the manual display set-up, as defined previously, this graph will show a single straight line. If the graph shows three RGB lines with separation, that shows the LUT is correcting grey scale/white balance.

The LUT Image Viewer can be used to view the LUT on any image within ColourSpace, which will show any possible issues with the generated LUT.

Active LUT Verification

With the LUT still held within ColourSpace the actual calibration result can be verified using the 'Active LUT' function.

  • Make sure the external TPG is still connected, and ColourSpace is still talking to it
  • Verify the setups under Settings and Probe Options, and set as required, setting Min & Max Luma target values to the display's actual values, if desired
  • Within Manual Measure, or Characterisation, use the drop-down Active LUT menu to select the generated LUT from within ColourSpace
  • Select the desired profile mode to apply through the Active LUT

With Active LUT selected, all calibration patches sent to the display via ColourSpace, or the external TPG, are pre-processed via the LUT, so verifying the 3D LUT Calibration. After the profile is complete the various graphs with ColourSpace can be used to assess the LUT accuracy.

LUT Upload Verification

The final phase in Calibration is to Upload the LUT into its final location for use, and to re-run the the previous verification procedures to ensure there are no issues with the LUT upload and application.

The actual LUT Upload procedure will be different for different LUT devices, and may use the in-built Hardware Upload feature of ColourSpace, 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 verification within ColourSpace 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. Use the Video Scale options within ColourSpace, if needed.