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Hardware calibration of LCD monitors

When LCD displays first came out the last thing you ever wanted to do was to touch their internal color settings. Color temperature on an LCD is defined by the color of its backlight, which cannot be adjusted. There are no electron beams to alter the voltage of. The only control that there is over the backlight is in its overall brightness, which does not affect the color.

Initial attempts to apply color changes to these LCD displays were implemented in the form of a gamma adjustment. The result of this was to leave the white point exactly where it was and then change the color of the grays. This caused horrific color shifts, which meant that the calibration software had to make two opposing color corrections, producing channel crossovers and a general mess.

SGI at one point had an interesting approach to this problem. They created an LCD that integrated two differently colored backlights. By changing the mixture of the two backlights you could change the overall color temperature of the display, including white. This display went the way of SGI but it was a solid approach at solving the problem.

Today's 10, 12 and 14 bit LCD displays work a bit differently. Basically they "pretend" to be CRTs. You'll see terms like gains and maybe even bias being applied but that is merely to use words that are familiar. In reality they have one or more lookup tables, just like the ones in the graphics card. Applying a correction through this internal lookup table does exactly the same thing as applying a correction in the graphics card, except for one important thing, they are 10, 12 or 14 bit. If the display luts were 8 bit, like your graphics card is, then the affect of the correction would be identical to applying it in the graphics card and there would be nothing gained. You'd still loose the same amount color resolution and would have the same banding issues. Because the internal display luts are higher bit levels there is more room for correction. Going to 14 bit luts gives you enough resolution to even be able to flip the correction curves over and have very fine detail over the shadow end, which is generally ignored.

Most of the controllable LCD displays are using the DDC/CI protocol for communication. This means that they can be controlled right through the display cable. This makes it very convenient but it does also mean that the graphics card will need to support DDC/CI in addition to the display. Though not a problem on Macs, this is something that you'll need to pay attention to on PC systems. NEC has a pretty good list of DDC/CI supporting graphics cards. (http://www.necdisplay.com/naviset/naviset_compatibility.htm) Its a few years old but I haven't found a better list yet. The Eizo displays are different in that they use a proprietary USB interface to communicate with the displays. This avoids the display card compatibility issues entirely.

With these new displays with higher bit luts you are absolutely better off calibrating them internally and leaving the graphics card correction to a minimum.

Bob

On May 19, 2005, at 6:32 AM, email@hidden wrote:


The Eizos (many of them not just the ColorEdge series) have adjustable
color temp also. The adjustment is not accomplished with a graphic card
LUT adjustment as the adjustment is made from the monitor. The question
of "how" still exists though. Doesn't the monitor have a LUT of it own,
hence the "14 bit" and "10 bit" internal LUT claims? I agree with your
premiss that you can't vary the color of the actual light output as
there is only a single source. It must change the appearance of the
white point by altering the color of all pixels.

David 

I'd like to know the answer of that one too. Maybe someone from Eizo
monitoring the list could do everyone a favor by explaining how this is
done? If this is not a commercial secret.

Roger Breton  |  Laval, Canada  |  email@hidden
http://pages.infinit.net/graxx




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