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Author Topic: iSis 2 XL - dual scan vs single scan deviation  (Read 2156 times)

Zachary Goulko

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iSis 2 XL - dual scan vs single scan deviation
« on: October 28, 2017, 03:44:17 pm »

I've been testing the iSis 2 XL over the past week and noticed a pretty big deviation between scanning in single and dual scan modes.
To test I used a single 2945 patch chart and measured it once in dual scan mode (M0, M1, and M2), and another scan in single scan (M2 only).
When comparing the M2 data between the single scan and dual scan mode there's a significant max delta E difference of 2.89 (average of 1.18) versus comparing two scans in dual mode which gives a max delta E difference of 1 (average 0.14). So the question is, which scan mode is more accurate if I only need the M2 data, and why do the dual vs single scan measurements differ so much?
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Zachary Goulko
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Doug Gray

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Re: iSis 2 XL - dual scan vs single scan deviation
« Reply #1 on: October 28, 2017, 04:32:08 pm »

I've been testing the iSis 2 XL over the past week and noticed a pretty big deviation between scanning in single and dual scan modes.
To test I used a single 2945 patch chart and measured it once in dual scan mode (M0, M1, and M2), and another scan in single scan (M2 only).
When comparing the M2 data between the single scan and dual scan mode there's a significant max delta E difference of 2.89 (average of 1.18) versus comparing two scans in dual mode which gives a max delta E difference of 1 (average 0.14). So the question is, which scan mode is more accurate if I only need the M2 data, and why do the dual vs single scan measurements differ so much?

M2 single scan is more accurate. I had a Super A3+ shift 10 dE with about 5 patches over 3 dE in dual scan mode comparing two, dual mode runs of the same target. Turned out the major reason was that the paper wasn't lying flat all the way through. Near the end, where the deviations were the greatest, the top corner was curling over the edge of my table which created assymetical drag when the paper is back hitched for the uV reading pass.

That issue aside, the back hitching will, even with perfectly flat, low drag surfaces produce some shift in where on the patch the spectro reads.  Other measurements I've made indicate the effective area read is only about 2mm by 3mm. So paper irregularities and tiny inking differences over a small spatial shift can result in significant dE. Because the M2 single pass mode has less spatial shift, the passes tend to match more closely.

Added: Also, I made a checkerboard pattern of black/white 6mm squares which is a good way to check registration and that you aren't getting light leaking in from nearby patches. Look for small changes only in the black square L*. Anything over 2 is problematic and represents about 0.2% light leakage from an adjacent white square. Ignore the white square variation as the paper irregularities are much larger than that. The change  from L* 95 to 95.1 has the same % reflectance change as L* 4 to 6 on the black patches
« Last Edit: October 28, 2017, 04:58:00 pm by Doug Gray »
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Steve Upton

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Re: iSis 2 XL - dual scan vs single scan deviation
« Reply #2 on: October 28, 2017, 06:22:05 pm »

So the question is, which scan mode is more accurate if I only need the M2 data, and why do the dual vs single scan measurements differ so much?

Itís important to scan multiple times in each mode before you can draw conclusions about the difference between modes.

The iSis scans each row twice only when you want M0 or M1 (with the iSis 2) measurements. In one direction it uses the visible LEDs and in the other the invisible UV LED. But itís important to realize that there is no ďdual scan modeĒ for M2 measurements. Only one scan is required/used for M2 - that using the UV-less visible LED light source.

Iím curious what kind of variation you see when you scan each way multiple times. Hint: patch width makes a big difference....
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Doug Gray

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Re: iSis 2 XL - dual scan vs single scan deviation
« Reply #3 on: October 28, 2017, 07:02:34 pm »

Itís important to scan multiple times in each mode before you can draw conclusions about the difference between modes.

The iSis scans each row twice only when you want M0 or M1 (with the iSis 2) measurements. In one direction it uses the visible LEDs and in the other the invisible UV LED. But itís important to realize that there is no ďdual scan modeĒ for M2 measurements. Only one scan is required/used for M2 - that using the UV-less visible LED light source.

Iím curious what kind of variation you see when you scan each way multiple times. Hint: patch width makes a big difference....

In M2 mode iSiS 2XL horizontally scans a row with the white led, then moves the paper forward to the next row and scans horizontally in the reverse direction with the white led and again moves forward to the next row.  This is repeated until the last row is scanned.

For dual scan, it scans two rows as above, then, instead of advancing to the next row, backs the paper up to the prior row and scans the same two rows with a uV only led. Then the next two rows are processed, etc.

While scanning it makes micro adjustments, tiny fractions of a mm, to account for imperfect printing. It does not require any information such as color changes between adjacent patches. All the patches can be the same color and it will measure each of them based purely on their geometry relative to the black reference bar located above the top row and the diamond registration marks at the margin of each row.

Please see this post where large deviations occurred and the cause discovered (paper curving over table edge)

http://forum.luminous-landscape.com/index.php?topic=119448.msg1003106#msg1003106


« Last Edit: October 28, 2017, 10:56:35 pm by Doug Gray »
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Zachary Goulko

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Re: iSis 2 XL - dual scan vs single scan deviation
« Reply #4 on: October 28, 2017, 11:05:05 pm »

M2 single scan is more accurate. I had a Super A3+ shift 10 dE with about 5 patches over 3 dE in dual scan mode comparing two, dual mode runs of the same target. Turned out the major reason was that the paper wasn't lying flat all the way through. Near the end, where the deviations were the greatest, the top corner was curling over the edge of my table which created assymetical drag when the paper is back hitched for the uV reading pass.

That issue aside, the back hitching will, even with perfectly flat, low drag surfaces produce some shift in where on the patch the spectro reads.  Other measurements I've made indicate the effective area read is only about 2mm by 3mm. So paper irregularities and tiny inking differences over a small spatial shift can result in significant dE. Because the M2 single pass mode has less spatial shift, the passes tend to match more closely.

Added: Also, I made a checkerboard pattern of black/white 6mm squares which is a good way to check registration and that you aren't getting light leaking in from nearby patches. Look for small changes only in the black square L*. Anything over 2 is problematic and represents about 0.2% light leakage from an adjacent white square. Ignore the white square variation as the paper irregularities are much larger than that. The change  from L* 95 to 95.1 has the same % reflectance change as L* 4 to 6 on the black patches

Interesting. I will test out a checker pattern and see what results I get.
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Zachary Goulko
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Zachary Goulko

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Re: iSis 2 XL - dual scan vs single scan deviation
« Reply #5 on: October 28, 2017, 11:10:18 pm »

Hint: patch width makes a big difference....

I wonder if increasing the width of the patches dramatically to something like 10mm or even 12mm would result in more accurate readings. For instance if there are some surface imperfections, dust, scratches. wheel marks etc.. would that reduce the errors or does the iSis only scan the center of the patches.
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Zachary Goulko
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Doug Gray

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Re: iSis 2 XL - dual scan vs single scan deviation
« Reply #6 on: October 28, 2017, 11:29:06 pm »

I wonder if increasing the width of the patches dramatically to something like 10mm or even 12mm would result in more accurate readings. For instance if there are some surface imperfections, dust, scratches. wheel marks etc.. would that reduce the errors or does the iSis only scan the center of the patches.

Increasing the patch width does reduce reading error. Going from 6mm to 10mm approximately doubles the patch area read and reduces the RMS error. However, patch error from various sources in my test comparing 12mm widths against 2 charts with differently randomized patch locations resulted in significantly better results with the latter even though more overall area is read with the former. I suspect this was due to ink variance differing more when the areas read were not physically close.

Also note that I got significant, large dE increases on multiple passes with a Baryta Finestra fine art 300gsm. It is very sensitive to acquiring surface creases from the spacers.

Because these creases are just under about 1mm in width, increasing patch width does significantly improve dE consistency. Much more so on the Baryta than other papers. Another approach is adjusting the paper margins.

The changes can be quite high if the creases are in the center of the patches. I was able to effectively eliminate this by shifting the left printer margin slightly so that the creases were close to the patch transition instead of near the middle. This can be done with the patch width set at either 6 or 7 mm as the spacer separation is a multiple of both.

This is pretty much a non problem with other papers.

The "baking" approach:

I've been looking more closely at the Baryta Finestra (300gsm) luster paper's high sensitivity to color shifts from the i1iSiS guides and rollers. There was a large reduction in color changes due to tracking creases by first "baking" the target prints at 120F for 2 hours. This seems to harden the printed surface with dE reductions from pass creases reduced to that of standard glossy prints which is a negligible level. However, there is a slight cost. Patch color overall is shifted very slightly about .15 dE compared to just several days of drying at room temp.

Also, the Baryta is by far the most sensitive to these guide induced color shifts. Various other luster and semigloss papers have much less sensitivity to this even with very short drying times down to just half an hour at room temp. I don't know if this is characteristic of Baryta papers generally or that particular vendor as it's the only Baryta paper I've tested this on.
« Last Edit: November 01, 2017, 01:31:16 pm by Doug Gray »
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