Re: Accuracy of the Munsell Neutral Value Scale
Re: Accuracy of the Munsell Neutral Value Scale
- Subject: Re: Accuracy of the Munsell Neutral Value Scale
- From: Ben Goren <email@hidden>
- Date: Sat, 17 Apr 2010 10:58:56 -0700
Danny,
Thanks much for the clarifications!
> Note 3: You should not assume that N5 corresponds to L*=50. L* is generally around 10 * MunsellValue, but this is not an exact relation.
> N 4.75/ is in fact very close to L*=49, as you measure.
I *was* going to ask where one can find the means to convert between Munsell and Lab, as my Google skills weren't helping out much. Then I finally stumbled across BabelColor CT&A...but I still don't see in it any direct way of specifying HVC values...just RGB in various color spaces or Lab. Am I missing something? Is that only available in the non-trial version?
> The error between your measument and the nominal value for N9 is:
> 1.54 CIEDE94.
> Which is not as good, but the error is mostly in L*. This could be explained by :
> 1-The backing used for measurement.
> 2- A dirty i1 calibration plate. (clean with pure alcohol).
> 3- The i1 used many times in a row (some models will show slightly increased luminance when repeating a measurement, especially if ststing with a "cold" instrument).
> 4- And the manufacturing tolerance...
I had just used the swatch book itself for backing. The calibration plate looked fine and it hasn't been all that long since I last cleaned it, but I gave it a good cleaning just to be sure.
#3 certainly seems to be at least a marginal factor. Here's a sequence of readings of the reference tile, each taken a second or so apart:
Result is XYZ: 86.712801 90.127600 71.358082, D50 Lab: 96.049672 -0.348964 2.623734
Result is XYZ: 86.768327 90.186923 71.418420, D50 Lab: 96.074251 -0.351915 2.612415
Result is XYZ: 86.805921 90.225544 71.454580, D50 Lab: 96.090247 -0.351154 2.607828
Result is XYZ: 86.824961 90.246301 71.476349, D50 Lab: 96.098842 -0.352905 2.603288
Result is XYZ: 86.837370 90.259370 71.487486, D50 Lab: 96.104253 -0.353227 2.602715
Result is XYZ: 86.843630 90.265999 71.495016, D50 Lab: 96.106997 -0.353454 2.600751
Result is XYZ: 86.846647 90.267717 71.496233, D50 Lab: 96.107709 -0.350929 2.600896
Result is XYZ: 86.845584 90.267963 71.497499, D50 Lab: 96.107810 -0.353339 2.599946
So, here's the result of the N 9/ patch on top of a block of white polystyrene (which itself reads 92.179090 -0.282941 -0.564617) after making several readings of the tile and doing a re-calibration (by quitting spotread and re-launching):
Result is XYZ: 78.489035 81.736977 67.267302, D50 Lab: 92.458646 -0.637077 0.145746
My original reading was 92.528793 -0.681620 0.018600, which would seem to be well within rounding error.
I then made several measurements of my Watch Your White target, re-positioning the i1 each time:
Result is XYZ: 93.959538 97.478963 80.449838, D50 Lab: 99.016891 -0.052186 -0.032418
Result is XYZ: 96.472871 100.151009 83.018264, D50 Lab: 100.058361 -0.160183 -0.325402
Result is XYZ: 94.262579 97.819508 80.956733, D50 Lab: 99.150673 -0.096481 -0.217445
Result is XYZ: 94.378732 97.945708 81.105167, D50 Lab: 99.200172 -0.106092 -0.253500
Result is XYZ: 94.333062 97.896360 81.015258, D50 Lab: 99.180821 -0.102774 -0.213348
(That reminds me -- while I've got you on the line, is it safe to clean the target? If so, how?)
Considering that the (PTFE?) target is within the margin of error (if anything, a bit dim) yet the N 9/ patch is a bit bright, should the gray scale patches be considered suspect?
Thanks,
b& _______________________________________________
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