Luminous Landscape Forum

Raw & Post Processing, Printing => Colour Management => Topic started by: Jeff-Grant on June 16, 2017, 01:30:53 AM

Title: Comparing spectros
Post by: Jeff-Grant on June 16, 2017, 01:30:53 AM
I now have a new 1iPro2 on my io2 and a DTP 70. Everything that I have heard about the DTP 70 is that it is a superior spectro to the humble i1p2. Today I did a quick test on a 21x4 linearity chart. The attached chart from CTP tells the story. The first column is from the i1p2 in spot with two reads, the second is from the DTP, and the third is from the i1p2 in scan mode. The DTP shows a lower number on the deep blacks than the i1p2. That may well be true but I have no idea how to know which one is the more accurate. I would like to think that this is showing me the superiority of the DTP but it would be wishful thinking.

I have discovered the Chromachecker target which I can get for a mere $500. I'm wondering if that is the only choice as I don't want a service, I just want a chart with known values that I can use to compare.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 16, 2017, 08:03:57 AM
I now have a new 1iPro2 on my io2 and a DTP 70. Everything that I have heard about the DTP 70 is that it is a superior spectro to the humble i1p2. Today I did a quick test on a 21x4 linearity chart. The attached chart from CTP tells the story. The first column is from the i1p2 in spot with two reads, the second is from the DTP, and the third is from the i1p2 in scan mode. The DTP shows a lower number on the deep blacks than the i1p2. That may well be true but I have no idea how to know which one is the more accurate. I would like to think that this is showing me the superiority of the DTP but it would be wishful thinking.

I have discovered the Chromachecker target which I can get for a mere $500. I'm wondering if that is the only choice as I don't want a service, I just want a chart with known values that I can use to compare.

What you did is showing you the difference between a pair of those columns, not accuracy. You don't say which two were used to derive the last column that contains the dE values. And by the way, for this kind of testing I would use dE(76) rather than dE 2000 - you are looking for simple differences, not differences adjusted for human visual perception.

To do accuracy testing you don't need to buy anything. You need a target with known reference values and you can construct this yourself in Photoshop (see my recent article on Extended Grayscale, this website), and the reference values will be the ones with which you create the patches. The objective of your exercise will then be to discover the differences between the reference values and the printed values read with the spectros. Make sure the reference values you choose are within the gamut of the profile you will be using to print the target. You would use the same printer, same paper, same profile, and make sure the Rendering Intent is set to Absolute. Then read the target with the DTP70. Read it again with the i1Pro2, subtract each of these from the reference values and you will see which set produces the lowest dE(76), and that will be the more accurate one.

If you use Enhanced Matte paper, as you know it is rich in OBAs, hence important with the i1Pro2 to retain the three readings M0, M1, M2 and select the one that produces the most accurate results. It would be preferable to do the testing with a PK paper (wider gamut) that contains no OBAs (eliminates this complicating variable).

Perhaps you aren't aware, because X-Rite is such a dreadful outfit when it comes to documentation, that when the i1Pro2 is is in scan mode, it reads 200 samples per second and averages them. So if you scan the patches at the rate of one per second, you have an average of 200 samples per patch. If you do spot readings, it will sample the whole area of the black hole that the light shines through, taking 200 samples and averaging them.

Given this kind of performance and the fact that there is a 5 years technological gap between these instruments, I would tend to expect the i1Pro2 to stand up very well to a DTP70, both being higher-end professional instruments, but not having worked with the latter I don't know - so the research you are doing is interesting (to see whether new technology means better in this case) and I for one shall be interested in any further results you develop.
Title: Re: Comparing spectros
Post by: Ethan Hansen on June 16, 2017, 03:02:41 PM
Mark: Be careful about X-Rite's specs. Yes, the i1Pro2 makes 200 measurements per second. The question is how many of those measurements are valid. The measurement aperture is 4.5mm. Therefore when measuring a target with the default patch size of 8mm, less than half the measurements (44%) are reading the pure patch color. A reason for the ferocious sampling rate is to allow scanning at reasonable speeds while still detecting which measurements should be discarded due to corruption by neighboring patches.

We perform our own in-house calibrations of our aging fleet of Spectroscans. We use BCRA Series II tiles (the standard 12 pack of CERAM tiles plus black and white) as primary reflective standards. We now use the iSis for most measurements and do comparison tests on paper targets between Spectroscan and iSis. Our measurements are in rough agreement with the spec values - calibrated Spectroscans are more tightly grouped than the i1-based iSis (0.3 dE typical/0.8 max vs. 0.4 typical/1.0 max). We saw similar agreement with the spec values for iCColor units when we still used them (aside: if anyone has use for an iCColor or two, let me know). The DTP 70 was comparable to the iCColor in terms of inter-instrument agreement.

Jeff: There can be two causes of black measurements showing darker values. Obviously the blacks really could be darker. Unfortunately this is also a typical sign of the measurement lamp fading. Are you seeing lower L values with both UV included and UV excluded measurements?

Check a 2-D LAB color space plot of profiles made on from the same paper on the i1Pro2 and DTP 70. At L=50, an aging measurement lamp usually produces lower saturation in yellow through orange (gamut boundary at maximum b, a in +/- 50 or so range. A second test is to make prints using the profiles. Soft proof a step wedge in Absolute Colorimetric. Set the steps so you can visually distinguish the difference between adjacent values but not by much. Print each version and check under good illumination if the DTP 70 steps are all visible.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 16, 2017, 03:21:57 PM
Ethan - even if only 10 or 20 of those samples are usable, it's still enough to make a useful average for the purposes of eliminating undue impact from outliers. I was just trying to make the point that this isn't a slouchy instrument and is capable of very considerable accuracy.
Title: Re: Comparing spectros
Post by: Ethan Hansen on June 16, 2017, 05:43:39 PM
Mark - My contention was not that the i1Pro 2 is not capable. It is, but it also needs a large number of samples to reduce noise. X-Rite has made instruments faster and easier to use but in terms of absolute accuracy in reflectance measurements, none of the current generation are a match for two-decade old instruments.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 16, 2017, 06:20:03 PM
Mark and Ethan, many thanks to you both. How I ended up with the DTP70 is a long and sad tale, due in part to local Xrite folks not being interested in helping me with my Pro2 problem. I can now happily report that, once having found the right people in Xrite, I received stellar support. In desperation, I bought the DTP70 from a friend, having heard nothing but praise for it over the years. I also ended up buying a new Pro2 to use on my io so I now have 2 x Pro2, 1 x Pro, 1 x io and a DTP70. Overkill perhaps for someone doing this mainly for his own pleasure. If anyone is interested, I would be happy to part with a perfectly good Pro2 that won't work on an io at a good price.

I've spent a lot of time digging around with my new/old gear. Doing this, I arrived at the conclusion that I was better off using the Pro2 in spot mode as it would be less challenged with readings. I also increased the default patch size by 1mm, and use 2 reads. This is slow as molasses but is not an issue. I think that I am now getting the best that I can from the io/Pro2 combo.

My concern all along has been that the DTP70 may be showing signs of age. That's why I started out looking for a way of validating what I'm seeing. I'll now start by reading a patch set with UV in and out and report back.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 16, 2017, 06:52:27 PM
I've now come to a screaming halt. I pressed down on the little plastic lever that switches between UV in or excluded and it broke off. It's stuck in limbo so I can't measure anything. I'll report back if I get it going. Thanks again. This definitely isn't the outcome that I was hoping for.
Title: Re: Comparing spectros
Post by: Ethan Hansen on June 16, 2017, 08:15:46 PM
Sorry to hear that! I assume the head was positioned in the center of the measuring path? If not, positioning needs to be done through software rather than just shoving the head manually. Might be worth a try to see if the filter lever can be pried with a sharp implement and moved to a definite, final position.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 16, 2017, 11:45:26 PM
Done, I now have an M0 only device, and a query in to Xrite for if they do service and how much.

I dug out a couple of profiles from early on in my trials, one made with the DTP and the other with the i1P. The inner line in the red/orange area is the DTP which looks like what was predicted. I think that I'll wait to hear from Xrite before I make another move.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 07:35:26 AM
Mark - My contention was not that the i1Pro 2 is not capable. It is, but it also needs a large number of samples to reduce noise. X-Rite has made instruments faster and easier to use but in terms of absolute accuracy in reflectance measurements, none of the current generation are a match for two-decade old instruments.

That's interesting Ethan, as you obviously have a professional perspective on it. I've put out for some additional views of the matter and if anything useful turns up I'll mention it here.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 07:43:07 AM
Done, I now have an M0 only device, and a query in to Xrite for if they do service and how much.

I dug out a couple of profiles from early on in my trials, one made with the DTP and the other with the i1P. The inner line in the red/orange area is the DTP which looks like what was predicted. I think that I'll wait to hear from Xrite before I make another move.

For the diagramming it would have been easier to distinguish between the instruments had you chosen to represent each instrument with a different solid colour gamut map. You can do that in CTP. Anyhow, no big deal, one can see regardless that the gamut shapes and volumes are close enough that either instrument would probably do about as well (in respect of print quality) at least in respect of that variable. Profile accuracy however is another matter that cannot be inferred from the gamut shape alone.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 17, 2017, 07:48:56 AM
I'm really interested to hear what others have to say anout the i1p2. For years, i read that the DTP was a stellar device, and that the i1p as it was then was nowhere near as good. When the chancec to get a DTP came up I jumped at it. I am now using multi read spot mode on the io which looks like it should give the best result. My observation of the i1p2 is that it is very good on repeatability.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 17, 2017, 07:57:09 AM
For the diagramming it would have been easier to distinguish between the instruments had you chosen to represent each instrument with a different solid colour gamut map. You can do that in CTP. Anyhow, no big deal, one can see regardless that the gamut shapes and volumes are close enough that either instrument would probably do about as well (in respect of print quality) at least in respect of that variable. Profile accuracy however is another matter that cannot be inferred from the gamut shape alone.
Agreed, Mark, and apologies for not making it more obvious. From day one with the DTP, I wondered why it didn't seem to like darks, and now Ethan has pointed out the problem. On one target, if the first patch was close to black, it would give a weird reading, but if I moved the patch, it was better. I have an awful feeling that the repair price is going to be a deal breaker.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 08:04:56 AM
I'm really interested to hear what others have to say anout the i1p2. For years, i read that the DTP was a stellar device, and that the i1p as it was then was nowhere near as good. When the chancec to get a DTP came up I jumped at it. I am now using multi read spot mode on the io which looks like it should give the best result. My observation of the i1p2 is that it is very good on repeatability.

Much as I have no love-lost for XRite as a company, I have to say - I've been using my i1Pro2 for over a year now, done a considerable amount of work with it as you can peruse from the extensive printer and paper reviews I've researched and written for this website, and I think it is a very good instrument. It bundles the three luminance conditions (M0, M1, M2) in one set of readings and its accuracy/consistency appear to be of a high standard - at the very least high enough to make high quality profiles that deliver high quality prints, and consistent enough in its spot measurements to be analytically very useful. i1Profiler is of course part of the story and a less happy one - it does the needful from a technical perspective, but just a dog's breakfast with no manual.

I have been advised by several knowledgeable people when making profiles to print profiling targets with scrambled patches, so that the differences between them are more obvious to the spectro and therefore less error prone. Using MeasureChart in Spot mode does work very well for measuring various patch sets (making sure the patches are large enough for accurate unambiguous placement of instrument) or individual colours, but this of course is not for creating profiles.

As for servicing whic you mention just above, There are two hurdles with XRite. In their "infinite wisdom" they do choose rather aggressively when to discontinue support to "legacy" products, so the first hurdle is whether they will service at all, and then of course the cost.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 17, 2017, 08:17:10 AM
I'm in violent agreement with you on all of that. I'm still contemplating whether to use DropRGB but, despite the lack of documentation i1p does build good profiles. Using the io let's me do the spot read easily. I woukd not contemplate doing that hy hand. I have been using scrambled patches for years, not as pretty but seems to work well. I also up the patch size by at least 1mm from the default.
Title: Re: Comparing spectros
Post by: Rhossydd on June 17, 2017, 08:58:08 AM
To do accuracy testing you don't need to buy anything. You need a target with known reference values and you can construct this yourself in Photoshop
I'd agree that to do any sort of absolute accuracy testing with spectros needs a known reference target, but building a target yourself seems too unreliable.
It assumes that the profile is absolutely correct and that the printer can deliver output absolutely consistently. They are both big assumptions.
In my experience printers don't deliver results more accurately than spectros.

Comparing different spectros for absolute accuracy is a complex and difficult task, especially so if you need specially formatted targets for autospectros.

Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 09:10:40 AM
I'd agree that to do any sort of absolute accuracy testing with spectros needs a known reference target, but building a target yourself seems too unreliable.
It assumes that the profile is absolutely correct and that the printer can deliver output absolutely consistently. They are both big assumptions.
In my experience printers don't deliver results more accurately than spectros.

Comparing different spectros for absolute accuracy is a complex and difficult task, especially so if you need specially formatted targets for autospectros.

It depends on what you are testing and to what end.
Title: Re: Comparing spectros
Post by: Rhossydd on June 17, 2017, 09:20:57 AM
It depends on what you are testing and to what end.
The OP's original question is fairly simple;
"I have no idea how to know which one is the more accurate"

To know the answer to that requires comparing to a known accurate reference.

DIY charts have a use for comparative testing, but for absolutes they won't be any help.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 09:26:04 AM
Chasing absolute accuracy with any of this stuff is somewhat ephemeral, because everything in the chain can have small variances. Comparative testing suffices for the OP's purposes and that's the context I had in mind when I posted my response. I.E., amongst the options he has on hand, which is going to deliver the most correct and reliable outcomes would seem to be the practical question at hand.
Title: Re: Comparing spectros
Post by: Rhossydd on June 17, 2017, 10:49:25 AM
Comparative testing suffices for the OP's purposes
Well that might be your opinion, but his question isn't answered; "which one is the more accurate"

The only way to discover that is working from a known reference.
A CC24 might help get an insight, a Colorchecker SG might get a bit further, but there will still be a need to do some comparative testing to discover the repeatability of instruments to know what the limitations of the spectros measurements are.

I understand the interest in this issue having been through these enquiries myself when I first owned more than one spectro. After lots of testing and analysis my own opinion is that when the numbers start to drop below 1dE it's time to stop worrying about it and get on with making pictures. The CM tech will be good enough.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 11:51:26 AM
Well that might be your opinion, but his question isn't answered; "which one is the more accurate"
drop below 1dE it's time to stop worrying about it and get on with making pictures. The CM tech will be good enough.

Yes, he wants to know which of his options is more accurate, so that's a "comparative" matter and the advice I gave him is very workable for his purpose without spending a bunch of money. That will be it from me on methodology, unless the OP wants any operational advice.
Title: Re: Comparing spectros
Post by: Rhossydd on June 17, 2017, 12:35:46 PM
Yes, he wants to know which of his options is more accurate, so that's a "comparative" matter
Only in the sense of comparing to a known accurate reference.

If you have two rulers and one reads 306mm and one reads 309mm you can compare them as much as you want, but the only thing that will confirm which is closest to the correct distance is to compare one to a known accurate distance.

Yes, he's going to need access to or buy a reference. There's no way to avoid it if he wants to know which is more accurate.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 17, 2017, 01:19:32 PM
Not necessarily, there are different workable approaches; but I'll leave that for you to figure out; I'm just plain out of time; sorry.
Title: Re: Comparing spectros
Post by: Doug Gray on June 17, 2017, 01:20:40 PM
Only in the sense of comparing to a known accurate reference.

If you have two rulers and one reads 306mm and one reads 309mm you can compare them as much as you want, but the only thing that will confirm which is closest to the correct distance is to compare one to a known accurate distance.

Yes, he's going to need access to or buy a reference. There's no way to avoid it if he wants to know which is more accurate.

I actually had the same question come up when trying to, "accurately," measure printed ink distances. In addition to the mechanical issues of print head positional accuracy and repeatability there is the variation in paper since dimensions can change slightly over time, humidity, etc.

So how accurate were my steel tapes? And how consistent?

I had a SRX1 (surveyor's total station) which can measure angles accurately to about 1 second of arc but can only measure distance accurately to about 1.5mm.

By trading off smaller angles with increased distance one can find a point where an optimal accuracy is achieved. At 15m distance the angular and distance accuracy come out to about 0.1mm expected error across a 1m tape.

I found some steel tapes had cyclical variation of about .5mm but most were surprisingly good. I took the best one, which had errors of less than about 0.2mm, and use that with a loupe to measure printed ink distance.

I haven't found anything similar that would provide a reference standard for determining color measurement accuracy. There is considerable variation amongst the 3 I1's I have in the more saturated colors. But that variation decreases as colors desaturate. All are in reasonable agreement in or near neutral colors.

On another related point, X-rite, when it swallowed GMB, added some small changes in how the GMB (I1, etc) instruments read color so the company's product lines would more closely agree with each other. XRGA I think it's called. I have found essentially no documentation on the algorithms of this change but it involved differences around 2 to 3 dE on some colors.
Title: Re: Comparing spectros
Post by: aaron125 on June 17, 2017, 03:32:16 PM
If you search hard enough, you might find X-Rite's white paper about their XRGA standard. I remember reading it quite some years back when they bought GMB.


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Title: Re: Comparing spectros
Post by: Rhossydd on June 17, 2017, 03:50:42 PM
I haven't found anything similar that would provide a reference standard for determining color measurement accuracy.
Last time I looked into this Hutchcolor offered the most credible affordable reference targets. I think Wolf Faust also offered similarly individually measured and referenced targets.
At more affordable levels the Colorchecker SG is probably the best option.
The CC24 seems to have gone through so many slight variations in manufacture process that getting the most accurate reference data is trickier, but is still a good starting point for amateur investigations.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 18, 2017, 12:08:36 AM
The targets that you mention all appear to be scanner targets. What I need is a DTP70 formatted target that I can also read in spot mode with the I1p2. I'll attach a grab of a DTP70 target.
Title: Re: Comparing spectros
Post by: Rhossydd on June 18, 2017, 02:43:14 AM
The targets that you mention all appear to be scanner targets.
No, they're just reference charts. You can use them for scanner profiling, plus cameras and other systems.
The point is that they can be used to ascertain the accuracy of the handheld spectro and once you've an idea of limits of that, you can then measure your own autospectro charts to see how they compare.
It's not perfect, but the most affordable way to get an idea of what each instrument is capable of.

Quote
What I need is a DTP70 formatted target that I can also read in spot mode with the I1p2.
I've never found a DTP70 reference chart for sale anywhere. I'd assume that only X-Rite would have any. They might sell you one, but I would have thought it extremely unlikely that they still would have any they would be prepared to sell and if they did it would be uneconomically expensive.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 18, 2017, 02:50:33 AM
Got it. Validate the accuracy of the i1p2 first and go from there. Here's a link to the DTP compatible chart. Unfortunately, it's $500 as it includes a subscription fee: https://chromachecker.com/info/en/page/A2

Do you have a recommendation of a chart to select?
Title: Re: Comparing spectros
Post by: Rhossydd on June 18, 2017, 03:14:13 AM
Here's a link to the DTP compatible chart. Unfortunately, it's $500 as it includes a subscription fee:
'on-line shop not available' Interesting, but the information shown suggests that chart might be $500 additionally to the $4k annual subscription.
It might be worth emailing them to see if they'd sell a chart subscription free with no support given. I'd be interested to know what they reply.

Quote
Do you have a recommendation of a chart to select?
Whatever you have available ? A CC24 comes with i1Profiler, colorchecker SG came with PMP5.
If you have neither, a CC24 or X-Rite passport would be the one of the easiest ways to get a reference. Both could also be used for camera profiling too.
X-Rite also supplied a reference target for scanner profiling with i1Match in the past. You could get involved with some reference file hacking and DIY to use it as a reference for the DTP70, but that would be quite challenging.
Title: Re: Comparing spectros
Post by: GWGill on June 18, 2017, 09:32:34 AM
Ethan - even if only 10 or 20 of those samples are usable, it's still enough to make a useful average for the purposes of eliminating undue impact from outliers.
Number of samples doesn't mean much if each sample has a poor signal to noise ratio. Total light collected is typically what counts. What can improve S/N is slowing your scan down, but in terms of repeatability, the incandescent lamp in the i1Pro is its main limitation. Thermal effects cause noticeable reading to reading variation. In contrast, the ColorMunki spectro has much better repeatability, comparable or even superior to the SpectroScan.
Title: Re: Comparing spectros
Post by: GWGill on June 18, 2017, 09:40:15 AM
XRGA I think it's called. I have found essentially no documentation on the algorithms of this change but it involved differences around 2 to 3 dE on some colors.
Persusing the xrga_equations table in spectro/xrga.c in the ArgyllCMS source code should provide all the technical information you are after.
Title: Re: Comparing spectros
Post by: Doug Gray on June 18, 2017, 10:20:58 AM
Persusing the xrga_equations table in spectro/xrga.c in the ArgyllCMS source code should provide all the technical information you are after.
Perfect!  No idea why I didn't think to look there instead of searching for pdfs.

Thanks.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 18, 2017, 10:29:03 AM
Number of samples doesn't mean much if each sample has a poor signal to noise ratio. Total light collected is typically what counts. What can improve S/N is slowing your scan down, but in terms of repeatability, the incandescent lamp in the i1Pro is its main limitation. Thermal effects cause noticeable reading to reading variation. In contrast, the ColorMunki spectro has much better repeatability, comparable or even superior to the SpectroScan.

Yes, I've tested for reading to reading variation with my i1Pro2 under two conditions: (1) re-reading a patch without moving the spectro at all, and (2) taking numerous readings from different parts of the patch (in this case patches being at least a square inch large so one can move around in them such that each reading is unique. So I know where this matter stands and variances are there but livable. Approach (2) of course doesn't totally isolate the spectro because it is possible that ink laydown isn't totally uniform accross the patch, and indeed, the variances from (1) tend to be much lower than those from (2). I made such measurements on medium to high density patches, as well as on paper white. The outcome of all this was that with approach (1) dE(76) outcomes were within a narrow range of several 10ths of a dE, whereas with approach (2) the dE outcomes had a wider range but none greater than 1.
Title: Re: Comparing spectros
Post by: Rhossydd on June 18, 2017, 12:02:42 PM
with approach (2) the dE outcomes had a wider range but none greater than 1.
This is the sort of result (I've seen the same with testing here) that tells us that a degree of pragmatism is needed when idealising about which instrument is better.
If printers have a variability significantly greater than the instruments that measure their output, it's not worth getting too obsessed about a small amount of variability of readings.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 18, 2017, 12:24:51 PM
This is the sort of result (I've seen the same with testing here) that tells us that a degree of pragmatism is needed when idealising about which instrument is better.
If printers have a variability significantly greater than the instruments that measure their output, it's not worth getting too obsessed about a small amount of variability of readings.

Totally agree.

And it gets to be even more fun when you look at prints made with different printers, different profiles, but the same paper and have a hard time seeing the difference between them - because your colour management set-up is good enough to do what it is supposed to be doing.
Title: Re: Comparing spectros
Post by: Doug Gray on June 18, 2017, 01:40:47 PM
Totally agree.

And it gets to be even more fun when you look at prints made with different printers, different profiles, but the same paper and have a hard time seeing the difference between them - because your colour management set-up is good enough to do what it is supposed to be doing.

This is the sort of thing I simply expect from good color management. When it doesn't happen it's almost always because I did something wrong in the underlying color management process. At least apart from significant BP variation and that is usually minimal with the same paper between different printers.
Title: Re: Comparing spectros
Post by: GWGill on June 18, 2017, 09:41:07 PM
Last time I looked into this Hutchcolor offered the most credible affordable reference targets. I think Wolf Faust also offered similarly individually measured and referenced targets.
At more affordable levels the Colorchecker SG is probably the best option.
The CC24 seems to have gone through so many slight variations in manufacture process that getting the most accurate reference data is trickier, but is still a good starting point for amateur investigations.
None of these charts strike me as being suitable for checking reflection instrument accuracy. None of them typically come with individually measured reference files measured using a reference grade instrument.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 19, 2017, 12:51:22 AM
I've just heard back from Xrite. The DTP 70 is well and truly discontinued so I guess that is the end of this little exercise, unless someone out there is repairing them. I did the exercise of testing my new i1p2 on the ColorChecker. I'll attach the results.

Thanks for all the help .
Title: Re: Comparing spectros
Post by: Rhossydd on June 19, 2017, 01:07:06 AM
None of these charts strike me as being suitable for checking reflection instrument accuracy. None of them typically come with individually measured reference files measured using a reference grade instrument.
The key word was "affordable". Individually measured targets are VERY expensive.
Whilst not perfect, the charts mentioned are probably getting as close to spec. as is needed for the OP's purpose.
Title: Re: Comparing spectros
Post by: GWGill on June 19, 2017, 07:54:50 PM
The key word was "affordable". Individually measured targets are VERY expensive.
Whilst not perfect, the charts mentioned are probably getting as close to spec. as is needed for the OP's purpose.
Hmm. Care to point to an available chart at any price that is measured to a traceable reference grade ?
BCRA/CERAM tiles etc. yes. But a chart produced using graphic arts type technology ?
- the charts mentioned so far in this thread seem to be of the type that typically use i1Pro2 grade instruments as their references, so unsuitable for checking the accuracy such instruments.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 19, 2017, 10:20:22 PM
Here's the target that I was looking at initially: https://chromachecker.com/info/en/page/A2

Certainly not cheap, but more trustworthy?
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 20, 2017, 01:15:30 AM
I also discovered today that Colorport puts out an amount of bad spectral data. I don't know how much but on the patch that I discovered it on, it literally makes black white. I've changed to PM5 output, and the problem that I know of goes away. Anyone know of another target reading app that supports the DTP70?
Title: Re: Comparing spectros
Post by: Rhossydd on June 20, 2017, 01:28:49 AM
I also discovered today that Colorport puts out an amount of bad spectral data.
I've been using Colorport with a DTP70 for years and never had a problem with the combination.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 20, 2017, 02:55:02 AM
I've been using Colorport with a DTP70 for years and never had a problem with the combination.

I envy your good fortune or superior knowledge. I'll attach a couple of grabs. The second is from CTP which shows L of 86.79 for patch 1. The first is from the Colorport CGATS file which shows L of 6.28.

If you have a DTP70, would you measure a target for comparison? That would be very interesting for me.
Title: Re: Comparing spectros
Post by: Alan Goldhammer on June 20, 2017, 08:23:46 AM
Here's the target that I was looking at initially: https://chromachecker.com/info/en/page/A2

Certainly not cheap, but more trustworthy?
I went to the website and it appears to be part of a kit with a $500 price tag on it.  I couldn't find a link that allowed for separate purchase though I imagine if you contact the company perhaps you can. 
Title: Re: Comparing spectros
Post by: GWGill on June 20, 2017, 10:15:31 AM
Here's the target that I was looking at initially: https://chromachecker.com/info/en/page/A2

Certainly not cheap, but more trustworthy?
Given that there seems to be no indication as to whether  a reference file is supplied or not, never mind whether it is individually measured or what instrument/standard it is measured to, I don't see why you think it might be used to establish the accuracy of  DTP70 or i1Pro class instrument.

On the contrary, Looking through their marketing material, it seem that all they are offering here is a target for relative measurement - i.e. tracking how a particular instrument changes, or comparing between your different instruments. They aren't providing a high accuracy baseline measurement against a reference.
Title: Re: Comparing spectros
Post by: Mark D Segal on June 20, 2017, 11:00:46 AM
I'm making an assumption that the O/P's objective is to select the profile-making solution that will help him print file values as accurately as his equipment and materials will allow. In that context, to put a bit of perspective into this discussion, I think it's important to be realistic about what's important and hence what's needed. Spot-on accuracy with prosumer or lower-end professional equipment and materials is not likely to be achievable, but for most people using this stuff it also isn't important, because beyond a certain point human visual perception has limits that make some levels of tolerances acceptable; and these levels can vary depending on what colours and what aspects of the colours one is looking at. For this class of equipment and materials, most peoples' needs would be adequately served using methods that are capable of ranking which produce lower dE than others relative to known reference values for the patches on the target being printed. So, for example, if I print and measure a printer evaluation target with a resulting dE average of 0.8 (and low dispersion around the mean) versus another (say, using another profile) with average dE of 2.5 and higher dispersion, I would tend to prefer the 0.8 result, even knowing that part of the difference could also include for variances in the performance of the instrument being used for the measurements. So going back to the O/P, if he were to use his DTP-70 and i1Pro2 to make profiles with which he prints evaluation targets having known references values and measures them with the same equipment used to make the profiles, he can find out enough to make a workable choice for his future work.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 20, 2017, 08:35:00 PM
Given that there seems to be no indication as to whether  a reference file is supplied or not, never mind whether it is individually measured or what instrument/standard it is measured to, I don't see why you think it might be used to establish the accuracy of  DTP70 or i1Pro class instrument.

On the contrary, Looking through their marketing material, it seem that all they are offering here is a target for relative measurement - i.e. tracking how a particular instrument changes, or comparing between your different instruments. They aren't providing a high accuracy baseline measurement against a reference.

Thanks. That's why I asked the question. It would appear that such a product for measuring accuracy does not exist.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 20, 2017, 08:55:30 PM
I'm making an assumption that the O/P's objective is to select the profile-making solution that will help him print file values as accurately as his equipment and materials will allow......  So going back to the O/P, if he were to use his DTP-70 and i1Pro2 to make profiles with which he prints evaluation targets having known references values and measures them with the same equipment used to make the profiles, he can find out enough to make a workable choice for his future work.

Thanks Mark, that sums it up well. I've been down a lot of rabbit holes along the way, mainly, I now suspect due to Colorport rather than the DTP70 itself. I started this thread because I could not reconcile what I was seeing from the two devices.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 20, 2017, 09:44:35 PM
I may be missing something very basic here and would love to be enlightened. I have discovered that Colorport gives quite different measurement files depending on the output format. Measuring a 2033 patch target I discovered that each file is different. The worst example is between PM5 output and CGATS. The first two patches have a de of 79 and 87 respectively and then 0 for the other 2031 patches. Other variations produce differences but not as dramatic. I have measured the patch set a number of times and the PM5/CGATS difference is repeatable so it wasn't just one reading.

It's probably my own lack of knowledge at play here but I would not have expected to see differences when saving the same set of data in different formats. I have put the files on Dropbox if anyone want's to see them: https://www.dropbox.com/sh/qv3mu764lrbrmrc/AADSRQl5TlSeaDI5TRMSkL2Va?dl=0
Title: Re: Comparing spectros
Post by: Doug Gray on June 21, 2017, 12:07:24 AM
I may be missing something very basic here and would love to be enlightened. I have discovered that Colorport gives quite different measurement files depending on the output format. Measuring a 2033 patch target I discovered that each file is different. The worst example is between PM5 output and CGATS. The first two patches have a de of 79 and 87 respectively and then 0 for the other 2031 patches. Other variations produce differences but not as dramatic. I have measured the patch set a number of times and the PM5/CGATS difference is repeatable so it wasn't just one reading.

It's probably my own lack of knowledge at play here but I would not have expected to see differences when saving the same set of data in different formats. I have put the files on Dropbox if anyone want's to see them: https://www.dropbox.com/sh/qv3mu764lrbrmrc/AADSRQl5TlSeaDI5TRMSkL2Va?dl=0

I looked at the 4 files with PatchTool.

The CGATS Data XRGA 380.txt file differs from the others in that it also has computed LAB values in addition to the spectral values.

The other major difference is that it's spectral numbers are scaled to 100 whereas the others are scaled to 1.

It also appears to be spectrally identical to ProfileMakerCGATS XRGA.txt except for the scaling factor of course.

Other than that they are reasonably close. The CGATS Data XRGA 380.txt median error with ProfileMakerCGATS Gretag.txt is  0.54, and with ProfileMakerCGATS None.txt 0.24
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 21, 2017, 12:41:26 AM
Thanks Doug. I understand the differences. What I don't understand is why there are differences. These files come from the same scan.
Title: Re: Comparing spectros
Post by: Doug Gray on June 21, 2017, 01:18:46 AM
Thanks Doug. I understand the differences. What I don't understand is why there are differences. These files come from the same scan.
The conversion to XRGA adjusts the spectral data slightly so that the product lines that got merged into x-rite would produce more closely aligned data. That's what produces the biggest difference. I'd guess the smaller change is just specific to the instrument with a small adjustment so the instrument more closely matched some internal reference between different products in their line.
Title: Re: Comparing spectros
Post by: Rhossydd on June 21, 2017, 01:43:59 AM
I now suspect due to Colorport
Colorport is fine and works well. The problem here is your understanding of the data it produces and what you're doing with it.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 21, 2017, 02:11:40 AM
The conversion to XRGA adjusts the spectral data slightly so that the product lines that got merged into x-rite would produce more closely aligned data. That's what produces the biggest difference. I'd guess the smaller change is just specific to the instrument with a small adjustment so the instrument more closely matched some internal reference between different products in their line.

Thanks Doug.
Title: Re: Comparing spectros
Post by: Pat Herold on June 21, 2017, 07:23:43 PM
The kinds of differences you are referring to sound exactly like the kind introduced by XRGA.  I could not find any recent info on xRGA on the X-Rite website.  The link to the white paper referred to here is no longer active:
http://whattheythink.com/articles/53636-x-rite-xrga/

But I will include our own copy here.  You'll have to read through it and decide which flavor of export is more appropriate for your purposes.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 21, 2017, 07:54:43 PM
Thanks Pat.
Title: Re: Comparing spectros
Post by: Ethan Hansen on June 23, 2017, 12:29:34 AM
Regarding XRGA, this standard came about because of X-Rite's acquisition of GretagMacbeth. Both companies calibrated their instruments to internal reference standards. This led to instruments from CMB producing slightly different readings on the same substrate as did legacy X-Rite devices. As a unified entity, the goal was to define a common reference point and calibration standard that minimized systematic differences between existing instruments and would be the baseline for future spectrophotometers.

If you look at the average post-XRGA measurement differences between instrument families (see the pdf Pat posted above), they are in roughly the same range as inter-instrument variation for each instrument. In other words, don't fret it.

Back to the original topic of this thread, I pulled the measurements we made comparing various spectrophotometers. I only included measurements made directly on BCRA tiles (hauled around the country to different facilities), not secondary standards such as plastics or prints that fit in scanning instruments. We measured eight devices for all but the Spectrolino and I plotted average and maximum average dE for the 14 tiles. We had measurements for 19 Spectrolinos. The chart is attached below.

A general trend emerged: higher-end instruments have somewhat lower average inter-instrument dE and dramatically lower maximum dE. Even with the higher sample size, the old Spectrolino exceeded the capability of newer instruments in terms of maximum measured differences.

Mark brought up whether these differences are significant in the real world. If you use larger than minimum patch sizes with an i1Pro and scan slowly, the argument degenerates into trivial quibbling. Minimum size patches and speedy scanning are another matter, but this comes down to instrument limitations, not inherent accuracy.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 23, 2017, 03:15:25 AM
Thanks Ethan. That picture is worth a thousand words. I have taken to using my i1p2 on the io in spot mode after reading some of the previous posts. I have increased patch size by 1mm each way. Do you have a suggestion for an optimal size?
Title: Re: Comparing spectros
Post by: Ethan Hansen on June 23, 2017, 12:53:51 PM
Jeff - Patch size matters when you use the i1 in normal scanning mode. As Graeme mentioned earlier in this thread, the signal-to-noise level in the i1Pro appears to be lower than in other instruments. Modern cell phone cameras have pixel resolution comparable to a mid-range DSLR. Which one are you going to use for your next job? Throw in the need to determine which measurements only contain colors from a single patch and problems can arise.

If you use the i1p2 in spot patch mode, you should not need to worry about patch size. As long as the layout detection is accurate, you essentially turn the i1iO into a Spectroscan. Move, stop, measure, repeat. Your main worry is heat. An improved dark current sensor makes the i1p2 less sensitive to thermal variations than the original i1Pro. Nevertheless, it still exists. Our only use for the i1p2 is measuring spot colors or ambient light in remote locations. A more regular user can offer better advice than I. A simple minded test would be to measure a page of a single color. Compare readings and see if there is variation for patches read immediately before and after visits to the white calibration tile.
Title: Re: Comparing spectros
Post by: Jeff-Grant on June 23, 2017, 10:58:33 PM
Thanks Ethan. I just tried a repeatability test on the i1p2 on io2. I printed a 140x210mm page with half at RGB 15,15,15 and the other half at 245,245,245. Using the measure chart in i1p to give me 600 7mm patches. Choosing dark and light seemed to be a good test as the i1p seems less comfortable at the ends of the spectrum. I scanned in spot with two reads per patch. The max de of 0.61 works for me. I can live with that.