Manual Display Calibration

LightSpace CMS Guide to Manual Display Calibration

LightSpace CMS can be used for manual Display Calibration, using any display's in-built Colour Management System (CMS) controls to accurately calibrate the display as far as is possible with the display's provided controls.

Manual Calibration can be performed with the Free LightSpace ZRO version of LightSpace CMS.

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 Manual 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 manual display calibration.


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)."

What to Calibrate?

What can be manually calibrated actually depends on the manual controls provided by the display manufacturer. Often, these controls are next to useless, and the only real approach available for accurate calibration is to 'turn off' (or null) just about all the manual controls, and turn to an external 3D LUT box to provide the most accurate calibration possible.

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.

But, for the sake of this guide, we will assume the display in question has good internal manual CMS controls, and provide a step-by-step overview to the 'best-practice' approach for manual display calibration.

With this acceptance, there are six major calibration steps to be taken:

Black & White levels
Peak White
Best 'Picture Mode'
Gamma
Grey Scale/White Balance
Gamut/Colour

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, 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 display calibration 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 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
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.

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.

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.

Iris
projectors often have an Iris control (and possible bulb power) which is equivalent to 'Backlight' on an LCD display, and should be used in a similar way.

A second function of Backlight is to move between 'Night' and 'Daylight' settings, if you cannot accurately control the viewing environment (limit daylight contamination). Changing the Backlight setting can usually be performed without (badly) affecting other 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.

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. Colour should be set to maintain internal gamut colour saturation accuracy, and not to try to extend the display's maximum gamut.

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. With most modern displays this should be left at 'null'.

Hue
Hue can be present as an alternative to Tint, and changes the overall screen colour based on a vector rotation. As with Tint, this should be left null if better CMS controls are available. On some displays the 'Hue' control is just a 'Tint' control, again just altering the ratio of green to red.

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.

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 displays, especially those that are selected by users looking for decent image display capabilities, have additional 'Advanced Settings'. Theses controls usually include more accurate CMS (Colour Management System) capabilities, 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.

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.

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.

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.

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 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 Manual Display Calibration are basically the same as for 3D LUT calibration, as the initial steps are essentially the same.

LightSpace ZRO
The free LightSpace ZRO version of LightSpace CMS can be used for all the manual display calibration steps outlined within this guide.

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, especially when you consider the actual level of accuracy possible with Manual Display Calibration.

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 ZRO to compare the measured values with the actual target vales for the required colour space. 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.

If it is desired to use an external Patch Generator, and a purchased license for LightSpace CMS is being used, 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, the Raspberry Pi based PGenerator, and madVR HTPC system, plus many others. See the Tips & Tricks pages for more info.

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.

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'.


Initial Display Setup

With the required tools at the ready it's time to start the Display Calibration process. 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.

Extra Delay

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.

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 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.

Ted's LightSpace CMS Calibration Disc

To use Ted's LightSpace CMS Calibration Disc for a Quick Profile you will need to know the Maximum time taken for the reading of dark patches.
(The only way to verify the actual timings required for DIP Mode is to 'time' the duration any given probe/display combination requires when measuring dark patch colours - not just black, but dark R, G, and B patches. The longest time, plus one second, must be used as the time for all patches in DIP Mode. To take such time duration measurements use the live 'Measure' mode with colour patches displayed manually via whatever source is connected to the display to be profiled.)
(Here are instructions for measuring probe read times using the 'Meter Time per Patch Finder Chapter' of Ted's Calibration Dics.)

Then simply set the disc to the correct Quick Profile chapter for Primary and Secondary patches, and from within LightSpace's 'Calibration Interface' window select 'Profile', then select 'DIP Mode', and also 'Primary & Secondary' from the drop-down list.

To start the profile sequence press 'Play' on the Blu-Ray player, and the last 'OK' button on LightSpace (shown when the 30 second timer is counting down) at the same time. Both the Blu-Ray and LightSpace CMS will run 'in sync'.

Point Select

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!


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, from turning off Display Modes that cause serious issues, to using the Manual Controls to provided better calibrated accuracy.

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.


Manual Display Calibration

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


Set Black & White levels

The first Manual 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 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.

As before, we will initially assume you are using the Direct HDMI connection from the LightSpace CMS laptop, using Extended Desktop mode to display the patches and CalImages on the display being calibrated. Using this technique the EDID data will re-scale the images to be the correct TV Legal range - 16-235 (64-940), meaning there will be no 'below black' or 'above white' bars as used from Ted's Disc source, as outlined below.

BrightnessCal Brightness
ContrastCal Contrast

For more information on understanding Black and White level issues, and the use of direct HDMI from the LightSpace laptop seethe Data vs. Legal TV Levels, and Direct HDMI pages of the website.

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).

Ted's LightSpace CMS Calibration Disc

Brightness
Contrast

On some home TVs the way the Contrast control functions prevents Super White levels from being clipped, regardless how bright the TV is made. On such TVs just concentrate on getting the correct Peak White value, and ignore Super White clipping.

TV Legal Black and White Level Setup

There is a lot of confusion regarding the correct setup for Black and White Levels, based specifically on the concept of Super White (or Whiter Than White - WTW) signals that can go beyond the TV Legal level of 235 8 bit (940 in 10 bit). With Black Levels it seems to fairly well understood that home TVs should be set to 'clip' at 16 (64).

While in the world of production and post-production many cameras and post-systems can have valid image data beyond and below TV Legal levels (black and white), there are (presently) no consumer sources that ever contain valid content that is below 16 (64), or above 235 (940), so calibrating a home TV to 'allow' for Super White is just limiting the display unnecessarily, with no 'image' benefit at all - in fact, the exact opposite.


Data Range Black and White Levels

With displays that are not expecting TV Legal input sources, such as 'computer monitors', the correct Black and White levels will depend on the source in use - often 0-255 (0-1023) - but, depending on the sources in use, may be 16-235 (64-940).

For on-set & post-production displays the correct 'calibration' for Black and White levels will depend on the workflow in use, which many indeed be Full Range data signals, or TV Legal. It is imperative the the correct workflow, and therefore expected Black and White levels, are known and understood before calibration commences.

Set Peak White

With the Black and White levels set using the 'CalImages', the next step is to set the set the Peak Luminance 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 Nits 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...


Select The Best Picture Mode

Note: With some TVs, each independent Picture Mode can have its own Backlight, Contrast, Brightness etc, controls (there are no 'Global' adjustments), so Picture Mode should be selected before Black and White levels are set.

With The Black & White levels set, as well as the Peak White Luminance value, the next step is to select the best 'Picture Mode' to obtain the best initial start point for final calibration.

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.

The following Quick Profile results compare two different Picture Modes from the same home TV. Direct comparison of the results shows that 'Movie Mode' is the more accurate.

Picture Mode 'Standard'

Picture Mode 'Movie'

After selecting the best Picture Mode remember to go back and re-check the Black & White levels as well as the Peak White luminance value, as it is likely these will have changed.


Set Gamma

With the best Picture Mode selected, and all unnecessary modes turned off, the next stage of calibration is to verify the display Gamma and set it as accurately as possible to the desired colour space standard.

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

If the display has multi-point Gamma capability each point can be set using the continuous Measure mode within LightSpace, matching the 'Actual' probe measured Nits values to the 'Target' value, as defined by the Min & Max Luminance settings via 'Setups/Options'.

Calibrated Gamma Gamma
Calibrated Differential Gamma Gamma

Set Grey Scale Balance

With the Gamma set correctly, the same basic approach is used to set Grey Scale Balance, this time focusing on the colour component, rather than the Luminance values.

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.

LightSpace CMS Flexibility

The way the same basic procedure and tools are used within LightSpace CMS for Gamma and Grey Scale calibration shows the very open and 'free-flow- approach LightSpace CMS uses, without the need to adhere to rigid pre-set structures or workflows, with the inherent limitations such rigid approaches suffer.

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 Widget, and Active Measurement 'Actual' values.

If the display has 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.

The RGB bars within the Bars Widget show both Colour Balance and Luminance Offset. If the RGB bars are equal height, but above or below the centre line it shows the Colour is correct, but the Luminance is incorrect (high or low). If all three bars are at 'zero' it shows both Colour and Luminance are correct.

The White Bar shows Delta-E, so when all bars are at zero the calibration for the colour being measured is accurate.

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 Low adjustments.


Set Gamut & Colour

Gamut calibration sets the display to correctly map given peak primary input colour values such that the on-screen colour (chromaticity) for 100% Red, 100% Green, and 100% Blue is correct for the expected colour space standard of the source images. For example, High Definition TV images 'expect' to be viewed within a Rec709 or BT1886 colour space, which as part of their specifications set the Gamut (the Gamut for both is identical).

Peak Colour x Chromaticity y Chromaticity Y Luminance
Red - 255, 0, 0 0.6400 0.3300 0.2126
Green - 0, 255, 0 0.3000 0.6000 0.7152
Blue - 0, 0, 255 0.1500 0.0600 0.0722
White - 255, 255, 255 0.3172 0.3290 1.0000

Luminance (Y) is specified as a percentage, as there is actually no set value for Y within any colour space standard. As mentioned previously, Rec709 for example specifies peak white for a grade-1 display as being between 80 Nits and 120 Nits. And for home TVs the 'ideal' peak white may need to be a lot higher to overcome environmental considerations.

Additionally, Secondary colour chromaticity is also not specified with any given colour space standard, as they are a direct calculation from the primary colours, and should fall into place when Primary colours are set accurately. However, due to the usually poor colour management processing within home TV displays, CMY Secondary colour control is often (incorrectly) provided, so know the target values can help.

Peak Colour x Chromaticity y Chromaticity Y Luminance
Cyan - 0, 255, 255 0.2250 0.3290 0.7874
Magenta - 255, 0, 255 0.3210 0.1540 0.2848
Yellow - 255, 255, 0 0.4190 0.5050 0.9278

Colour calibration means that the volumetric colours within the specific Gamut are also correct. As with Secondary colours, all the values for volumetric colours are a direct calculation from the RGB primary colours, and each colour target is calculated within LightSpace CMS on the fly, as each colour patch is generated - seen here for a 'Yellow/Orange' colour.

After calibration has been set as above, run Quick Profiles for 'Gamut Sweep' and 'Memory Colours', and verify volumetric colour alignment, with the targets being the small circles, and the crosses the actual colour measurements, with the centre crosses being the grey scale.

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.

Displays with Advanced colour calibration capabilities offer a greater level of colour control. including Gamut (within the physical constraints of the display), and can be calibrated to a greater level of colour accuracy. This is usually defined as 'Colour Space', and provides a 'User' option for advanced adjustment of each colour in turn - Red, Green, and Blue primary colours, and often Cyan, Magenta, and Yellow secondary (although as stated previously, such secondary adjustments should not be required in a well designed display!).

Different displays have different controls for colour adjustment - RGB, HSL, xyY, but the use of them is the same regardless. The aim is to always make the 'Actual' measured readings match the 'Target' values, as accurately as possible, using the two CIE diagrams and Yxy or Luv values, depending on the chart chosen.


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

For most displays the best setting for 'Sharpness' id 'Off', or '0', although that is not always the case, so it is bets to verify with the SharpnessCal image.


Calibration Report

After calibration has been completed run a full set of Quick Profiles and verify the results are as accurate as possible. To export a PDF of the profile data enter 'Manage Colour Spaces', select the desired profile, and 'Display' it. The 'Export to PDF' button can be found at the bottom of the 'Display' window.

Post Calibration Report