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Author Topic: Shortcut to determining good spectro results when they disagree  (Read 543 times)

Doug Gray

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I recently read a collection of 8k patches with an I1iSis XL 2 and noticed some peculiar "dents" in the wire frame gamut I1Profiler draws. This is never good.

My target image has registration bars at the top and bottom so I can read the same charts in reverse order. This reads the last page first, then bottom to top and right to left instead of the normal sequence. The result is the same patches but reversed in order.

This also is a way to detect errors that sometimes occur when the same chart is read in the same automated way.

So I read the chart in reverse and looked at the dE76 values. Worst case was over 10. The first 200 largest dE76's were over 1.0.

I've seen this before. It's because of a bit of lint or dust reflecting rather small amounts of light. This doesn't normally cause a problem because most of the spectra have reasonable reflectance. But here's where it matters. Near the gamut edges on the lowish L* side, there are portions of the spectral reflectance values that are only a few percent ( .01 ). Sometimes less than .01.  If a tiny dust particle happens upon the focused light, which is only a few mm's across, it can easily reflect enough light to increase the measured reflectance from say, .01 to .02.  This may not seem like much but it is because conversion to Lab is non-linear.

So what I did was compare spectral plots from the largest 20 dE readings which range from about 11 down to 6. In each case the reverse readings were lower than the forward readings and, in each case the spectral values affected were the small values below .04.

So, when you have two scans with large differences between them, you should select the ones with the smaller minimum in the spectra since lint/dust will nearly always result in an increase in measured reflectance. OTOH, if the piece of lint happened to be quite dark, it would hardly effect anything. Blocking 1% of the light has almost no affect on dE. But adding 1% is a killer.

So I cleaned the iSis and re-read the 8k charts in both directions. dE on all patches was under 1 and averaged about .10.  Comparing the dE between these and the prior bad pair set, but selecting the patch values that had the smallest minimum spectrum reflectance from the bad pair set produced nearly identical results. From a situation where 200 patches all had dEs over 1 to where everything was under 1.

Probably a good idea to always do a second pass (in reverse order if you have the tools) and look for signs of dust/lint messing up readings. But, if one only has the two sets of readings and some of them differ significantly, select the ones with the lower spectral minimum. Overall, this will work better than averaging them.

But best to clean the equipment and re-do.
« Last Edit: May 07, 2019, 09:09:14 pm by Doug Gray »
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Pat Herold

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Re: Shortcut to determining good spectro results when they disagree
« Reply #1 on: May 07, 2019, 07:36:53 pm »

What a great example of what some have called "digital noise" in measuring dark patches.  Yes, those darker patches are very susceptible to a stray piece of something that would reflect light back and mess up the accuracy of the black measurement.  It does not take much!

Many in Lulu are mostly interested in measuring for profiling purposes, but we have dealt a lot with companies that print control charts daily to ensure that they are printing to within tight tolerances.  In these settings, precision in measurement and repeatability are, if anything, more important.  For those situations, increasing patch size to about 1/2 inch is great for reducing noise and ensuring accuracy on an iSis.  It's certainly too large a patch size for practical profiling purposes, but it has its uses as I've said.

The iSis has been on the market for many years.  If you have one that's been around awhile and you've never opened it up to clean it - let Doug's results be a lesson to you!
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-Patrick Herold
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Doug Gray

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Re: Shortcut to determining good spectro results when they disagree
« Reply #2 on: May 07, 2019, 09:40:55 pm »

What a great example of what some have called "digital noise" in measuring dark patches.  Yes, those darker patches are very susceptible to a stray piece of something that would reflect light back and mess up the accuracy of the black measurement.  It does not take much!

Many in Lulu are mostly interested in measuring for profiling purposes, but we have dealt a lot with companies that print control charts daily to ensure that they are printing to within tight tolerances.  In these settings, precision in measurement and repeatability are, if anything, more important.  For those situations, increasing patch size to about 1/2 inch is great for reducing noise and ensuring accuracy on an iSis.  It's certainly too large a patch size for practical profiling purposes, but it has its uses as I've said.

The iSis has been on the market for many years.  If you have one that's been around awhile and you've never opened it up to clean it - let Doug's results be a lesson to you!

Hi Patrick,

Yep, increasing the patch size does reduce noise errors.  Some time back I did some experiments with special targets that let me determine the actual area scanned by different size patches. Both horizontal and vertical size changes.

Wider patch sizes helps because, as you point out, the region scanned is larger. Longer patch sizes doesn't because the effective scan length wise is fixed. It can provide increased margin to separate patches from those above or below. This mostly comes into effect scanning large papers. Especially when scanning M0,1 and 2. which involves a back hitch and this can introduce positional error. I've seen it but it's rare. For large papers it's critical to have the paper laying flat as it feeds all the way through the iSis XL. If one end goes off a table it can introduce greater resistance on the back hitch and get a bad read. Of course increasing the patch length from 6mm to 8mm helps this too but at the cost of much more paper. I rarely do M0/1 scans and found  just scanning M2 sees little benefit from longer patch sizes. But M2 does benefit from wider patches.

Generally, rather than wider patches, I invert the prints and read them in reverse but I have a program that prints the black registration bar on both the top and bottom of the charts.  Comes in handy at times.

Ultimately, I decided on keeping the 6mm size but reading them forward/reverse and checking the stats for critical measurements.

Couldn't agree more on the need to keep the instrument clean as well as having consistency checks.
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