How digital screens display black color

Monitor calibration

Monitor Calibration: An Introduction

One distinguishes between calibration and characterization (or Profiling) of the monitor. Calibration comprises everything that you can set on the monitor independently of the computer - in the past using rotary knobs, today mostly using buttons and menus displayed on the screen. In addition to the geometric adjustments of the display, these are above all Color temperature, brightness andcontrast. Strictly speaking, it belongs to one made via the graphics card Gamma setting also about it. However, using the graphics card for this is not recommended. It is better to use a special monitor calibration program (with a measuring device) or a makeshift software tool such as Adobe Gamma.

The calibration is used to put the monitor in a defined state, which at the same time makes optimal use of its technical possibilities. Calibration is a prerequisite for optimal color representation and should therefore always be the first step.

In the second step - profiling - the properties of the monitor are communicated to the operating system. In this way, programs such as Photoshop can for their part correct non-optimal monitor displays, but only within the limit values ​​that are specified by the calibration.

The goals of calibration and profiling are the best possible use of the monitor color space, the visually uniform display of tonal value gradations (as on the gray scale shown below) and - as a high school, so to speak - the color-correct display of images including a preview of their display on other output media ( e.g. printer). You should at least get close to the first two goals with the help of this and the following pages.

What the monitor setting cannot do: It does not guarantee that your exposed or printed digital photos are exactly the same as the monitor image. Since the printer color space is generally smaller than the monitor color space, this would not be desirable at all - it would mean artificially cropping the monitor color space. The opposite is correct: optimization of the monitor display, then optimization of the print output in order (if at all possible) to match the print result to the monitor display.

Uniformly graded gray wedge with 18 gray fields from tone value 0 to 255,
Gradation of 15 tones.

 

This and the following statements refer primarily to CRT monitors, in German: CRT monitors (CRT = Cathode Ray Tube). The modern flat LCD monitors (Liquid Crystal Display) still cannot keep up with good CRT monitors in terms of color display (at least not within similar price ranges). In addition, some of the settings described below are not even possible with them.

In case you're not sure if you have a CRT or LCD monitor - here is the ultimate test:

Look at the adjacent picture (possibly from a greater distance). Are the left and right halves the same shade of gray? Then you are sitting in front of an LCD screen. But if the left half of the image is significantly darker than the right, then you are (probably) sitting in front of a CRT monitor.

This test has a very serious background. The two halves of the picture show exactly the same picture, but rotated by 90 degrees. It is a grid of 1 pixel wide black and white stripes. In the left half the stripes run vertically, on the right horizontally. With optimal monitor resolution and a greater viewing distance, the stripes should no longer be distinguishable; instead, the eye sees a uniform gray. The individually controlled transistors of the LCD monitor do not care whether the stripes run vertically or horizontally. But not the electron beam of the CRT monitor: It is guided line by line (horizontally) across the screen and, in order to be able to display the vertical stripes, has to switch from 0% to 100% radiation intensity and back at a very fast rate (more than 100 megahertz) become. No cathode ray tube can do this completely seamlessly. The 100% target radiation intensity is no longer reached at all when you wipe the spot where the white pixel is to be displayed for a short moment - the grid therefore appears overall darker than intended.

Such grids generally serve as "reference gray" in the visual monitor setting. But they must always run horizontally, otherwise the eye on CRT monitors will find a wrong, too dark reference and the entire setting will go wrong. For the same reason, reference patterns made up of black and white pixels distributed like a checkerboard (as they can sometimes be found on websites for monitor calibration) are unsuitable for CRT monitors.

No rule without exception: If you are sure that you have an LCD monitor and still see the two gray fields in different brightness, then it is probably a monitor with special, non-rectangular color pixels. Such pixels are used in expensive monitors in order to increase the image quality and the viewing angle, but they produce direction-dependent effects similar to those of the CRT monitor.

Brightness (black point)

The brightness setting influences the basic brightness of the monitor, i.e. the brightness of black (!). This should be a little lighter than the black when the monitor is switched off. If the brightness settings are too low, tone value gradations can no longer be recognized in dark areas of the image. If the black point is too high, such differentiations are possible, but the picture looks flat. Setting aids for the black point can be found here.

Contrast (white point)

The contrast setting only influences the maximum brightness of the monitor. This of course also influences the contrast (since the minimum brightness is not changed). A high contrast makes it easier to recognize small tonal value gradations. The maximum setting is still not recommended as it can shorten the life of the monitor. Lower settings are better (and easier on the eyes). This also allows the contrast to be readjusted later (the maximum brightness of CRT monitors decreases with age) and thus guarantees a reasonably constant screen display for many years.

Due to the great ability of the eye to adapt to differences in brightness, an exact visual setting of the contrast is hardly possible. For this purpose, measuring devices are required which measure the brightness objectively, but which cannot be discussed at this point.

A test image with which you can assess the tonal value differentiation in the area of ​​the greatest monitor brightness can be found here.

Color temperature

The second important property of monitor white (besides brightness) is color. What we perceive as white is quite flexible - the eye normally makes a white balance on the brightest areas of the image within fractions of a second, which is why we perceive a sheet of paper in the yellowish incandescent lamp as white as in the bluish light of the midday sun. Our eyes correct even a strongly colored monitor image, so that we only notice the error in direct comparison with a neutral environment.

The color of white is measured as the color temperature in Kelvin. A monitor color temperature that corresponds to that of the ambient light under which we also view reflective documents (photos, printed images) would be ideal. In most cases, however, this cannot be achieved (the ambient light alternates between daylight and incandescent light, for example) - but it is not particularly critical because of the quick adaptability of the eye.

The color temperature of 9000 Kelvin (and more), which is usually preset for brand new monitors, is, however, far too bluish. In the printing industry, the standard light D50 (5000 Kelvin) is often used, which in turn appears unusually yellowish. A good compromise - and also standardized - is 6500 Kelvin (standard light D65).

An exact setting of the color temperature again requires a measuring device. In most cases, however, the selection of the desired color temperature in the monitor settings menu is completely sufficient. Color casts in shades of gray - which are often particularly annoying - can also be eliminated to a limited extent by gamma correction of the individual color channels. A test image for assessing the gray display can be found here.

Other important settings:

Monitor resolution

Only with LCD monitors is the number of pixels displayed by the graphics card (logical resolution) exactly set to the number of existing liquid crystal cells (physical resolution). With CRT monitors, the two values ​​differ more or less from one another. The manufacturers usually give an "optimal resolution", which one should also choose.
The optimal resolution can be determined from the dot pitch (distance between the phosphor triples or strips) and the screen dimensions: A dot pitch of 0.28 mm results, for example, with an image width of 36 cm (19 inch screen) a physical resolution of 1286 pixels in width. In this case, choose (if possible) 1280 x 960 pixels as the optimal logical resolution.

Color representation

Here you should always choose 24 bit (true color) or better (if available) 32 bit.

Room lighting

Although this is not directly part of the monitor settings, it plays a major role in the brightness and color perception of the monitor image. Room lighting that is good from a health point of view (poor lighting conditions can lead to headaches, concentration can decrease) does not necessarily correlate with the optimal lighting for the monitor. That is why monitors are often shielded from lighting from the side by means of screens. For the lamps and luminaires themselves, those should be chosen whose light spectrum is similar to that of daylight. Halogen lamps and the new RGB LED technology are therefore better suited than incandescent lamps or energy-saving lamps. If it is to be fluorescent tubes, pay attention to the last three digits in the name: The first denotes the light quality (7 is bad, 8 average, 9 very good), the last two stand for the light color in Kelvin. The number "940" means: Good quality of light with a light color of 4000 Kelvin.