Best DIY method to compensate for meter drift - AVS Forum
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post #1 of 38 Old 11-04-2011, 03:27 PM - Thread Starter
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All,

Meter drift seems to be the achillies heel of DIY calibration but I can't find any posts that recommend how to tackle it.

I'm considering buying my third meter (a D3 packaged with either Chromapure or Calman, not decided) because after a year or so I don't trust the meters because of drift (or potential drift), as I don't have easy access to a reference meter to check the accurancy of mine.

So I was thinking about techniques to compensate for meter drift, and want to get feedback from this forum about the best approach.

1) Buy the "pro" meter from Chromapure and send it back each year for reference adjustment. This costs almost as much as a D3 plus the hassle of shipping (I'm not Stateside)

2) Find a local calibrator and check mine against his reference, creating an offset table to use in Calman/Chromapure. Trouble is the local calibrators aren't interested in this service even at a fee.

3) When I first get the meter, do a WRGBCMY level check on a Laptop monitor and write down the results. In a year time, do the same and create an offset table based on the difference, which represents the amount the meter has drifted. Only problem is that the laptop LCD backlight might have drifted in that time.

4) Purchase a photographers reference white tile and do a WRGBCMY level check against the tile (with a good light source like sunlight at midday). As white contains RGB I should be able to get the right readings. Do the same in a years time and the difference in the reading becomes the offset. This looks to be the best option.

5) Keep buying new meters each year. Expensive & a waste!

I'm no calibration expert so not quite sure what will be the best approach, I suspect #4 but would like validation from folks who know more about this than I do.

Thanks!
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post #2 of 38 Old 11-04-2011, 04:09 PM
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The problem you run into which you pointed out and why this has not been solved easily is the reference. Validating with a reference is straight forward. So the answer is you need is a reference you can trust over time to not drift. Well the short answer is they all do drift to some degree over time. In our NIST calibration lab we do use reference displays and they are checked and adjusted with our CS-2000 before each meter calibration run. Even the high-end reference displays we use drift and take time to warm up usually 30 minutes even for LCD CCFL before you can get a stable reading. You also have many environmental factors, ambient light conditions, room temperature and humidity, mechanical alignment with the meter and display to minimize angler issues. We have over $50k in jigs just to deal with mechanical alignment.

Now this is not to say it is impossible to do just not very cost effective at this time. When I first start with video calibration many years ago this was one of the first projects I worked on but never came up with an affordable solution I’m still looking for the answer.

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post #3 of 38 Old 11-04-2011, 07:31 PM - Thread Starter
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Hi Derek,

Wouldn't a white tile be a stable reference? Assuming the color itself didn't fade over time (if its kept in a dark environment) then it doesn't matter if its not perfectly white to start with as you are looking for the difference between the old reading and the current, not the absolute color. You do have to be careful with the light source, but if you use sunlight you should be OK?

Also, it would be good if your C6 meter had a cheaper options, perhaps only calibrated for the main display types (UHP projector, LED LCD panels).
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post #4 of 38 Old 11-04-2011, 08:51 PM
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Quote:
Originally Posted by deandob View Post

Hi Derek,

Wouldn't a white tile be a stable reference? Assuming the color itself didn't fade over time (if its kept in a dark environment) then it doesn't matter if its not perfectly white to start with as you are looking for the difference between the old reading and the current, not the absolute color. You do have to be careful with the light source, but if you use sunlight you should be OK?

Also, it would be good if your C6 meter had a cheaper options, perhaps only calibrated for the main display types (UHP projector, LED LCD panels).

What are you going to light the tile with?

Nothing has color until light is bounced off of it.
A stable light source in a controlled enviroment would be required to even begin the process.

The other thing would be that a simple white measurement isn't good enough to create a correction table. While that would give you an x,y offset, the Colorimeter is measuring colors in X,Y,Z space where each value is measuring an overlapping area of the spectrum, so you need measure segments of the spectrum that roughly correlate to the X, Y and Z areas which is why when you profile a meter in CalMAN we do an RGB and W reading.

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post #5 of 38 Old 11-04-2011, 09:03 PM - Thread Starter
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What about using a photographer reference card like this:
http://xritephoto.com/ph_product_ove...=1234&catid=28

I was thinking as these cards are carefully printed for color accuracy they could be used as a reference to check the meter accuracy initially, however the problem is the quality of the light source, even sunlight is a different color at different times of the day and not D65. But this approach should still be usable for checking for drift, if you measure at the same time of day cloudless around the same time of year (ie. do this yearly) as we are looking for changes. You could use red, blue and green tiles as well - again the absolute color of these tiles may not be perfect, but we are looking for changes so the absolute accuracy shouldn't matter.

This seems too simple - I'm sure there is a flaw with my thinking.

Are there any accurate light sources that could be used instead of sunlight (eg. do laptop LCD backlights drift with age?) as you need to keep the light source consistent between measurements.
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post #6 of 38 Old 11-04-2011, 09:13 PM
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Quote:
Originally Posted by deandob View Post

What about using a photographer reference card like this:
http://xritephoto.com/ph_product_ove...=1234&catid=28

I was thinking as these cards are carefully printed for color accuracy they could be used as a reference to check the meter accuracy initially, however the problem is the quality of the light source, even sunlight is a different color at different times of the day and not D65. But this approach should still be usable for checking for drift, if you measure at the same time of day cloudless around the same time of year (ie. do this yearly) as we are looking for changes. You could use red, blue and green tiles as well - again the absolute color of these tiles may not be perfect, but we are looking for changes so the absolute accuracy shouldn't matter.

This seems too simple - I'm sure there is a flaw with my thinking.

Are there any accurate light sources that could be used instead of sunlight (eg. do laptop LCD backlights drift with age?) as you need to keep the light source consistent between measurements.

You need a way better light source than the sun.

A neutral Gray card, RGB cards and a jig to align them and you'd be close.

The other issue is of course the reason we have individual profiles is that displays can have very different spectral signatures.

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post #7 of 38 Old 11-04-2011, 09:37 PM - Thread Starter
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Yes, but the advantage of this method is we are checking for differences, not absolute values. I agree that this approach is not ideal for calibration, but if we take a new D3 meter which should be pretty accurate (oreven better Chromapure's "pro" D3, or your C6 meter) then we don't care how close our gray and RGB cards are, as we are looking for shifts (differences in reading) over time.

If you can use a light source that is consistent over time, and use quality cards that don't fade or shift colors over time, this method should work. I'll assume that we can find cards or tiles that are reasonable representations of RGB and have stable colors if kept in a dark dry location, so the problem is finding a stable light source (even if the source is not true D65).

This method should work keeping the meter accurate and won't depend on the display type you are using to calibrate.
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post #8 of 38 Old 11-04-2011, 09:41 PM
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A colorimeter has sensors that are suppose to match the XYZ curves, X roughly aproximates red, Y green and Z blue, but it's much more nuanced than that.

Both of these monitors are D65

Dell u2410


Dell 2408wfp



What you are doing with creating a correction table is learning where the filters don't match the curves in those graphs. The corrections therefore are specific to the spectral output of the display.

Even if you created profiles off those cards, the profile would really only be good for those cards, since the corrections wouldn't correctly hint the software for the changes in the sensitivity of the filter for the corresponding XYZ curves.
LL
LL

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post #9 of 38 Old 11-04-2011, 09:47 PM
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Don't get me wrong, it's probably better than nothing, but you're talking about buying 4x $70 card + a $70 lamp, all of which are going to drift and fade, for $300+ dollars.

After a year your D3 won't have drifted far, after 2 it might be a good idea to re-run the thing, and you might get a decent correction. After another 2, the lamp and the cards will likely have drifted faded as much that you won't be able to get a reliable 2nd correction.

So you're talking about spending $300 to get an incremental improvement in 2-3 years from now, when the cost of a new meter now is $300 and likely to be less 4yrs from now.

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post #10 of 38 Old 11-04-2011, 10:24 PM - Thread Starter
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Thanks Sotti, I think I understand, the problem is the profile / offset tables are designed around a specific light source, so if I go looking for changes using a different light source (eg. the sun), what I'm reading won't be relevant for a UHP projector lamp.

So because its not (DIY) possible to have a reference UHP light source, or even one that doesn't drift over time, my suggested method won't work.

The only thing that I think could work is if I checked the offset on a projector bulb (after 1/2 hour warm up), then put that bulb away for a year and then rechecked with the same bulb I should be able to compile an accurate offset table, assuming the other variables in the system don't change (eg. voltage to the bulb). This is a hassle, as you say its probably easier to just buy a new meter.
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post #11 of 38 Old 11-04-2011, 10:43 PM
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Right, as Sotti said you would have an offset for how it has drifted when using the sun as a light source, but not when using an LED, CCFL, plasma, CRT, etc...

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post #12 of 38 Old 11-05-2011, 08:18 AM
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Quote:
Originally Posted by deandob View Post

This is a hassle, as you say its probably easier to just buy a new meter.

You don't have to buy a new meter. If you have ChromaPure, you can recalibrate/convert the D3 into the PRO version for $150 (same fee for both services). This service gives you enhanced accuracy and allows you to maintain that accuracy by redoing the service every 2 years or so. I believe SpectraCal might offer a similar service for their C6, but if cost is an issue the D3 PRO is about $300 cheaper.
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post #13 of 38 Old 11-05-2011, 08:42 AM
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Originally Posted by PlasmaPZ80U View Post

You don't have to buy a new meter. If you have ChromaPure, you can recalibrate/convert the D3 into the PRO version for $150 (same fee for both services). This service gives you enhanced accuracy and allows you to maintain that accuracy by redoing the service every 2 years or so. I believe SpectraCal might offer a similar service for their C6, but if cost is an issue the D3 PRO is about $300 cheaper.

We do offer NIST re-certs on our C6 and just about everything else we sell. The re-certs are not just recalibrate and check but a proper NIST cert. SpectraCal has one of the few labs for providing NIST cert's for every popular tristim made.

The C6 costs more because we do more and it can be field upgraded. So when we add more display types you don't have to send the meter back in for the update, CalMAN will update it for you. We just added LED RGBY, so for those that already have a C6 it is now calibrated for the Sharp Elite and other quattron's. You only need to send the C6 back in for NIST re-cert's.

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post #14 of 38 Old 11-05-2011, 09:00 AM
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Quote:
Originally Posted by derekjsmith View Post

We do offer NIST re-certs on our C6 and just about everything else we sell. The re-certs are not just recalibrate and check but a proper NIST cert. SpectraCal has one of the few labs for providing NIST cert's for every popular tristim made.

The C6 costs more because we do more and it can be field upgraded. So when we add more display types you don't have to send the meter back in for the update, CalMAN will update it for you. We just added LED RGBY, so for those that already have a C6 it is now calibrated for the Sharp Elite and other quattron's. You only need to send the C6 back in for NIST re-cert's.

Could you go into more detail why/how you can add more display types without sending the meter back in? If you have a new method of calibrating meters that is truly superior to the traditional method why keep this info secret? More people would buy the C6 if they fully understood what differentiates it from other calibration solutions like the D3 PRO.
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post #15 of 38 Old 11-05-2011, 10:54 AM
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Great question and one that I wish I could go in to extreme detail on how it is done because X-Rite has truly came up with a very advanced way of calibrating the D3 family. We are under NDA with X-Rite for much of the details but I will try and cover as much as I can without violating that.

We started working with X-Rite just about two years ago on what is now the D3 family. We went to the design team at X-Rite and had meetings with their color scientist team including some of the guys from the original Sequel Imaging group. We had a long laundry list of requests we felt the industry was going to need in the near future as display technologies were changing so fast. Some of those requests were: The ability to read at very low light with repeatable results because displays and projectors are getting so dark. The meter needed to have a tripod mount embedded in its body because displays are getting much bigger and hanging the meter cord over the top of the display is not a good option anymore. Accessories or components that can’t be lost or misplaced like with the D2 and C5 so a attached diffuser with detector to tell us if it is properly in place. A design that dealt with highly polarized and angular light that LCD manufacturers seem to be doing to focus more light into a narrow vertical but wider horizontal viewing area. Sealed optics and sensors to minimize drift over time. The ability to add display characterization data to the meter in the field to help make the meter future proof over a longer period of time.

What are the two things that make a meter obsolete? Sensor drift over time and display technology changes. Both of these problems are interrelated. You need a very stable and accurate filter array to solve the drift over time issue and once that was solved you can take advantage of it. With a stable and accurate filter/sensor array you can characterize each meter at the factory with a high degree of accuracy.

The second issue that can make a meter obsolete is changing display technology specifically the spectral response curve. Here is an example of a spectrum sweep with the 1931 2 deg standard observer curve on it.



A tristimulus meter through its filters and sensors try’s to mimic the standard observer curve in this case the 1931 2 deg CMF. Which if all display spectral response curves match the 1931 2 deg observer and the filters in the meter did the same we would not need to calibrate or characterize the meter. But in real life the meters filters are never perfect and have deviations. I have not seen a display that perfectly matches the 1931 2 deg standard observer. So in the past we had to create a calibration table for each major type of spectral response curve to cover our bases. As part of that processes of creating additional tables for each spectral response curve it also took into account the deviation in the meters filters. This is how it’s been done for a very long time and is the recommend method by NIST for calibrating a tristimulus device. But now that X-Rite has eliminated the need to deal with the filter/sensor deviation we can solely concentrate on just the display spectral response not matching the standard observer curve. So with a highly stable and factory characterized meter and the ability to add display characterization data outside the factory you have a field upgradable tristimulus meter.

As part of the work with X-Rite we worked out a deal to be able to provide these field upgrades with the SpectraCal C6. Our inhouse lab that does all of this work is quite extensive starting with our Konica Minolta CS-2000 which is only one of two devices certified by X-Rite for this work the other being a CS-1000. The lab is temperature and humidity controlled. Each display device has went through a very rigorous process of testing before being used in the lab and is calibrated before each meter or research run. The mechanical jigs used to line everything up for calibration or testing is controlling tolerance down to or even some times under 1mm.

I hope this helps everyone out and I wish I could go into a lot more detail because it is really very cool color science X-Rite really did their homework.

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post #16 of 38 Old 11-05-2011, 11:25 AM
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Derek,

I'm surprised you didn't mention requirements for new meters including being able to take readings through 3D glasses and from light reflected from a projection screen... most puck-type meters can't do readings through 3D glasses or of light reflected from projection screens. A DIY person looking for a meter today might be more likely to go for a meter that could measure light from a projection screen if they have an idea they will be getting a projector in 3 or 5 years or whatever... they'd know they could use the same meter. This would be especially attractive if the meter could be kept in calibration over many years.

And of course, taking readings through 3D glasses makes the requirement of measuring very low light levels even MORE important/critical.

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post #17 of 38 Old 11-05-2011, 11:55 AM
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Quote:
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Derek,

I'm surprised you didn't mention requirements for new meters including being able to take readings through 3D glasses and from light reflected from a projection screen... most puck-type meters can't do readings through 3D glasses or of light reflected from projection screens. A DIY person looking for a meter today might be more likely to go for a meter that could measure light from a projection screen if they have an idea they will be getting a projector in 3 or 5 years or whatever... they'd know they could use the same meter. This would be especially attractive if the meter could be kept in calibration over many years.

And of course, taking readings through 3D glasses makes the requirement of measuring very low light levels even MORE important/critical.

Again good points:

The D3 family has the optics, filters and sensors to be able to read from a direct light source or reflected as in front projection off of a screen. The requirements for this are a narrow field of view normally less than 15 degrees. As you point out most puck style meters don’t have any form of optics and as a result have a very wide FOV they see light from all angles not what you want when reading off of a screen. The C6 has a FOV of around 10 degrees so more than suited for front projection screen work. Heck it even has a tripod mount built in. You would be surprised at how much push back we got when we said the meter had to have a tripod mount.

As for calibrating through the glasses for 3D the only meter we currently recommend is the i1Pro or other spectro device. As I pointed out in my previous post the accuracy for a tristim is all about its ability to mimic the standard observer curve with filters and based on known display spectral response curves. Well when you shoot through a pair of 3D glasses it does change the spectral response curve so that becomes an unknown. With a spectro they mimic the standard observer response curve using a high sample rate 40 to 1000 samples and math. As a result they are very accurate but are also much slower and don’t read down to the low light most displays are capable of today.

Is there a perfect meter no. All have their tradeoffs. Tristim’s are fast, read low light but the accuracy subject to the calibration tables they are based on. Spectro’s are very accurate but do poorly at lower light and are slow. Our CS-2000 at very low light can take up to 4 minutes for a single reading. That’s ok for doing research and lab work but not very practical if you used it to calibrate with.

So what do we do to get the best of both worlds you profile a target meter to a reference meter when needed. This involves setting both a spectro as the reference and a tristim as the target up to take readings from the exact same spot and field of view on the display. CalMAN will show a series of patterns White, Red, Green and Blue for each meter and then run those 8 readings through some math to come up with a corrective matrix. Now you have a tristim that is as accurate as the spectro for the display you just profiled with.

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post #18 of 38 Old 11-05-2011, 02:17 PM - Thread Starter
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Only problem with the training approach is that it requires you to buy & maintain 2 meters, not very DIY friendly, and eventually the spectro will drift and require recalibration anyhow.
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post #19 of 38 Old 11-05-2011, 03:00 PM
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Quote:
Originally Posted by deandob View Post

Only problem with the training approach is that it requires you to buy & maintain 2 meters, not very DIY friendly, and eventually the spectro will drift and require recalibration anyhow.

If you wish to only use one meter you can buy a stock D3 and replace it when it drifts too much or have it converted into the D3 PRO (assuming you have ChromaPure) OR Get the C6 and have it re-certified/recalibrated when the time comes. You can also get the D3 PRO now and have it recalibrated when the time comes. Those are the only single meter options you really have for the D3.
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post #20 of 38 Old 11-06-2011, 07:45 AM
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Quote:
Originally Posted by PlasmaPZ80U View Post

If you wish to only use one meter you can buy a stock D3 and replace it when it drifts too much or have it converted into the D3 PRO (assuming you have ChromaPure) OR Get the C6 and have it re-certified/recalibrated when the time comes. You can also get the D3 PRO now and have it recalibrated when the time comes. Those are the only single meter options you really have for the D3.

Not the only options you also have the X-Rite i1Display Retail which CalMAN v4.4 also supports. So the best option would be to find a X-Rite i1Display Retail locally for the best price. That way you don't have to pay for any additional shipping or duties. The filter design in the D3 family should be good for a couple of years if taken care of. After a couple of years sell it off and buy a new one.

Another option is you can rent a ColorMunki Spectro from us once a year but I think the shipping cost both ways to Australia would be prohibitive. This option works out really well when shipping costs or duties are not a factor.

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post #21 of 38 Old 11-06-2011, 08:03 AM
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Quote:
Originally Posted by PlasmaPZ80U View Post

If you wish to only use one meter you can buy a stock D3 and replace it when it drifts too much

I have a problem with this statement....
How and when do you know that the meter has drifted too much?
Only way to know, is to have it check against a reference meter.
I rememeber way back when, I first got into calibartion I had an i1D2 that I thought was correct cause it was brand new....sad truth was...it was reading wrong but I never knew that till I compaired it to a CS-200.

Just a word to the wise......ALL METERS ARE CORRECT in your mind..till they are compaired to a reference meter....then you see the errors.

Just my $.02
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post #22 of 38 Old 11-06-2011, 08:32 AM
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Quote:
Originally Posted by Rayjr View Post

I have a problem with this statement....
How and when do you know that the meter has drifted too much?
Only way to know, is to have it check against a reference meter.
I rememeber way back when, I first got into calibartion I had an i1D2 that I thought was correct cause it was brand new....sad truth was...it was reading wrong but I never knew that till I compaired it to a CS-200.

Just a word to the wise......ALL METERS ARE CORRECT in your mind..till they are compaired to a reference meter....then you see the errors.

Just my $.02
RayJr

well if you know the typical lifespan of the meter to be 2-3 years (for example) and you store it properly, you should be able to make a good decision as to when to get it re-calibrated/re-certified or replaced

having a reference meter to check the D3 against would be more precise but if that's not an option you have to do the best you can to determine when the meter is no longer returning reliable readings
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post #23 of 38 Old 11-06-2011, 09:58 AM
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Quote:
Originally Posted by PlasmaPZ80U View Post

well if you know the typical lifespan of the meter to be 2-3 years (for example) and you store it properly, you should be able to make a good decision as to when to get it re-calibrated/re-certified or replaced

having a reference meter to check the D3 against would be more precise but if that's not an option you have to do the best you can to determine when the meter is no longer returning reliable readings

You are missing the meaning behind my post.....The meaning is...even a NEW meter can be wrong..but you don't know it unless it is checked against a reference meter.

RayJr
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post #24 of 38 Old 11-06-2011, 10:38 AM
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Quote:
Originally Posted by Rayjr View Post

You are missing the meaning behind my post.....The meaning is...even a NEW meter can be wrong..but you don't know it unless it is checked against a reference meter.

RayJr

Yes, that is the case with the stock D3, retail or OEM. However, the D3 PRO and the C6 are both checked against reference meters. This is exactly why I decided to upgrade my OEM D3 into the PRO version.
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post #25 of 38 Old 11-06-2011, 11:52 AM
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Assuming someone bought a meter primarily to calibrate one display, here was my question on the subject http://www.avsforum.com/avs-vb/showt...1#post19191171 Really the difficult thing about this line of thinking is that there's not much hard data to use as a reference for forming an opinion related to expected drift of different devices over time. I mean that I personally am not aware of examples of people explicitly documenting drift of colorimeters and spectros over time, in order to get an idea of worst-case scenarios. Personally my few year old colorimeter didn't vary gray by any more against my new i1pro than it had varied against an identical model, but I cannot say how Colorado climate might affect a colorimeter drift differently than other regions. My personal opinion is that I seem to find differences in relative light output more noticeable than variation in absolute color. Since gamma is essentially not standardized, and no one really seems to care about practical gamma differences, for my own use I'm just not aware of any clearly-documented information to worry about spectro or camera drift over time.
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post #26 of 38 Old 11-06-2011, 03:15 PM
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Quote:
Originally Posted by derekjsmith View Post

As for calibrating through the glasses for 3D the only meter we currently recommend is the i1Pro or other spectro device. As I pointed out in my previous post the accuracy for a tristim is all about its ability to mimic the standard observer curve with filters and based on known display spectral response curves. Well when you shoot through a pair of 3D glasses it does change the spectral response curve so that becomes an unknown. With a spectro they mimic the standard observer response curve using a high sample rate 40 to 1000 samples and math. As a result they are very accurate but are also much slower and don't read down to the low light most displays are capable of today.

Based on your above statement, can I profile the i1D3 through 3D glasses against i1Pro through 3D glasses and get similar result compare to my i1Pro?

My i1Pro doesn't drift much after 3 years as I compare to my friend's i1Pro that had been recertified recently.
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post #27 of 38 Old 11-06-2011, 04:15 PM
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Quote:
Originally Posted by rjyap View Post

Based on your above statement, can I profile the i1D3 through 3D glasses against i1Pro through 3D glasses and get similar result compare to my i1Pro?

My i1Pro doesn't drift much after 3 years as I compare to my friend's i1Pro that had been recertified recently.

That is not something we have tried but I will ask our lab to check it out for us and see.

Derek

CTO / Founder - SpectraCal Inc.
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post #28 of 38 Old 12-15-2011, 07:55 AM
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post #29 of 38 Old 12-15-2011, 08:45 AM
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The issue with your research is the hardware you have access to.

Those meters, use tungsten lamps to produce a constant predictable, known spectrum.

Because they know with relative certain the spectrum of light that is being reflected, they can accurately measure the paper.

The original post was about measuring the paper or tile with a colorimeter and an unknown light source. It's the light source that makes all the difference.

Joel Barsotti
SpectraCal
CalMAN Lead Developer
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post #30 of 38 Old 12-15-2011, 11:41 AM
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