Anyone does contrast comparisons? - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 11-24-13, 05:03 PM
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Re: Anyone does contrast comparisons?

I get that. Here's why:

Let's take our above example of a PJ/screen that gives you 20 ftl at 100 ire. Take a Gamma of 2.3. at 10 ire you're looking at .1 ftl. an error of .002 ftl is completely invisible to the naked eye. Now if you're sharp, you MIGHT be able to detect a difference of the two if they're side by side. But the gamma curve being off by .002 ftl will have zero impact on the image we see. Color balance is another matter. if there's a green push, a lot of people will see it even at .1 ftl. (Not everybody.) As we go up the gamma scale, .002 error becomes even more irrelevant. We can't even control accuracies that low. The built in gamma controls on my Epson 6020 certainly can't adjust anywhere near that levels of accuracy. My Lumagen XS-3D will get closer, but I don't believe even it can adjust luminosity to within .002 ftl. So, an error of .002 ftl can be considered outstanding-world class--when it comes to calibrating a display.

Now let's go with contrast calculations. My post above shows our calculations for a PJ with a 5,000:1 CR putting out 20 ftl at 100 ire. Black level should be .004 ftl. (BTW, this is good native contrast and a very bright screen. A PJ with excellent contrast and not as bright will have much lower black levels.) Our error of .002 which was meaningless for calibration now creates a huge problem for CR calculations. With a .002 ftl margin of error, we can only say our CR is between 3,333:1 and 10,000:1. A Klein K10-A could measure it, but even it would have a margin of error of between 4,000:1 and 6,666:1. Double that CR to 10,000:1, or drop 100 ire to 12, and even the Klein can no longer accurately measure our CR. The Klein can measure below .003 ftl, but with NO guarantee of accuracy according to Klein's own specifications.

Bottom line: A meter can be a world class calibration tool and still be worthless for contrast readings. Every meter has a margin of error. Every meter. They're rarely specified for calibration tools because tiny errors won't affect our calibration. I'm not saying you can't try and measure CR with an i1D pro. What I'm saying is that if you don't know the specific accuracy at the level you are reading, you have absolutely no idea what your margin of error is. Look again at the Klein's specs, they show not just an absolute error, but a percentage error. You add those two errors to find your accuracy at any level. Now you know what your margin of error is. If two CR calculations overlap in their margin of error, you have zero idea of how far apart they are. If the results are repeatable, you can safely assume one is higher than the other, but you have no idea by how much. If the black level you are reading is below the device's published specifications, you don't even know if one really is higher than the other. The meters output curve might not be linear below the published minimum accuracy.
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Old 11-24-13, 05:26 PM
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Re: Anyone does contrast comparisons?

Yes Nak, i can see your point. Your point is to measure a CR with extreme precision and without any doubt or margin of error. Two things to consider though...

From the moment that even the Minolta 20000 has a margin of error, wouldn't be a little utopic serching for perfection with a non existing device?

And secondy...immagine that there is such device with minimum margin that narrows the error of reading between 1500:1 and 2000:1 CR. Do you believe that you will be able to tell the difference in intrascene contrast based on such figures?

Personally i don't...and didn't see them with 2 projectors side by side such as Benq 1070 and Optoma 25.
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Old 11-24-13, 11:14 PM
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Re: Anyone does contrast comparisons?

No, not at all. That isn't my point. My point is to know what the margin of error is, and to whethor or not your calculations have any meaning. I'm not talking about +-250:1 CR accuracy as in your example. I'm pointing out that without very expensive gear, CR measurements likely have errors of +- 3000:1 or even 8000:1, or even greater. I'm not recommending that anyone buy such gear. After all, our CR is what it is. We have no ability to affect it other than to worsen it through improper calibration. Even testing screens is pretty much useless, as screens that hot spot horribly will measure quite well using such a measure.

For calibration, knowing your exact accuracy is not important, because small errors will not affect your calibration. For CR calculations, it is absolutely critical to know specifically what your accuracy is, because tiny errors will have a huge impact on your calculations. Testing CR without knowing your margin of error is nothing more than an exercise in hope. If you know your accuracy, you can calculate your margin of error. Then you could actually determine the usefulness of the result. If you have no idea what your margin of error is, your result is utterly meaningless.
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Old 11-25-13, 08:33 AM
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Re: Anyone does contrast comparisons?

Hi Nak!

To my knowledge, there is no such thing as perfection. Even the best out there can give an error of 100:1 CR wrong with an error margin of 2%.

http://sensing.konicaminolta.us/prod...troradiometer/

On the other hand, Klein gives an error of 1% at least for values lighter than 0.01 nits.

http://www.kleininstruments.com/colorimeters/k10-a/

Now, if you allow me...i believe your examples are somehow exagerated. You are not speaking for margins of error, you are talking about...Grand Canyon!!

It's totally another thing an error of 2%, 5% or even 10% with the extreme margins shown above. A 5000:1 to a 10.000:1 CR means 100% error!!!

So my idea for measuring validly a CR on a projector with a budget meter is the following:

Firstly, i look up the specs and it's ability to go down on dark readings. For contrast readings, i take 10 or 20 readings and see it's behaviour. If it's stable and the values corrispond to manufacturer's specs, then i am happy and quite certain that the CR of this certain projector is around there. If using this method come up with a CR of 2000:1, it's irrelevant to me if it's really 1931:1 or 2076:1. If on the other hand the sensor can't handle the black or the white levels (out of spec), then i don't care what it reads since it's readings are not full proof and therefore i can't come up with a valid CR.

Last edited by Yiannis1970; 11-25-13 at 08:39 AM.
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Old 11-25-13, 12:20 PM
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Re: Anyone does contrast comparisons?

Hey Yiannis!

I do hope you realize that I respect your opinion, and that I'm not dissing you. We're just having a technical argument that I find enjoyable. I enjoy it because your a smart and level headed guy. Anyways, I just wanted to make sure you knew where I was coming from. Back to the subject:

Actually, my examples aren't exaggerations. Show me where in my math I'm exaggerating? And you're right, very small accuracy errors in luminance result in Grand Canyon errors in CR calculations. That's exactly why CR measurements require very expensive instruments.

With the Klein, you're ignoring half of the accuracy specification. You missed the +-1 LSB. At very low light levels the +-1 LSB is going to have a much, much, larger impact on accuracy than the percentage error.

It's not about getting extremely accurate CR results. It's about getting CR results that aren't completely worthless. The nature of the math involved in the CR calculation requires extreme accuracy at the low end to get any kind of result that is useful at all. Don't you think if X-rite had a meter that was accurate to +-.001 ftl they'd be mentioning it in the specs? Hell, they'd be splashing that all over their marketing materials. Even that number really isn't good enough to be checking CR ratios.

It's not "If using this method come up with a CR of 2000:1, it's irrelevant to me if it's really 1931:1 or 2076:1." The reality is at best your margin of error is most likely between 1000:1 and 3333:1. That's not an exaggeration, that's math. That's assuming that the accuracy of your meter is +-.003 ftl. That's .01 nit. Actually, I doubt it's that good, so your error is most likely much greater. Again, not an exaggeration, just math. If you disagree, can show the math that makes you believe I'm wrong?

We could argue about this all day, so instead I chose to call X-rite. I did this just after I wrote the last paragraph... I just spoke to one of the engineers and he quite plainly said that the i1D Pro is in no way accurate enough at low light levels to be used for contrast calculations. He--the X-rite engineer--said any contrast calculations done with readings from an i1D Pro would be wildly off. He stated that the accuracy was more than sufficient for calibration purposes--in fact the accuracy is excellent for calibration purposes he said--but that contrast readings would require a different instrument. I told him it was just a hobby, and asked if I couldn't get useful information just for comparison purposes? He actually laughed, and said "No." He point blank said that the i1D Pro should absolutely NOT be used for contrast readings. It is not designed for such work, and is useless for such work. That's the position of one of the engineers responsible for the i1D Pro.

EDITED TO ADD: I do want to point out we're talking about PJ On:Off CR. The i1D Pro can be used to adjust cr DOWN on monitors to 300:1. This is a technique used in photographic work, to match the CR of the monitor to the CR of photo paper. But such adjustments result in MUCH higher black levels than we get with even inexpensive PJs.

Last edited by Nak; 11-25-13 at 12:37 PM.
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Old 11-25-13, 12:58 PM
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Re: Anyone does contrast comparisons?

Quote:
Nak wrote: View Post
We could argue about this all day, so instead I chose to call X-rite. I did this just after I wrote the last paragraph... I just spoke to one of the engineers and he quite plainly said that the i1D Pro is in no way accurate enough at low light levels to be used for contrast calculations. He--the X-rite engineer--said any contrast calculations done with readings from an i1D Pro would be wildly off. He stated that the accuracy was more than sufficient for calibration purposes--in fact the accuracy is excellent for calibration purposes he said--but that contrast readings would require a different instrument. I told him it was just a hobby, and asked if I couldn't get useful information just for comparison purposes? He actually laughed, and said "No." He point blank said that the i1D Pro should absolutely NOT be used for contrast readings. It is not designed for such work, and is useless for such work. That's the position of one of the engineers responsible for the i1D Pro.
Good idea to call them! They've always been very forthcoming when I've talked to them in the past. And this was pretty much as I had suspected all along.
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Old 11-25-13, 01:12 PM
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Re: Anyone does contrast comparisons?

Thanks Mech!

Yeah, we were just going round & round with no one really having the data to settle the debate. It seemed a lot more efficient just to go to the horse's mouth. BTW, very nice guy, took the time to really explain it to me. Excellent customer service! Another good reason to buy i1 meters!
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Old 11-25-13, 01:27 PM
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Re: Anyone does contrast comparisons?

Quote:
Nak wrote: View Post
Hey Yiannis!
I do hope you realize that I respect your opinion, and that I'm not dissing you. We're just having a technical argument that I find enjoyable. I enjoy it because your a smart and level headed guy. Anyways, I just wanted to make sure you knew where I was coming from. Back to the subject:
Same here my friend!!

I believe we have a constructive (if the term is wright) argument here and not a useless ''battle''.

Quote:
Nak wrote: View Post
Actually, my examples aren't exaggerations. Show me where in my math I'm exaggerating? And you're right, very small accuracy errors in luminance result in Grand Canyon errors in CR calculations. That's exactly why CR measurements require very expensive instruments.
The margin of error you used in your examples is extreme. When you state that a sensor instead of 0.002 FL reads 0.004 that means that its error is 100%, not 1%, 2%, 5\$ or even 10% like Minolta's or Klein's specs state, but 100%. Thats why i called your margin used in your example an exageration. With 2% in error Minolta gives as a CR +/- 100:1, with 20% in error in luminosity values it would give us a wrong CR of ~1000:1. I agree with you that we don't know how much the sensor drifts when measuring near its limit but to tell you the truth i wouldn't guess an enormous drift. When after 20 or 30 measurements a sensor seems stable enough, i consider its readings safe enough (always speaking doing so whithin its range).

Quote:
Nak wrote: View Post

We could argue about this all day, so instead I chose to call X-rite. I did this just after I wrote the last paragraph... I just spoke to one of the engineers and he quite plainly said that the i1D Pro is in no way accurate enough at low light levels to be used for contrast calculations. He--the X-rite engineer--said any contrast calculations done with readings from an i1D Pro would be wildly off. He stated that the accuracy was more than sufficient for calibration purposes--in fact the accuracy is excellent for calibration purposes he said--but that contrast readings would require a different instrument. I told him it was just a hobby, and asked if I couldn't get useful information just for comparison purposes? He actually laughed, and said "No." He point blank said that the i1D Pro should absolutely NOT be used for contrast readings. It is not designed for such work, and is useless for such work. That's the position of one of the engineers responsible for the i1D Pro.
You 've done well, but if i were me on the phone i would have asked him two questions (one good one and the other ''bad one'').

Ok mister XRite....i own a Benq 20000 and put my ID3 one meter away off its lens. I display the white pattern and then the black pattern. I read in black pattern 2 nits (~7 lux). Can you explain to me how in such levels the sensor is wrong on its reading? Are you trying to tell me that it can't read well 7 lux of illuminance? Do you know mr. XRite in what IRE corrisponds such a figure doing a a proper calibration off the screen with a white level at 45 nits? Do you really claim that i cannot measure CR on my projector using this method?

So, mr. XRite...it is not designed to measure well the CR but ''its accuracy is excellent for calibration purposes''.....well, what to respond on this nonsense??

Don't you know mr Xrite that your super duper sensor which is excellent for calibration purposes is a piece of junk underestimating the red color in grayscale measurments? 300K, 500K, 700K in difference!! Where do you live? On Mars? Have you tried to calibrate your own monitor with yours software, provided by mom XRite (i1 profiler)? Don't you know, that in order for your super duper sensor to work properly on grayscale, half of the world has pubbliced correction matrices?? Haven't you heard that the ''8th miracle'' does not work well even correcting the probe with these on any display??

So, dear Nak, seems better that you have talked with him and not me because i would probably call him with various ''names'' about the sensor that is excellent for calibration purposes!!
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Old 11-25-13, 02:28 PM
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Re: Anyone does contrast comparisons?

Quote:
Yiannis1970 wrote: View Post
The margin of error you used in your examples is extreme. When you state that a sensor instead of 0.002 FL reads 0.004 that means that its error is 100%, not 1%, 2%, 5\$ or even 10% like Minolta's or Klein's specs state, but 100%. Thats why i called your margin used in your example an exageration.
No , the margin of error I used is very tiny. +-.002 ftl is less than even the Klein claims. Remember, there are two parts to the accuracy. Percentage, and absolute. The Klein specs are thus:
Quote:
Luminance +- 1% Y, +- 1 LSB (displayed value > 0.01 nits)
There is a percentage error: +-1%, and an absolute error: +-1 LSB. That is typical specifications for any measurement instrument; percentage plus absolute. At high values the percent error is controlling, at low values the absolute error is controlling. Now, Klein didn't publish what they are using for their Least Significant Bit, but since they are using Nits in their specs, it's reasonable to assume that .01 nits is Least Significant Bit. That gives us an error at low light levels of .003 ftl. That's pretty incredible if you think about it. Accuracy down to .003 ftl. Amazing. And yes, at .003 measured ftl, the Klein could be off by 100%. But 100% is still only .003 ftl. For calibration purposes this is an insignificant number. For Contrast Ratio calculations, it's quite large. Remember, percentage accuracy is only half of the equation. There is ALWAYS an absolute measure of accuracy as well.

It's all about the right tool for the right job. I have a Stihl Chainsaw. Best consumer chainsaw in the world. Outstanding for cutting trees down. But I don't use it to cut tile. If I tried to cut tile with it the results would be horrendous. But how can this be? It's outstanding at cutting trees! Why can't it cut tile well? Well, for cutting tile I use a tile saw. It's really bad at cutting down trees. But amazingly, it cuts tile really well. I really, really, want the Stihl to be great at cutting anything. I mean, I really want this. I only want to buy one saw, because saws are expensive. A good tile saw is really expensive. So I don't want to buy one. But it doesn't matter what I want. No matter how bad I want otherwise, my Stihl would still do a horrible job of cutting tile. So, when I need to cut tile, I rent a tile saw.

I guess when I bought a colorimeter I just assumed I would have to profile it against a spectrophotometer. That's the way it's always been, and I figure it always will be that way. At least in this price range.

If you get the meter close enough to the lens, maybe you could get the black level high enough to do a CR measurement. But you'd have to know what the accuracy specs are to know how high you'd need to be. With the Klein, you have the spec. That allows you to calculate your margin of error. Without that spec, you have absolutely no way of knowing how accurate your CR calculations are. Again, the math shows us that extreme accuracy is needed for CR calculations, much much higher than you need for calibration. Even a hundredth of a ftl error is not going to be visible in a calibration. Hell, even a tenth isn't. That's why we can buy affordable probes to calibrate displays. If we required accuracy down to .003 ftl, there'd be a lot fewer calibration hobbyists.
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Old 11-25-13, 03:00 PM
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Re: Anyone does contrast comparisons?

Quote:
Nak wrote: View Post
No , the margin of error I used is very tiny. +-.002 ftl is less than even the Klein claims. Remember, there are two parts to the accuracy. Percentage, and absolute. The Klein specs are thus:
There is a percentage error: +-1%, and an absolute error: +-1 LSB. That is typical specifications for any measurement instrument; percentage plus absolute. At high values the percent error is controlling, at low values the absolute error is controlling. Now, Klein didn't publish what they are using for their Least Significant Bit, but since they are using Nits in their specs, it's reasonable to assume that .01 nits is Least Significant Bit. That gives us an error at low light levels of .003 ftl. That's pretty incredible if you think about it. Accuracy down to .003 ftl. Amazing. And yes, at .003 measured ftl, the Klein could be off by 100%. But 100% is still only .003 ftl. For calibration purposes this is an insignificant number. For Contrast Ratio calculations, it's quite large. Remember, percentage accuracy is only half of the equation. There is ALWAYS an absolute measure of accuracy as well.
Correct me if i am wrong but, ''Luminance +- 1% Y, +- 1 LSB (displayed value > 0.01 nits)''

means not necessarily 0.1 but 0.09, 0.08.....0.02, therefore LSB could mean also 0.02.

Profiling is an option for those who seek to be the most accurate possible and not a must thing, especially when you have the need for minor correction and not when your monitor turns out all red. But, if you feel ok with XRite's policy to whistle around (i haven't read something relative to apologize somehow...) and enjoy your probe despite that major deficiency, it's fine with me.

And i like to apologize for my outburst but, at least to me, 200 Euros is a considerable amount of money and i can't stand to be worked around by any XRite. When i buy a car, the salesman doesn't tell me that it comes without tyres and have to buy (profile) them apart...
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