Display Calibration - Initial Set-up

Pre-Calibration display set-up for final LightSpace CMS 3D LUT calibration.

Unlike alternative calibration systems there is very little required when it comes to setting up a display for LightSpace CMS LUT based calibration.


Initial Display Set-up Procedure

Display Calibration

The following steps should be followed to gain the best from any final display calibration. If these steps are not followed correctly any final calibration cannot be guaranteed. If anything is not fully understood, STOP, and research the problem fully before commencing with calibration.

The key here is to use the minimum number of controls within the display, including not using, or setting to 'off' any in-built calibration capabilities such as 'THX/ISF' type set-ups and controls, and setting display gamut to 'wide' mode if there is an option (wide gamut mode usually means using the maximum gamut available for the given display). The reason for this is that such in-built controls can distort the underlying display's colour balance (RGB Separation for example), introducing distorted colours and cross-coupling errors, making later accurate calibration next to impossible.

One of the very annoying realities of display calibration is that Home TVs are often far more difficult to calibrate accurately than professional displays, due to the use poor image processing electronics, and the implementation of totally unnecessary 'image enhancement' features, all of which need to be disabled.

Problems with Displays 'Out Of The Box'

One of the very annoying realities of display calibration is that Home TVs are often far more difficult to calibrate accurately than professional displays, due to the use poor image processing electronics, and the implementation of totally unnecessary 'image enhancement' features, all of which need to be disabled.

As mentioned above, the key here is to use the minimum number of controls within the display, including not using, or setting to 'off' any in-built calibration capabilities such as 'THX/ISF' type set-ups and controls, setting the display gamut to 'wide' mode, to minimise distortion of the underlying display's colour balance (RGB Separation for example), to prevent the introduction of cross-coupling errors.

The following image shows such a problem, where the display's internal colour management is causing colours to fold back on themselves as the internal calibration is only controlling the peak colours, not the colours in the darker range.

Bad internal colour management
Bad internal display colour management

The following image shows the poor RGB Separation as a result of the display's bad internal calibration, making the display next to impossible to calibrate accurately.

Bad RGB Separation
Poor RGB Separation

The best initial pre-calibration set-up for any display is to maximise the screen's native capabilities by turning off all in-built colour management, and setting the display's Brightness & Contrast controls as required.

If it is impossible to turn off all a display's internal colour management, the gamut should be set to be as large as possible. It is impossible to increase gamut beyond the display's manufactured limits without greatly reducing the display's brightness, and altering it manually - specifically colour independent adjustments of RGBCMY and Saturation - can induce problems with later accurate calibration, as seen above.

Next, download the Light Illusion Calibration Images from the Downloads page.

Make sure you know the expected image levels required, and make sure the display chain is set correctly - data or TV legal. the BarsAndBlack image will help with this.

For calibration it may be necessary to set the image system output to re-map data levels to legal TV if using TV legal range monitoring as all calibration patches need to be visible. See the 'Data vs. TV Legal Levels' information below.

Before starting the calibration procedure make sure all 'dynamic contrast' or similar functions are deactivated as they will defeat any attempt at accurate calibration.

Make sure the display and any probe to be used has fully warmed up before commencing calibration. 20 minutes minimum.

Also make sure the display is in a dark environment, with no reflections or light contamination, and remains so throughout the whole calibration process.

To start the calibration process use the downloaded Calibration Images to set black and white using the display's own controls. All we are interested in here is settings the black point (Brightness control) and white level (Contrast control). If either is set incorrectly part of the image level will be missing. Brightness should always be set before Contrast, and then checked again after Contrast is set as the two controls operate interactively.

Initial Display Set-up for LightSpace CMS Calibration

The advanced calibration algorithms of LightSpace CMS means any display can be accurately calibrated without the need for intricate optimization of internal display settings - simply turn off any internal display colour management, set the black and white points as required, and let LightSpace do the rest. No requirement to pre-set colour temperature, gamma, or gamut, as with alternative calibration systems!

Data vs. TV Legal levels

All calibration assumes that the entire range of levels are visible - regardless of the workflow being Data or TV Legal range.

This means it is imperative the workflow to be used for real work is well understood and the calibration path set-up accordingly.

With most DI systems this means understanding how to re-scale the image workflow as required - for example, mapping 0-1023 to 64-940 as required.

The needs for understanding this cannot be stressed enough, and more information can be found on the Data vs. TV Levels page. If in doubt ask, and if necessary pay a consultant to check and verify everything before performing any calibration.

As a quick guide, if the BarsAndBalck image is clipping at the black patch, and there is no 'surround' on the white patch (and no 'step' in the colour bars), you have an incorrect workflow for calibration. Alternatively, if the Black patch is looking washed-out, and the white patch looking 'grey', there is incorrect scaling being applied, which again is incorrect for calibration.

Data Full Range
Full Data Range, or re-mapped TV Legal Range
(This is the correct set-up for all calibration)

TV Legal Range
wrong - TV Legal Range Clipping

TV Legal Range
Wrong - Incorrect Scaling


Black and White Levels

Setting accurate black and white points (levels) is a key component to having a display that will accurately show material as it is intended.

With the majority of displays the black point is simply the minimum black the display is capable of showing, when the 'Brightness' control is correctly set, as outlined below. However, some newer display technologies, such as OLED, can produce a black level that is too dark for accurate shadow detail representation, especially when used for grading. Such displays should have the minimum black level set to the standard Grade-1 display black level of 0.03 to 0.05 Nits.

When grading, if the black level is too low it will result in shadow detail being 'lifted' to overcome the too deep blacks, causing them to be further lifted when viewed on a more standard TV, with black levels at 0.05 Nits, or higher.

White levels should set to the standard grade-1 value of 100 Nits, although a brighter level may be needed if the viewing conditions are less than optimal, as with TVs in a home environment. However, the final white setting must prevent clipping, as outlined with setting of the 'Contrast' control below.


Backlight

Many of today's flat screen displays have a 'Backlight' control that is independent of 'Brightness' and contrast' controls. With such displays the 'Backlight' control will affect both the black and white levels, and should be set to give an accurate 100 Nits white point, with the best black level as close to 0.03 to 0.05 Nits as possible.

The Brightness and Contrast controls are then used to set the 'clipping' as defined below, which may mean the Backlight needs to be re-adjusted in rotation with the Brightness and Contrast controls.


Brightness

Set the display's Brightness control so that all the patches within the BrightnessCal image are just visible (the top left patch should be almost/just/not really sure visible!), but the background black is not lifted. What you are aiming for here is to prevent image clipping, or turning black to grey.

Note: the images shown here will ONLY look correct on a correctly calibrated display - a real chicken and egg situation!

Brightness
Correct - the darkest patch is 'almost just' visible

Brightness Clipped
Wrong - many patches are clipped

Brightness Lifted
Wrong - all patches are lifted far too much


Gamma

Using the 'BrightnessCal image you can also do a quick check on gamma or with some displays 'Black Stretch'. The separation between each patch should not be too 'obvious'.

Note: Gamma will be checked technically and more accurately later in the calibration process.

Gamma Wrong
Wrong - the separation between patches is far too great


Contrast

Set the display's Contrast control so that all the patches within the ContrastCal image are visible, while simultaneously making the peak white value approx 80 to 120 nits for TV monitoring, and 50 to 55 nits for cinema film work. The peak white value will need to be measured with a probe and LightSpace CMS. You may need to compromise on the peak white value to prevent clipping of the white patches within the ContrastCal image.

Contrast
Correct - the brightest patch is 'almost just' visible, with no colour clipping

Contrast
Wrong - one or more of the RGB colour channels is clipping
(The overall peak white level will need to be reduced)

Contrast Clipped
Wrong - many patches are clipped
(The overall peak white level will need to be reduced)

Contrast Low
Wrong - all patches are far too visible (this can also signify an incorrect gamma setting)

Note: it is rare for contrast to be too low, it is usually clipping too much that is the problem. The correct value is found by setting the the correct clipping point with the ContrastCal image, and then measure the actual peak white point and set accurately, BUT without any clipping, so this may be a compromise on some displays.


Gamma, Gamut & Colour Temp

With brightness and contrast set to get blacks and whites correct, the next step is to check the display colour temperature and measure its colour gamut, which will require the use of a probe and the LightSpace CMS Calibration option to perform direct measurements and adjustments as required, as well as performing a Quick Profile to see the overall settings for Colour Temperature, Gamut, and Gamma.

Gamma can be set as accurate as possible, although this is not a truly necessary step, while Gamut needs to exceed the target colour space, ideally set to the maximum gamut of display as defined by the manufacturer.


Colour Temp

Colour temperature can usually be left at the display's default, as this will be fully corrected by the LightSpace CMS calibration LUT. However, some users may prefer to set this to be more accurate initially, using the display's available controls for colour temp.

One point to note is that if Colour Temp is corrected via the generated calibration, 3D LUT the display's peak white point will be reduced to compensate for the colour temperature difference.

On simple LCD monitors the control available for colour temp setting are usually limited - a selection of presets, that are usually not that accurate - and custom RGB independent controls that work only with gain. More professional monitors will have bias controls as well, which affect colour temperature throughout the grey scale range. If bias controls are available they can be used to first set the correct colour temperature in the blacks/shadows, and then use gain for highlights/whites. Re-check low and high colour temp cyclically.

White Point Measurement
The centre circle shows the target colour temp, the cross the actual measured reading

Setting the monitor's white point as close as possible to the desired target does minimise the work the calibration LUT has to perform, although as stated previously the final calibration LUT will correct the colour temp, so pre-adjustment of the colour temperature is often not required.


Bias and Gain

A display's Bias and Gain controls can be confusing, but in simple terms work just like contrast and brightness, but for RGB separately. Gain controls 'contrast' and Bias the 'brightness'. This allows the colour temperature of the blacks and whites to be set independently, and therefore throughout the whole grey scale.

As a rule of thumb, add Bias, and subtract Gain. This prevents the RGB channels from clipping. also, adjust Bias before gain, and then re-check Bias after adjusting Gain.

The reason to adjust Bias first is that Bias has 100% effect on black levels, reducing throughout the grey scale range to have 0% effect on white levels, but often still has some small effect on white levels. Gain has 100% effect on white levels, reducing to 0% at black, and rarely has any effect on black levels.


Gamma

Gamma needs to be checked using real profiling, with the Quick Profile function of LightSpace CMS.

The aim is to have the measured gamma within an acceptable range, with no clipping (as should be the case in the black and white points have been set correctly), using the basic controls of the display - for example, do not use any multi-point internal (ISF type) controls as the final LUT will correct the gamma accurately, and any pre-multi-point configuration will conflict and 'fight' with the 3D LUT calibration.

Gamma Measurement
An example of an accurate enough Gamma plot


Colour Gamut

Colour gamut (saturation) it should be set to be larger that the target gamut, ideally the maximum the display can produce - looking at the below image the aim is to have the crosses beyond the circles, with the measured gamut triangle being larger than that of the target... which is the case here, although blue is only 'just' outside of (larger than) the required gamut.

Gamut Measurement
A close to perfect pre-calibration Gamut plot

If gamut can be controlled be sure not to reduce the display gamut below the target as the later calibration LUT will not be able to increase the gamut again. Calibration LUTs can only work by subtracting to align colours, hence the preferred approach of setting the display to its maximum native gamut.


Re-Check Settings

Before finalising the manual settings re-check everything as later adjustments may have altered previous settings.


Perform Full Characterisation and LUT Building

When manual calibration setup is complete perform a full Display Profile, and then generate a final calibration LUT...

Note: If the display shows good RGB separation and reasonable RGB Balance (white point), with a wider gamut than the target, a Quick Profile can be used to generate a 3D Calibration LUT. If the display's RGB Separation is poor, has poor RGB Balance, and/or a low gamut a full Display Characterisation profile (cube based profile, providing full volumetric colour information) will be required for accurate calibration LUT generation.

For further info on accurate calibration techniques see the Probes - User Guide page. Pay special attention to the What Can't Be Corrected section!

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