Hardware calibration of LCD monitors
Hardware calibration of LCD monitors
- Subject: Hardware calibration of LCD monitors
- From: Bob Burnett <email@hidden>
- Date: Thu, 19 May 2005 09:48:37 -0700
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
---
[This E-mail scanned for viruses by Declude Virus]
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden