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2009.06.02

Sadly, not much information can be found about this important subject. It truly deserves a long discussion but – until somone writes up something longer – you will have to settle for this blog post!

Color-calibration is important to see accurate image colors and to best visualise photographs. Often, we get comments from people who notice that their images have different colors on different monitors. This happens because monitors are not all capable of showing the same colors and because not all monitors show the same color for the same input – even for the colors which they can display. The former problem is determined by the color gamut of a display, while the latter is controlled by color-calibration, or lack thereof.

The simple solution is to tell people to calibrate their monitors. So, a lot of users search the net and find ways to calibrate their video cards! Unfortuntaly, these are not equivalent, although most online articles refer to both as Display Calibration. The difference is subtle but quite important for users of LCD displays. For those lucky enough to still use a high-quality CRT, it is much less important.

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Calibration works by translating the color-value of each image-pixel into the color-value used to display it on the monitor so that it appears as its intended color or so. Given an sRGB image and a perfect sRGB display, the translation should not change any values. However, since displays are generally not perfect, some translation is often necesary. Image pixels are represented as tripplets of numbers, one for each of the red (R), green (G) and blue (B) additive color primaries. In most cases, these numbers are 8-bit values running from 0 to 255. All but two current LCD displays only accept 8-bit values as well. This is a limitation of the DVI-D connection which carries in its protocol at most 8-bits per color-component. Analog signals used by CRT monitors do not have this limitation and neither does the new DisplayPort connector, which supports up to 16-bits-per-component, but only if the display can accept it.

The difference between calibrating a display and a video card comes from these numbers. Since almost every LCD on the planet can only accept 8 bits-per-component, calibration of the video card can only send 8-bit values to the display. This gives rise to precision problems since a translation from 8-bits-per-component to 8-bits-per-component has to create gaps and overlaps in its output. For example, say the red component is 1/4 too bright, red image values 7 and 8 will be both transalted to 6 for the display. Such precision problems will appear as banding on the display. The opposite – posterization – is also possible when the translation does not use all possible display values.

Display calibration avoids this problem to a large extent by doing higher-precision translation after the signal is sent to the display. In this case, image pixel color-values are sent unchanged from the computer to the display. All 8-bits-per-components are used, so banding and posterization are not introduced in the signal. Instead, the display translates from the 8-bits-per-component signal to a 10, 12 or 14-bit precision using an internal calibration table. To set this table one must calibrate the display and not the video-card. This requires a calibration device which plugs into the monitor directly and usually model-specific software to control it.

The catch is that not all monitors are capable of doing this. It used to be the case that only expensive high-end monitors had this ability but this is no longer the case. The cheapest calibratable LCD available now retails for $499 USD or $585 CDN. That would be the NEC Multisync P221W which has 10-bit internal tables.


6 responses to “Calibrate Your Display, Not Your Video Card!”

  1. Zak says:

    Very interesting, I didn’t realize there were different things that could be calibrated. What I still don’t get though is what happens to colors the monitor can’t display?

  2. Itai says:

    The gamut of modern LCD monitors varies but some models actually cover all of sRGB, AdobeRGB and NTSC color-spaces. This means that all colors represented by an image file from a digital camera can be represented.

    For the majority of LCD monitors which cover between 60% and 85% of sRGB, there are a good deal of colors which cannot be represented. Those colors have to be mapped to similar colors that the display can actually show. It is up to the calibration solution to make the decision but there are generally two approaches:
    (1) Match colors that are displayable exactly and match the ones that are not with the most similar color.
    (2) Find similar colors while maintaining the relationship between colors at the expense of accuracy.

    The first approach is more accurate but can show artifacts such as blotching for areas with a lot of similar-but-out-of-gamut colors. The latter may show the wrong colors but generally display gradations better.

  3. Dagge says:

    What other monitors, apart from the Nec Multisync P221W, supports the 10-bits (or better) tables?

  4. Itai says:

    NEC has both the 30″ NEC MultiSync LCD3090WQXi and the 26″ NEC MultiSync LCD2690WUXi2 which support 12-bit tables and cover at least 97% of the AdobeRGB color-space.

    They also have the 21” NEC MultiSync LCD2180WG-LED which features 10-bit tables and 10-bit input for future-compatibility. This means that future graphics cards will be able to able to output 10-bit per-color-channel directly from the graphics card. This particular display covers 107% of the AdobeRGB color-space.

    Eizo has several models supporting 12-bit tables such as the ColorEdge CG301W, the ColorEdge CG242W, the ColorEdge CG241W, the ColorEdge CG221 and the ColorEdge CG211. All these Eizo models cover 98% of the AdobeRGB color-space.

  5. laptop says:

    Are there any calibrated laptops?

  6. Itai says:

    Laptop displays in general have very poor gamuts, although this is improving with LED-backlit displays becoming more common. Most laptops support external displays via DSUB, DVI or DisplayPort connection. In this case you can always calibrate the external display as recommended here.

    There are laptops with color-calibration built-in but their gamut is limited to 72% of sRGB which is quite poor. So, at best, when calibrated you would get 72% of colors being accurate. These laptops are the Lenovo W700 and W700ds, the dual-screen version. These are quite expensive laptops, so it is highly more practical to buy an external monitor and hook it up when color-accuracy is needed.

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