Display calibration

The display is our window to the digital world and therefore to the world of digital images. But who wants to see the world through a muddy or color-falsifying window? A decent display and its optimal set-up belong to the basic conditions of correct image processing.

Display calibration: An introduction

There is a difference between calibrating and characterizing (or profiling) the display. Calibration means everything you can set up on your display independently from your computer. In most cases this is done by using the display's panel in combination with menus shown on the screen. Beside geometrical alterations there are in the first places: color temperature, brightness and contrast. Technically, setting the gamma value that is in the graphics card belongs also to calibration. Using the graphic card's set-up tool for this job is not recommended, though. It is much better to use a special display calibration program (comes with calibration devices) or - lacking one of these - a tool like Adobe Gamma.

The goal of calibration is to get the display in a well-defined state in which at the same time its technical possibilities are used optimally. Calibration is the precondition for optimum color rendition and therefore should be the first step of the whole process.

Through the second step - profiling - the operating system gets to know the display's characteristics. After that programs like Photoshop are able to correct for non-optimal color rendition, but only within the boundaries that are set during the calibration process.

The goals of calibration and profiling are optimal use of the displays color gamut, the visually smooth rendition of tonal value shifts (like the grayscale gradient below) and - the fine art of color management - the color-correct rendition of images, including previews of how the image's colors would look like on other output media (like printers).

You should come close to at least the first and the second goal with this and the following pages.

What display calibration cannot do for you: It does not guarantee that your prints will match your monitor output. This is not even desirable because displays can produce colors that no printer could ever match. So instead of restricting the display's gamut the following way is the right way: Optimizing the display's output, then optimizing the printer's output in order to bring the print results as much as possible into the direction of the original.

The following sections are related especially to CRT displays (CRT = Cathode Ray Tube). Most modern flat LCDs (Liquid Crystal Display) still cannot match very good CRTs when it comes to color rendition. Also, they cannot be set up in ways a CRT can be.

If you are unsure whether you have a CRT or an LCD, here comes the ultimate test:

Look at the image to the right from a sufficient distance. If both halfes appear to have the same brightness, then you are sitting in front of an LCD. If the left halfe is considerably darker than the right, then in all probability you own a CRT.

This test indeed has a no-nonsense background. Both halfes show the exact same image, yet one halfe is turned by 90 degrees. It is a grid consisting of 1-pixel-wide black and white stripes. In the left halfe, the stripes are vertical, in the right halfe they are horizontal. Considering sufficient display resolution and viewing distance, you should not see any difference between them. Both should be seen as solid gray. The transistors of an LCD are controlled separately and therefore it is the same wether the stripes are horizontal or vertical. With a CRT's electron beam things are different: The beam is guided over the screen area line by line (horizontally). In order to display the vertical stripes the beam's power has to change abruptly from 0% to 100% and back. No cathode ray tube is able to do so perfectly without leaving any trace. More so, the short time during which the single white pixels are drawn is not enough for the beam to reach 100% of its power. Therefore, the whole grid appears darker than planned.

Such grids normally serve as a reference gray for gamma calibration. As you can see, it is important that these grids have horizontal lines, otherwise on CRT monitors the reference would be too dark and the procedure would fail. For the same reason reference patterns consisting of chessboard-like positioned black and white pixels (like those to be found on some web-sites regarding display calibration) are not suited for CRTs.

Of course there are always exceptions to the rules: If you are sure that you have an LCD and nevertheless both gray areas look different (from a sufficient distance...), then you probably are the owner of a display with special non-square color pixels. Such pixels are used in expensive LCDs to enhance the rendering quality and the viewing angle, but they do cause direction-dependent effects quite similar to the ones of a CRT.

Brightness (black point)

The brightness setting influences the basic brightness of the monitor, i.e. the brightness of black(!). It should be a little brighter than the black of the turned-off display. If the brightness is too low, differences of tonal values near black cannot be distinguished. If it is too high, these differences can be distinguished very well, but the image looks dull. Aid for setting the black point can be found here.

Contrast (white point)

All the contrast setting does is alter the maximum brightness (brightness of white) of the monitor. This also means that the contrast changes because the minimum brightness stays the same. A high contrast allows for enhanced distinguishing of small tonal value shifts. Using the maximum contrast setting is not recommended, though, because it can shorten the monitor's life-span. Better use a more moderate setting which at the same time is more friendly to the eye. This also allows for adjustments later-on (the maximum brightness of a CRT is declining with the time) and therefore permits a nearly constant color rendition over many years.

Because the eye easily adapts to brightness differences setting the contrast visually is far from exact. To do this exactly, you will need a measurement device that can measure the brightness objectively, but these are unfortunately not covered here.

You can find a test image here to examine tonal value differentiation in the region of the maximum monitor brightness.

Color temperature

The second important characteristic of the monitor's white (besides its brightness) is its color. What we perceive as white is in fact very versatile - normally the eye does perform a white balance procedure on the brightest parts of an image within fractions of a second. That is why we perceive a sheet of paper under yellowish tungsten lighting and under the blueish light of the midday sun as being equally white. Our eyes are able to adapt even to a monitor image with a heavy color cast, therefore we can only notice such errors in direct comparison with a neutral environment.

The color of white is measured as the color temperature in Kelvin. A color temperature corresponding to the temperature of ambient light, under which we examine photos or prints, would be optimal. In most cases, this cannot be accomplished (e.g. when the ambient light changes from daylight to tungsten light) - but fortunately this is far from being critical because of the eye's ability to adapt easily.

On the other hand, the factory setting of most monitors is often above 9000K, which is way too blueish. The printing business often uses the norm light D50, which corresponds to 5000K and is therefore quite yellowish (and takes some getting-used-to). A good compromise is the norm light D65 which corresponds to 6500 Kelvin.

Setting the color temperature exactly is only possible with a measurement device. In most cases though, selecting the preferred color temperature using the corresponding display setting is fully sufficient. Color casts of mid-tones that can be quite disturbing can also be eliminated (to a certain degree) by correcting the gamma values of the RGB channels separately. Here you will find a test image for examining gray rendition.

Other important settings:

Display resolution

Only with LCDs the number of pixels displayed by the graphics card (logical resolution) matches the number of available liquid crystal cells (physical resolution) exactly. With CRTs both values differ more or less from each other. The manufacturers mostly recommend an "optimal resolution" which normally should be selected.
You can calculate the optimal resolution yourself from the dot pitch (the distance between the phosphor triples or stripes) and the screen dimensions: E.g., a dot pitch of 0.28 mm and a screen width of 36 cm (of a 19-inch CRT) result in a physical resolution of 1286 pixels in the horizontal dimension. Therefore, the optimal logical resolution would be 1280 x 960 pixels.

Color rendition/Number of colors

This should always be set to 24 bit (true color) or better 32 bit, if available.

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