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.
Full Data Range, or re-mapped TV Legal Range
(This is the correct set-up for all calibration)
wrong - TV Legal Range Clipping
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.
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.
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!
Correct - the darkest patch is 'almost just' visible
Wrong - many patches are clipped
Wrong - all patches are lifted far too much
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.
Wrong - the separation between patches is far too great
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.
Correct - the brightest patch is 'almost just' visible, with no colour clipping
Wrong - one or more of the RGB colour channels is clipping
(The overall peak white level will need to be reduced)
Wrong - many patches are clipped
(The overall peak white level will need to be reduced)
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 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.
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 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.
An example of an accurate enough Gamma plot
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.
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.
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 has a linear response to changes in the input signal, shows good RGB separation and reasonable RGB Balance (white point), a Quick Profile can be used to generate a 3D Calibration LUT. If the display's response is not linear, has poor RGB Separation and RGB Balance, 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!
Additional Technical & Support Info.
German language version of this page: displaycalibration.de