Monitor Calibration

One could easily expect that modern computer systems would handle the simple task of displaying digital images correctly and reproducible from system and/or platform to another. The fact is that, without proper calibration, they don't, not even close. Instead of getting the 24 bit color performance that is equivalent to 16,7 Mil color codes we only get heavily distorted luminance with some 17 bit equivalent pixel depth that converts to about 6.1 Mil color codes. 

In the above illustration the inner squares match with their outer square in the linear gamma 1.0 space, this illustration has to be viewed at such distance that the dither of the outer squares are averaged by the eye, this is about 1.5 meter to 3 meter. But the monitors are not linear devices instead they have a gamma 2.5 transfer function.

What is Monitor Gamma

Gamma is the transfer function of Cathode Ray Tubes (CRT). A CRT is like any other electron tube, they all have this non-linear behavior by nature, it is due to the non-linear relationship of the grid voltage to the anode current. In CRT the anode current relates to pixel intensity and grid voltage changes the intensity of the anode current. 

Continuous tone images on the monitor screen are produced by a scanning electron beam (anode current) that is focused on the phosphors on the screen mask. The intensity of each pixel is adjusted by changing the intensity of the beam with the grid voltage. In a color monitor there are three different phosphor dots for each pixel in the screen mask, one for red, one for green and one for blue, see a microscopic photograph of a typical screen mask if you wish. The whole image and all the colors that is seen on the monitor screen are produced by mixing the intensity of these red, green and blue dots.

The role of the display driver card

Monitor gets the image in form of voltage signal from the Display Driver Card that is sitting inside the computer.

In case where the display driver hardware does not have gamma correction ability or the correction is disabled then these voltage levels are equally spaced, in other words the range of the output voltage is linearly divided between the digital levels of the RGB codes.

In a common Display Driver Card there are 256 voltage levels for each three primary colors, Red, Green and Blue (RGB).

Monitors however do not convert these equally spaced voltage levels into equally spaced intensity levels, instead they produce intensity that is related to the input voltage by the below power law equation:

The physics inside the CRT monitor dictates the value of the monitor Gamma and it is quite accurately in the range 2.45 to 2.55. 

This power expression has two very visible and unwanted effects on displayed images.  Firstly monitor Gamma causes the whole image to appear too dark, with the emphasis of this being at black to midtowns. 

Secondly the colors on the monitor are mixture of red, green and blue. When these components are coded non-linearly then there will be hue shift towards that primary color that happens to be the dominant in each pixel separately. 

The effect of the Monitor Gamma can be seen from the graph on right. Input excitation at level 128 (50% light) will give output at level 45 on a typical monitor, only 18% light.  The compression effect of the Gamma at the shadows is so heavy that the 21 first input levels are squeezed into the first single output level, which by the way happens to be the pure black. That alone represents a 8,2% loss of the image information and there is much more compression in the shadows. At the highlights there is expansion that leaves some intensity levels without any information. These both produces quantization noise to the image and in overall the image appears to be dark.

It is important to note that the Gamma also causes hue shift as the components of any hue (red, green and blue) are highly non-linear. Because of this images on uncalibrated monitor seem to have strong emphasis towards the primary colors and more precisely towards that primary color that happens to be the dominant (in each pixel, separately). When the color wheel shown on right is viewed on a gamma 1.0 calibrated monitor then each of the six named color areas seem to be equal in size. On an uncalibrated monitor at gamma 2.5 the areas where a primary color (red, green or blue) is dominant are much wider than the areas where a secondary color (cyan, magenta or yellow) is dominant.

LCD and TFT displays that are the primary display devices for laptop computers are inherently quite linear. It is often possible to compensate their small non-linearity using gamma correction controls even if the nature of the non-linearity is not the same. However some of the manufacturers (try) to match the transfer characteristics of such displays with some average CRT monitor gamma making calibration even more difficult.

Affect of the Gamma to Continuous Tone colors and Half-toned or Dithered Colors

It is worthwhile to note that the Gamma affects only to continuous tones (i.e. to the range between but not including the pure black and pure light). From the graph above we can see that intensity levels 0 and level 255 are not affected by the gamma 

Halftoning or dithering is a process of cheating the eye by faking different intensity levels. This is done by changing the distribution of only two kind of basic image elements (pixels), pure black and pure light (white). For example dithering is employed in laser- and ink-jet printers since they are not capable to change the intensity of the basic elements. This kind of dithering is not affected by gamma at all. 

The calibration images on this page takes advantage of dithering. These images should be viewed at such distance that the eye fully averages the pixels, this is about 1,5 meters to 3 meters, depending on the monitor, display resolution mode and the eye of the observer. 

Continuous tone color, like those shown on the monitor, are affected by the Gamma in two ways. Gamma causes non-linearity to intensity that makes the images to show too dark and hue shift to all of the other colors than the three pure primary and secondary colors, pure white and pure black.

Signal Levels of the Display Driver Card and the Monitor

There are always some variations in the electronic components of the display driver card and monitor for this reason every monitor has the Brightness and Contrast controls.

Their naming can be confusing actually the function of these controls is to set black-level (Brightness control) and gain (Contrast control) of the signal amplifier in the monitor, and they should be regarded as such.

The Black-level (set by the Brightness control of a monitor) is the voltage level in the monitor input that produces no output signal (no light emission on the monitor screen). 

The Gain control (set by the Contrast control) affects to the intensity range that the monitor outputs. In the early monitors a too high Gain could saturate either the signal amplifier or the phosphors of the monitor screen, but today's monitors usually perform well at maximum contrast. The Contrast control can be used for limiting the maximum lightness of the monitor but in general it is best to set it to maximum.

Technically there is only one position for the Blacklevel control (Brightness) that is the proper one.

Monitor Calibration Process for Liner Domain (Gamma 1.0)

These steps should be performed sequentially. Monitor calibration is not platform dependent, every system or platform can be calibrated using these guidelines. 

Note 1: This will end up with linearly calibrated system, (Gamma=1.0). It is the only setup that can code and display colors correctly unless the ICC color-management is employed. By using any other system gamma value image manipulation will suffer from various unnecessary degradation's.

Note 2: ICC color-managed system like the one provided by Photoshop v.6.0 and later combined with the AdobeGamma or other system calibration utility completely detach the color system of the hardware from the color-system or the working-space inside the ICC managed application. When using ICC color-management it is not necessary to set the system space to linear domain, instead a linear working-space profile can be specified in the editing application In case you are using Photoshop 6.0 or later jump here.

Choices for Calibration

There are two methods or ways to calibrate the imaging system linearly.

System level gamma calibration may be available, If this is the case it is built into the display driver hardware and -software and the calibration is in effect system wide, for every application. Also AdobeGamma provides system level calibration. It is far better choice to use system level calibration. 

Calibration inside the image manipulation software. If there is no system level gamma calibration available then it is possible to use the corresponding controls of an image manipulation software. In this case the calibration is only effective when the images are viewed using that software. 

Only one method should be used for monitor calibration. If you choose to calibrate using the display driver software then verify that the monitor calibration section of your image manipulation software is disabled. If you choose to calibrate with the image manipulation software then disable the calibrations that are provided by the display driver card. If both calibrations are active then there are two conversions (of the 8-bit data, so two times the round-off error) in the display path and a chance that calibration is not correct.

Preparations for the Calibration

This calibration is for display driver calibrated systems.

The gamma setting and the Black-level control (Brightness) and/or the Gain control (Contrast) of the monitor most often affect each other so several calibration iterations are usually required. 

The calibration of a monitor can be done very accurately using a properly designed gamma chart. While doing the calibration the color- or gamma- calibration dialog of the display driver software (or image manipulation software) should be open and visible. 

If the system is to be calibrated using the display driver software then it is sufficient to view the gamma chart in the browser. If the calibration is done using the image manipulation software then the gamma chart must be downloaded and viewed using the software, since in this case the calibration is only in effect when images are viewed in such application.

 

The Monitor Calibration Chart

This monitor calibration chart is very accurate it use a novel dithering technique that allows visual gamma evaluation over the complete intensity range. Gamma chart must be viewed at such distance that the line dithering of the gamma sections are fully averaged by the eye so that individual lines can not be discerned. Un-focussing the eyes or squinting (narrowing) the eyelids may also help.

Monitor calibration chart icon

There must be good overall match between the dithered and continuous tone portions of all the four gamma swatches in order the calibration to be valid.

Note that the gamma chart itself is not enough for calibration since it is affect by the Brightness setting (black-point) of the monitor, see step 2 below.

Monitor Calibration

Step 1: Set up Proper Viewing Situation

The first step of the monitor calibration is to set up proper and pleasant viewing situation. In general keep the room lightning constant from session to session, keep reflections from the monitor surface at minimum and have the the intensity of the interior lighting more towards dim than bright. The interior lighting however should be as pure as possible, in other words the color temperature of the light source should be 5500K or above. What comes to the room lighting, in most cases one needs to take what is given but if you have the choice, use it.  Make sure that the monitor has been on for at least one hour in order it to stabilize.

Step 2: Adjust the Brightness and Contrast of the Monitor

You need to read this page to accomplish this step accurately.

Step 3: Getting close

Now view the gamma chart in the browser and adjust the gamma control of the display driver in "locked" mode so that the red, green and blue gamma-sliders are moved together. Adjust the gamma control for a close gamma match between the dithered and continuous tone portions in all the four gamma swatches.

Step 4: Fine-tune gray balance (remove color cast)

Now, the task is to adjust the gamma sliders separately so that the chart will appear as neutral gray as possible.

To start with, make a large changes to the red gamma slider in order to familiarize how it affects to the gamma chart. In addition to an overall gamma change you will notice hue change in both the continuous tone portion and dithered portions of the gray swatches.
 
Then use the red slider to remove -or- to balance as much as is possible the reddish tint between the continuous tone portions and in the dithered portions of all the gray swatches.
 
Now use the blue slider to remove -or- balance as much as possible the bluish tint between the continuous tone portions and in the dithered portions of all the gray swatches.
 
Repeat both the red slider and blue slider adjustments. 

If you now determine that you need to change the overall gamma then increase/decrease the green slider a little.  This will offset the color balance so you need to repeat the red-slider, blue-slider then again red-slider and again blue-slider adjustment as described above. 

Adjust & evaluate until accurate.