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LED displays: some comments and observations.
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LED displays: some comments and observations.


  • Subject: LED displays: some comments and observations.
  • From: tom lianza <email@hidden>
  • Date: Thu, 06 Dec 2007 08:25:51 -0500

Hi to all,

There certainly has been a lot of speculation on this forum about Huey, i1Displays, filters, fields of view etc. I don't want to talk about specific combinations of product with a particular customer. Rather than get into trouble for something I might say that may violate an NDA, I will simply correct Graeme Gill's comments when I feel he has made a mistake. So here goes...actually, there really aren't any corrections at this moment, but I'm keeping an eye on the guy :) I want to emphasize that to the best of my knowledge, Graeme has no affiliation with X-rite. He is a very knowledgeable researcher and while I may occasionally disagree with him, he is normally right on target.

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.

The increase in Gamut achieved by the LED backlight is found mostly in the green. Rather than narrowing the bandwidth and increasing the output, physics has demanded that the dominant wavelength of the LED shift to a lower dominant wavelength than a filtered CCFL lamp. (Roughly 550nm -> 520-535). This results in an increase of gamut, but there are some real problems lurking here. The range of dominant wavelengths for the LED are a strong function of temperature and chemistry. For any given LED, the center wavelength and optical bandwidth of the signal changes with temperature. From cold start to maximum operating temperature, I have measured an xy chromaticity change on the order of .013 . (Note, that's one zero beyond the decimal point, not two.) If you look at the spectrum of the various choices of green leds you will see that their rising edge crosses the rising edge of the y-bar CMF and falls squarely in the interaction region between the z,x and y cmf's . What does this mean? It means that it is very difficult to measure chromaticities accurately with any instrument at any price. The difference between two very expensive spectral instruments can easily be .008 when measuring the same source instantaneously. There is another issue as well. If you believe that the cmf's are based upon human visual experiments and represent an "average", one should expect a rather high degree of observer differences when looking at the same displayed color. In short, you got your gamut, but it's probably not the same gamut for the guy sitting next to you. If you have an issue with inter-instrument agreement now, matters will only get worse with these new technologies.

Now there have been some comments about issues of choice of bundled product by region. If you have ever done business on a world wide basis, you understand that your product sells in the context of a local currency and your profit is reflected in the currency of the nation of incorporation. The dollar is extremely low. An importer can buy a US made product very cheaply, and make a handsome profit by selling it in a region that has a high currency/dollar ratio. The same is not true for the US. The US market is killing the display industry. Prices for displays are falling quicker than the dollar. Keep in mind what Graeme has said about the performance difference between a Huey and and i1Display. In a bundled situation, on a high end display with custom software, there is essentially no difference in performance. It is not the case that the US is any less critical about color, it is simply a question of a manufacturer staying alive through this very tough time, by making intelligent technical decisions that respect the financial realities.

I hope this answers some questions and I suspect that it will generate more thoughtful comments as well.

Take care,
Tom Lianza
X-rite corporation.





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