Re: LED displays: some comments and observations.
Re: LED displays: some comments and observations.
- Subject: Re: LED displays: some comments and observations.
- From: Roger Breton <email@hidden>
- Date: Fri, 07 Dec 2007 07:55:05 -0500
Hello Tom,
Thank's for taking the time to share those valuable insights with this List.
I, for one, always appreciate reading what you have to say about color
management, especially displays metrology.
Please allow to ask you to clear a few things, if I may.
> The LED
> technologies do not optimally align with the filter sets of the LCD
> panels in use today.
Do you mean 'aligned physically' as in the LEDs being somehow "off axis"?
> ...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.
Sounds like a tough business model.
> The increase in Gamut achieved by the LED backlight is found mostly in
> the green.
No gains in the red or blue primary?
> 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).
Different chromaticity then.
> 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.)
So, the implication for metrology is to wait, what, a good hour before
calibrating and profiling?
> 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.
The difference between a PR-655 and a CS-1000 could easily be .008 in y
chromaticity? On the green primary alone? I guess it amplifies whathever x,y
errors in the instrument calibration in this region of the spectrum.
But in spite of this shortcoming you were able to find an acceptable set of
correction factors for the XL for your two filter-based colorimeters?
> There is
> another issue as well. If you believe that the cmf's are based upon
> human visual experiments and represent an "average",
We all have to believe that!
> one should expect a
> rather high degree of observer differences when looking at the same
> displayed color.
You mean 'observer metamerism'?
> 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.
Thereby accentuating the need to rely on the instrument as a independent
judge.
>
> Tom Lianza
> X-rite corporation.
Roger Breton
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