Monitor calibration
The monitor calibration is the exact setting of the color and brightness display of a screen, especially with the help of color management .
Purpose of calibration
A reliable color representation is required for color-critical applications. Photographers and image editors must be able to trust the image display on their monitor in order to correctly assess image properties and, if necessary, make corrections. The same applies to the color-predictable creation of print templates, the design of graphics of all kinds and the assessment of the color reproduction of video films.
Most monitors, however, are delivered with quite arbitrary settings for colors, contrasts and brightness distribution. Even if a monitor has been properly preset by the manufacturer, the settings will need to be improved after a certain time due to aging.
Variants and areas of application
There are two types of monitor calibration:
- Color space emulation, d. H. full adaptation of the monitor colors to a specific working color space
- Color space-independent calibration and subsequent determination of a profile for use in color management-capable application programs
Variant 1 is only used where the use of color management is not possible. This is especially the case where no computer is used as a player, e.g. B. on televisions and digital projectors with TV receivers and video players as the image source. Today they are mostly set to the Rec. 709 color space for HDTV playback.
Color space emulation is used less often for computer applications. This variant becomes interesting when exact color reproduction is required, but the software used does not support color management. This is especially true today for video editing programs. The emulation of a standard color space (mostly sRGB) can also be useful for office software and other non-color management applications that can incorporate images.
The biggest disadvantage of color space emulation is the definition of a single working color space; the capabilities of the monitor (especially if it has a large native color space) are curtailed from the start. It should also be noted that cheaper computer monitors do not offer all the necessary setting options for a complete color space emulation.
Variant 2 is today's standard for computer applications and can basically be used with any monitor; what cannot be set directly on the monitor is simply corrected via the graphics card.
The method consists of two steps: the actual calibration and the profiling. During calibration, the color space of the monitor is linearized (i.e. uniform gray levels are ensured), the white point (brightness and color temperature) of the monitor is set to specific values and a brightness distribution (e.g. a specific gamma value) is specified. However, the size of the color space remains untouched as far as possible in order not to unnecessarily restrict the capabilities of the monitor.
After calibration, a profile of the monitor is measured and stored in the operating system as a standard monitor profile. Application programs that have mastered color management can use the profile to adapt the colors of the images to be displayed to the monitor color space. This corresponds to the basic working method of today's standard ICC color management, for which two profiles (source profile and target profile) are required at all times.
With variant 2, calibration is only a preparatory step (which can even be omitted in exceptional cases), while the actual color space adjustment is shifted to the respective application programs - for which the measured profile is required. The advantage of the process is that the monitor's native color space can be fully exploited - which is particularly important when using large working color spaces and so-called wide-gamut monitors. But unfortunately the color-correct display is limited to application programs that can handle color management; This is true today for almost all image processing, graphics and prepress programs, but not yet for video playback, video editing, office and games. In the browser area, there is partial color management, but mostly incomplete.
Technical implementation
CRT monitors have settings for contrast and brightness. LCD monitors also offer controls for backlighting , gamma and color temperature . The graphics card configuration program has controls for contrast and brightness and allows these controls to be applied individually to the three primary colors. The operating system has a setting option for gamma. With these many controls, it is helpful to have an adjustment procedure for good playback.
Adjustment of contrast and brightness
The contrast setting is a gain setting. The input signal of the monitor is enlarged more or less. If there is too much gain or contrast, a light gray color at the input of the monitor is displayed as white. The brightness setting adjusts the black level. An incorrect setting of the black value either displays a black color as a dark gray color due to too high a brightness, or a dark gray color is displayed as black due to a too low brightness.
The settings for contrast, brightness and gamma are set once on an LCD monitor. The backlight is the only setting in daily use. The backlight is called the brightness setting on some monitors and there is no black level setting.
The first step is to restore the monitor, graphics card, and operating system to the factory default settings. Set the backlight to 50% intensity and the color temperature to 6500K. In the uppermost area eight light gray numbers are displayed on a white background, in the second area eight dark gray numbers are displayed on a black background. In the second step, the contrast in the graphics card and in the monitor must usually be reduced and the brightness increased in order to see as many numbers as possible in the white and black areas. There are other test patterns for contrast and brightness. The third step is to set the gamma. This also brings the color temperature into an acceptable range.
Setting gamma and color temperature (white point)
The gamma setting influences the average brightness values. The gamma of sRGB is 2.2. The Apple Macintosh used a gamma of 1.8. An Apple Macintosh image with a gamma of 1.8 is displayed too bright on an sRGB system with a gamma of 2.2.
Color temperature and white point have the same meaning for the monitor. A high color temperature makes the monitor image bluish, a low color temperature makes it yellowish. The standard sRGB color temperature is 6500 K. If the monitor does not have a color temperature setting, the color temperature is set with the brightness setting of the primary colors of the graphics card.
After the basic setting, the monitor display can be improved by first increasing the contrast, then correcting the gamma value, then changing the brightness and correcting the gamma value again. In monitors with TN LCD panels, the gamma value changes quickly with the vertical viewing angle. Changing the vertical angle of such a monitor is often the easiest way to adjust the gamma value. The differences in the primary color gamma are compensated for with the color balance of the operating system or the brightness setting of the primary colors of the graphics card. Further information on the test pattern is available under Gamma correction .
In most cases, the same result can be achieved by setting the monitor (contrast, brightness, gamma), the graphics card (contrast, brightness) or the operating system (gamma). In some cases, the usable range of the setting is different. In these cases, the setting on the monitor often results in a larger usable area.
A tristimulus colorimeter automates the setup process. High quality monitors have hardware color management and allow hardware calibration, i. H. the results of the colorimeter are stored in the monitor. Another approach is for the colorimeter to set the ICC profile in the operating system. The first approach gives the best results. Usually, however, the monitor and colorimeter must be from the same manufacturer. In the early days of color management, templates on photo paper were used, which the user should hold next to the monitor as a reference in order to then adjust the monitor's colors by eye. Individual photo laboratories still sell such test images to this day. In practice, however, they have not proven themselves; At best, you can judge the brightness distribution, but you can never correctly recognize the color range of a monitor (and even if you could, most monitors would not have enough setting options).
Calibration targets
There is no absolutely correct monitor display, as the human eye unintentionally adapts to different brightnesses and color references. The representation can at most be correct in itself, i. H. in relation to a medium gray of the monitor. For this reason, before calibration / profiling, the user can determine to which white point and which brightness distribution one is to calibrate.
Anyone who always concentrates entirely on the monitor image and uses a gray screen background as a reference for the eye can choose the values almost at will; if the same monitor is used in different lighting conditions, you have no other choice anyway.
However, it is more pleasant to adjust the monitor to the ambient light. In graphic studios and print shops it is even common practice to equip the room with special standard light and then to calibrate the monitor to its values. This has the advantage that the eye does not have to constantly switch between the ambient light and the monitor, and that even laypeople (who have not learned to consciously assess the colors within the monitor) get a correct display.
The most common lighting standards are D50 (5000 K color temperature) and D65 (6500 K color temperature), with the brightness mostly between 80 cd / m² (pure image processing environment, rather dark) and 120 cd / m² (normal room brightness, also for assessing paper originals suitable) is set.
If the room light does not correspond to any standard, the monitor white point will at least be approximated. It is important that the brightness is reasonable. In very bright surroundings, values significantly higher than 120 cd / m² can be necessary. If it gets too bright (e.g. daylight from large window areas), even bright monitors with their 300 to 400 cd / m² cannot keep up; No color-critical work is then possible there.
Another calibration target in addition to the white point is the brightness distribution - often referred to simply as gamma. A gamma value of 2.2 or the so-called sRGB curve is usually recommended, because this fits the majority of the unprofiled images. As soon as the application program masters color management, the brightness distribution is adapted automatically anyway; even if the monitor was calibrated to gamma 2.2, images with gamma 1.8 (e.g. ProPhotoRGB) or L * (e.g. ECI-RGB V2) are still displayed correctly.
Correspondence between monitor and paper photos
Calibration and profiling in conjunction with color management ensure that colors are displayed correctly in terms of the color space definition. However, this does not automatically correspond to the appearance of a paper print; For technical reasons, photo paper cannot display all the colors of the common working color spaces, and it has a lower contrast range than most monitors.
In order to simulate the appearance of a print or an exposed photo on the screen, a so-called softproof is used (also called print simulation); many image editing programs offer such a function. In addition to the profile of the working color space and the monitor profile, a profile of the printer or imagesetter is also required.
The soft proof shows a good preview of the image to be printed. If, for whatever reason, paper images or objects are to be held directly next to the monitor for comparison, the ambient light must also be adapted to the white point of the monitor (or vice versa). Ideally, standard light is used for this. If the effort for setting up the standard light in the entire room would be too high, you can use a standard light box as an alternative; in it, the paper template is illuminated with a brightness and color temperature that corresponds to the calibrated white point of the monitor.
known problems
- Classic calibration / profiling only works completely if application programs are used that support color management; furthermore, color management as such requires some expertise. The emulation of a standard color space (sRGB) would actually be more suitable for laypeople, but is not even available in connection with inexpensive monitors.
- Common colorimeters (which work internally with three color filters) can be misled by the light spectra of certain monitor backlights (metamerism) and therefore require suitable correction tables; this is a possible source of error because the unskilled user can choose the wrong type of lighting and because new correction tables must first be created for monitors with new types of light sources. So-called spectrophotometers (as an alternative to colorimeters) can measure colors absolutely, but are more expensive and not as reliable as colorimeters in darker color areas.
- The color filters of the colorimeter are subject to aging and lose their accuracy over time. Expensive professional colorimeters can be readjusted by the manufacturer; The effort is usually not worth it for cheaper models.
Web links
Individual evidence
- ↑ Eizo monitor test . Eizo. Retrieved December 8, 2018: "If in doubt, carry out the test with different browsers."
- ↑ Photo Friday: Photo Friday Monitor Calibration Tool . Photo Friday. Retrieved November 28, 2018.
- ↑ Han-Kwang Nienhuys: The Lagom LCD monitor test pages . Han-Kwang Nienhuys. May 18, 2008. Retrieved November 28, 2018.
- ↑ Kalliopi Monoyios: Gamma and White Point Explained: How to Calibrate Your Monitor . In: Scientific American Symbiartic , Scientific American, January 17, 2012. Retrieved November 28, 2018.