Subpixels in Plasma TV's - AVS Forum
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post #1 of 18 Old 07-18-2013, 02:54 PM - Thread Starter
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So from my understanding of plasma tv's, each pixel on a plasma tv contains three sub-pixels (also called phosphors). Those three are the red, green, and blue phosphors. Now I am having hard time researching/finding out what EXACT wavelength each phosphor produces. I don't care about the intensity/brightness/luminosity of the wavelength, I just want to find out what is the exact number (or the exact range) of the wavelength that each phosphor produces.

I know that the red phosphor produces a wavelength in the red region of the visible spectrum but what is the exact wavelength number or the range, if it is not exact, of that phosphor? (in nanometers, nm, please)
Similarly I want to know this about the green and blue phosphor.

So can someone help me out with this?

Thanks
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post #2 of 18 Old 07-18-2013, 04:14 PM
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I'll be honest I really have no idea but it sounds to me this would change depending on make& model.
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post #3 of 18 Old 07-18-2013, 07:52 PM - Thread Starter
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Thanks for the reply, that is what I figured.

But does anyone here know for a specific make and model? Just so that I can get an idea of how phosphors work. Mainly do they emit a range or is it an exact wavelength, and what is the value in numbers.

Thanks
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post #4 of 18 Old 07-18-2013, 08:26 PM
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Blue will be about 450 nm. Green will be about 550 nm. Red will be about 630 nm. Of course, these represent the peaks of a gaussian distribution for the most common phosphors. I am only knowledgeable about phosphors used for scientific equipment but I expect the phosphors used in displays to be in the same ballpark.

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post #5 of 18 Old 07-18-2013, 11:08 PM - Thread Starter
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So are you saying that the wavelength of each phosphor is a range, not an exact value, where the middle/average of the range are the numbers you gave (450 nm, 550 nm, 630 nm) and are the most emitted wavelengths?

If that is the case, from what I understand Plasma TVs control the intensity/amplitude of the wavelength by applying different amounts of voltage, so how do they control the range of each phosphor?

I would presume if the phosphor was emitting an exact value, it would be easier to control it. If you add the variable of range in the phosphor, how do you control that?
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post #6 of 18 Old 07-19-2013, 06:43 AM
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If I'm understanding him right it's peak. Like if u graphed the wavelengths the highest peak on the graph would be those #s. this link shows the graph of a tuning fork. Similar concept just in sound. There is 1 peak but that doesn't mean all other frequencies r excluded from being represented
https://www.google.com/search?q=tuning+fork+frequencies&safe=off&client=safari&hl=en&source=lnms&tbm=isch&sa=X&ei=MUHpUcCfOIvS8wSX-YGQAw&ved=0CAkQ_AUoAA&biw=1024&bih=644#biv=i%7C31%3Bd%7C-oklQsvDLns7cM%3A
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post #7 of 18 Old 07-19-2013, 12:53 PM
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The red phosphor may be a little noisy, like for CRTs:
http://static.newworldencyclopedia.org/thumb/2/29/CRT_phosphors.png/400px-CRT_phosphors.png

I would expect a similar curve to CRTs, because it allows for more accurate representation, though this may not be so true for modern plasmas.
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post #8 of 18 Old 07-19-2013, 05:26 PM - Thread Starter
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So how does a TV produce accurate colors, since I thought an RGB signal only sends sends one value for each phosphor like R: 255, G:200, B:150, where the values correspond to the intensity of the wavelength. This would mean the RGB signal is assuming that each phosphor only produces one wavelength and not a range. Am I correct on this, or is my interpretation wrong?
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post #9 of 18 Old 07-19-2013, 11:57 PM
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Quote:
Originally Posted by hcps-sulemanma View Post

So how does a TV produce accurate colors, since I thought an RGB signal only sends sends one value for each phosphor like R: 255, G:200, B:150, where the values correspond to the intensity of the wavelength. This would mean the RGB signal is assuming that each phosphor only produces one wavelength and not a range. Am I correct on this, or is my interpretation wrong?

Your eyes are not perfect and have lots of compensation.
http://www.normankoren.com/Human_spectral_sensitivity_small.jpg

Notice the significant overlap between green and red. The brain performs a trick on the red-green information to make it more distinct. (Incidentally, I have heard that the failure of this process is one of the causes of red-green colour blindness.)

Ultimately, the only way you perceive the visible world is through your eyes: so it doesn't matter how accurate the phosphors are -- just as long as they match the eyes closely.
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post #10 of 18 Old 07-20-2013, 03:18 AM - Thread Starter
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Okay I think I got it about the eyes making compensation for the accuracy, but what about the RGB signal from a binary code standpoint? Is the RGB signal assuming that the phosphors are producing only one distinct wavelength, or does the RGB signal code in and of itself account for the range of the phosphers?

P.S.
Also tom669, is it really true that the brain performs a trick on the red-green information to make it more distinct?
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post #11 of 18 Old 07-20-2013, 08:03 AM
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Colour is purely a perception, can you prove to me that the blue someone sees does not look like red to them? Trick or no trick, colour and vision is completely in your head, there's no physical reference for what red, green or blue -- or even brightness -- should look like.

From a standpoint of what the TV receives it depends on what colour standard the encoding uses. Typically the data is received in YCbCr format, which is converted to some form of RGB colour space before being displayed. Not sure what TVs use, but some computers use Adobe sRGB, which, with a calibrated monitor, will produce virtually identical colour to another calibrated monitor with the same RGB value displayed.
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post #12 of 18 Old 07-20-2013, 12:56 PM - Thread Starter
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You are right about the color perception part, and I agree with that. After googling about how our eyes perceive color, it has been said that we have three types of color cones (red, green, and blue) and one type of rod (that measures any amount of light regardless of color). But nowhere did I read that our brain performs a trick on the red-green information to make it more distinct. I know that there is an overlap between what colors each type of cone can see, meaning that the red cone is most sensitive to red color but can also sense other colors like green (as you showed on that graph). I was just wondering if the brain does even more processing of those colors as you implied with your red-green information brain trick.

But that is besides the point, I am just wondering if someone else knows what the RGB color space (After the original format, like the YCbCr format, is converted to RGB) in the TV is assuming when it is sending the signals to the phosphors. From a programming aspect is the RGB color space assuming that the phosphor emits only one wavelength or not?

I am only asking from a viewpoint of understanding the technology, not from what our eyes perceive and how it accounts for those differences or what TV technology is the best (I am not looking to buy a TV anytime soon).
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post #13 of 18 Old 07-20-2013, 01:18 PM
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It's probably related to ITU-R BT.601
http://en.wikipedia.org/wiki/YCbCr
to get the R,G,B levels.

As for what the phosphor wavelengths are, I'd guess that they are similar to CRT phosphors, as here: http://en.wikipedia.org/wiki/Phosphor#Cathode_ray_tubes

Red: Yttrium oxide-sulfide activated with europium is used as the red phosphor in color CRTs. The development of color TVs took a long time due to the long search for a red phosphor. The first red emitting rare earth phosphor, YVO4,Eu3, was introduced by Levine and Palilla as a primary color in television in 1964. In single crystal form, it was used as an excellent polarizer and laser material.

Green: Combination of zinc sulfide with copper, the P31 phosphor or ZnS:Cu, provides green light peaking at 531 nm, with long glow.

Blue: Combination of zinc sulfide with few ppm of silver, the ZnS:Ag, when excited by electrons, provides strong blue glow with maximum at 450 nm, with short afterglow with 200 nanosecond duration. It is known as the P22B phosphor. This material, zinc sulfide silver, is still one of the most efficient phosphors in cathode ray tubes. It is used as a blue phosphor in color CRTs.

Not sure what red phosphor wavelength is, but some research on the subject may reveal it.

I'd guess modern plasmas vary the phosphors slightly for improved efficiency and reduced afterglow.

If you have well defined phosphors then all you need to get right is the YCbCr to RGB.
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post #14 of 18 Old 07-20-2013, 05:15 PM - Thread Starter
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So from my understanding, the RGB color model assigns an intensity/amplitude of three colors only, ranging from 1 to 255. These different intensities for each color can create the perception of millions of different colors.

But the RGB model has to assume that the three colors are a specific wavelength. Does anyone here know what the specific wavelength number (nm) the RGB color model assumes for red, green, and blue? Or is my understanding of the model incorrect since I am having a hard time finding those numbers?
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post #15 of 18 Old 07-20-2013, 08:27 PM
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Quote:
Originally Posted by hcps-sulemanma View Post

So from my understanding, the RGB color model assigns an intensity/amplitude of three colors only, ranging from 1 to 255. These different intensities for each color can create the perception of millions of different colors.

But the RGB model has to assume that the three colors are a specific wavelength. Does anyone here know what the specific wavelength number (nm) the RGB color model assumes for red, green, and blue? Or is my understanding of the model incorrect since I am having a hard time finding those numbers?


You seem to be way off track. But it appears that you have a technical background. Google "Planck's blackbody radiation" and you'll find that the model for colorimetry is based on color temperature not color frequency (or wavelength.)

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post #16 of 18 Old 07-20-2013, 10:32 PM - Thread Starter
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Ok thanks Larry, its a little bit more complicated than what I had thought. That clears up the confusion. So I am guessing the phosphors in the plasma are also based on blackbody radiation? For example the Red phosphor gives of the Red color based on a black body radiation type and NOT of the electromagnetic spectrum red region, even though it may coincide with it?
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post #17 of 18 Old 07-21-2013, 01:47 PM
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Quote:
Originally Posted by hcps-sulemanma View Post

Ok thanks Larry, its a little bit more complicated than what I had thought. That clears up the confusion. So I am guessing the phosphors in the plasma are also based on blackbody radiation? For example the Red phosphor gives of the Red color based on a black body radiation type and NOT of the electromagnetic spectrum red region, even though it may coincide with it?


Not quite. It's the other way around. Don't get hung up on the phosphors. The characteristics of the phosphors are of secondary engineering relevance. Colorimetry is concerned with a methodology for describing the human perception of color.

The science that forms the basis for colorimetry started over 150 years ago from the works of Stewart, Kirchhoff, and Planck. Colorimetry itself wasn't fully developed and defined as an engineering methodology until the early part of the 20th century. (The history of physics is an interest and hobby of mine. Sorry if it seems a bit off topic but it is important to understand that all this happened long before digital electronics -- or, for that matter, electronics in general.)

Look up colorimetry, chromaticity, color temperature, and blackbody radiation for the scientific background information, Read Tom Hufmann's thread here on AVS for a terrific introduction into the specifics of display calibration: http://www.avsforum.com/t/852536/basic-guide-to-color-calibration-using-a-cms-updated-and-enhanced.

The information you are looking for is readily available. This topic really should be a discussion in the Calibration forum not here where the members are mostly interested in the overall performance of specific plasma TVs.

Larry
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post #18 of 18 Old 07-21-2013, 04:34 PM - Thread Starter
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All right thanks for that link. But I would say it have been a lot easier if the Color Model (Colorimetry) was based on the electromagnetic spectrum only, where the wavelength identifies the color and the amplitude identifies its brightness, even though I understand as you said that there are historical reasons for basing it on other criteria like black body radiation .
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