Re: LCD dot pitch and calibration devices
Re: LCD dot pitch and calibration devices
- Subject: Re: LCD dot pitch and calibration devices
- From: "tlianza" <email@hidden>
- Date: Tue, 8 Mar 2005 07:52:43 -0500
Hi to all
Jorgen Jannson wrote
Hi list,
Does the dot pitch of LCD screens have any effect on the measuring
accuracy of the most common screen calibration devices (EyeOne, Spyder,
X-rite...)?
Would the same instrument return the same values on two screens with
different dot pitch, assuming everything else is identical? If not, would
it matter if it's a colorimeter or a spectro?
A friend claims that quality control checks at the LCD factory is
performed from a 50cm distance to prevent interference from dotpitch
differences, and wants to know if it also affects calibration routines...
TIA
Jorgen Jansson
The 50cm distance certainly isn't found in any standard that I know of. It
sounds too short to meet the VESA specification (for many instruments) and
much longer than a QC product such as the Minolta CA210 is designed to work
at. We have found no problems due to dot pitch on LCD's with our products.
They have been tested on a wide range of LCD displays. The problem that you
are discussing is more common with large, low resolution plasma displays.
Here, the pixel pitch is at least an order of magnitude different than the
LCD.
In general, the need for the long distance measurement is based upon the
non-uniform light output of the display, with respect to both spatial
variation and spectral variation. By pulling the measurement away from the
screen, it is easier to meet the 2 degree- 2degree geometry constraint of
the VESA specification. This specification states that the entrance pupil
of the instrument should subtend 2 degrees or less, and that the Field of
view should be 2 degrees or less. If you have an instrument that measures
over 2 degrees or less, the number of pixels that it records is quite
dependent upon the distance from the screen. The 2 degree entrance pupil
requirement simulates the eye viewing axially at 250mm. All of the products
that you mention on this list depart significantly from the Vesa standard
geometry. The reason is that they are not QC products, they are calibration
products. I've written a number of white papers on this subject. It's a
particularly important detail in the medical field. Doctors don't view
axially. Specifications like the VESA spec are required for making
engineering and QC measurements of this display, but often can be very
misleading in calibration situations.
The approach that we take at GretagMacbeth COEDT, is to use geometries that
simulate the collection angle of standard binocular viewing. Humans don't
look at a display with one eye on axis. Other competitive offerings to the
EyeOne Display use an even wider field of view, which is valid under most
conditions suited for the device usage. One competitor also mentions an
asymmetric collection geometry in their literature. This is not something
that I agree with, but each technical organization has its own style. The
higher resolution, wide field of view, panels that are common in this
industry, minimize the need for stringent angular considerations. There are
certainly more significant factors than "interference form dotpitch
differences" that influence display measurements.
Tom Lianza
Director of Display and Capture Technologies
GretagMacbeth LLC
3 Industrial Drive
Unit 7&8
Windham, NH 03087
603.681.0315 x232 Tel
603.681.0316 Fax
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