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Squeeze it 'till it squeaks! - part 2 of 1 2 3

by Mike McNamee Published

Hahnemuhle Photo Rag

Three profiles were prepared, one with the X-Rite Pulse (on a 343 patch target), one with 729 patches from Monaco Profiler (on a DTP 41) and finally a tuned profile (see call-out box for tuning methodology). The modification required for the profile was a simple, all-round tweak to the saturation, to boost it by 4% in the middle of the range.

The gamut volume measured at 544,054 which may be compared with that from the Epson 3800 on the same paper of 541,293; this, in fact, is a very small difference, possibly brought about by the expansion of the gamut with the Vivid Magenta ink, but almost certainly compromised by the matt paper - the difference would show up more on a full gloss or lustre paper.

The tuned profile returned an error of 4.5LabDE/2.24 ΔE2000 which is the best result ever from all of our matt black ink results. The average of all results using a 3800 and matt black ink (ie on a matt paper by definition) is 3.4 (actually the same as the overall average) with a range from 2.4 to 4.3 ΔE2000. This is all for bespoke profiles. The main component of the error is usually the lightness channel and this was so in this case also, where it was actually 50% larger than both the saturation and hue errors. This reflects the limited Dmax of the matt papers.

Detailed Analysis - the foliage greens

For the type of work that Lukman specialises in, the accuracy of the foliage colours are important and almost universally present (flowers vary in colour but most leaves are actually quite similar in colour values). To investigate the detailed performance we made a special set of test swatches, selected to encompass all the colours we measured for the artists' medias plus a little leeway, further into the gamut. The clusters of colours were arranged in four brightness levels of 75%, 60%, 45% and 30% Lab lightness values.

The simplified data table shows that the error progressively rises as the Dmax of the black is approached, a value of 30% is only 10% shy of the maximum black that the ink/paper combination is capable of. Looking in more detail at the plot of errors across all the leaf tones also shows that the error increases with saturation that is towards the top left of each set of swatches. Again this is to be expected, especially as the residual errors we left in the profile was the saturation component, a further profile refinement might have improved things but just pinpointing the error distribution took half a day's work and we had returned home by then! The errors in the swatch sets at 75% and 60% brightness are just on the limit of detection for a human eye in controlled conditions. To put the error into perspective, a change of light source from D65 to Tungsten A was creating a difference of 17 ΔELab, that is about eight times higher than the error we achieved from the print. Even shifting from D65 to D50 was showing a difference of 5 ΔELab.

The bottom line to the analysis is that the colour error from the Epson 7880 was by now so small that it would not be practical to refine it much further, the limitations of metamerism were now controlling the result.

Boy in Turban - squeezing the magentas

The artwork for this sequence of tests was an original oil pastel by artist, Deborah Lomas. It was an ideal image as it contains vivid magentas, just the tones that the new ink set was designed to reproduce accurately! The original was first scanned on the Epson GT 10000 with a bespoke profile. Lukman then refined the scan with masked adjustment layers to correct the metamerism from the scanner and reproduce the pastel colour on screen as closely as possible. We then examined the image using a variety of colour-management tools to see just where we stood in relation to the 7880 gamut as defined by the profile for HPR. Monaco Gamut Works indicated that all of the magenta turban was in gamut but some of the orange back ground was out of gamut. Photoshop indicated that the entire image was in gamut. Plotting the colour values by hand showed that the turban was perilously close to the gamut boundary but just inside and that the same was true of the orange background. We did not have a bespoke test set of swatches but the closest visual match from the FOGRAGRAcol contract proof set had an error of just 0.99ΔE2000.

Analysis of the data simply legitimised what our eyes were telling us, the difficult magentas of the turban were reproduced so as to be quite indistinguishable from the original - it was an impressive demonstration

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last update 09/12/2022 14:59:15

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