Mailing Lists: Apple Mailing Lists
Image of Mac OS face in stamp
 
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: LED displays: some comments and observations.

Just to add to some of the items that Tom mentioned in his recent very informative post:


Date: Thu, 06 Dec 2007 08:25:51 -0500
From: tom lianza <email@hidden>

Led backlit displays are a relatively new technology and one should
expect that there will be a long learning curve in this market.  The LED
technologies do not optimally align with the filter sets of the LCD
panels in use today.  Changing the filtration in a panel is a rather
monumental, high risk, undertaking, so you are going to see this issue
for a relatively long time.  The reason the LED panels are hard to get,
is that they hard to build at any price, and it is nearly impossible to
make money at the price they are selling at.

A major problem with producing a "wide gamut LED monitor" by replacing a CCFL backlight with one based on RGB LEDs is the RGB color filter characteristics are not optimal, as Tom correctly stated. The standard blue filters let too much green though when combined with the lower dominant wavelength green LEDs.

What this means is that without using a customized RGB filter designed specifically for LED backlights, the blue output can actually end up having a smaller gamut than most standard CCFL based displays. The blue primary ends up being well inside the sRGB and AdobeRGB gamuts - meaning that it is impossible to faithfully display saturated blues and magentas of these colorspaces.

The countermeasure for this is of course to make a custom RGB filter set for the LED panel that has improved filtering of the blue to allow less green to pass through. While that sounds easy, in practice it is quite difficult and very expensive. The LCD RGB filters are printed by the millions for use on standard CCFL based displays - hence they can be churned out for just dollars each. Creating a custom filter design means a new custom ink formulation, which then needs to be tested to make sure it is stable over the lifetime of the display. Couple that with the very small market for wide gamut LED displays (in the tens of thousands), the resulting limited production runs, and the cost of the filter alone soon skyrockets.

This high startup cost was reflected in our first LED based display, the LCD2180WG-LED which has a custom RGB filter in order to achieve a gamut that meets and exceeds the blue of sRGB and AdobeRGB.

In addition to the RGB filter, there are also a whole host of other issues that need to be resolved when using RGB LEDs in order to make a true "professional reference" display - meaning one that is color stable, both in the short term (minutes and hours), but also over the lifetime of the display; and uniform in color and brightness across the screen.

It is not as simple as replacing the CCFL backlights with some RGB LEDs and a power supply. LEDs are inherently unstable and difficult to regulate. For example as you try to increase the brightness by increasing the power to the device, beyond a certain point the efficiency actually decreases, which reduces the light output. Red, Green and Blue LEDs age at different rates, and the rate of aging is a function of their ambient working temperature, so good thermal management and monitoring is essential within the backlight. Multiply these issues by the number of LEDs in the display and you can see it becomes very complex. Also the R, G and B light from the individual LEDs needs to be diffused together to achieve excellent color uniformity across the screen. Couple that with the temperature vs. dominant wavelength shift issues that Tom mentioned and it becomes no easy (or inexpensive) task to design a professional LED display.


Tying this blue filter issue in with a different topic discussed a couple of weeks ago relating to gamut sizes, and how the display industry currently has no standard for this specification: It is currently very easy to market a "wide gamut" display that claims >100% of AdobeRGB, or whatever, but is actually deficient in key areas such as the blue primary, by simply quoting a "gamut size" rather than "gamut coverage" value. I for one would like to see a standardized metric for color gamut in the display industry to avoid these loopholes.


As far as affordable devices to actually measure and calibrate these LED displays, we chose to go with a custom calibrated iOne Display V2 for a variety of reasons. The standard off the shelf device did a fairly good job of measuring the display (at least compared compared to some other devices), however in the end we decided to offer sensors individually re-calibrated by X-Rite to match the characteristics of the LED display. While the iOne Pro does an excellent job at measuring chromaticity on the LED display, it unfortunately suffers in other areas as discussed here recently, not to mention being rather expensive.

Will Hollingworth
Manager of OEM Product Design & Development Engineering
NEC Display Solutions of America, Inc.
http://www.necdisplay.com

_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list      (email@hidden)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/colorsync-users/email@hidden

This email sent to email@hidden


Visit the Apple Store online or at retail locations.
1-800-MY-APPLE

Contact Apple | Terms of Use | Privacy Policy

Copyright © 2007 Apple Inc. All rights reserved.