Is there a way to convert the gloss measurement at one angle to the gloss measurement at a different angle? For example, you may have a single angle gloss instrument and be required to report the gloss at a different angle to one of your customers.
The chart below is found in the BYK-Gardner catalog and is used in various PowerPoint presentations, but deserves yet another showing and explanation.
We state that if the gloss, when measured on a 60° meter, is less than 10 GU, an 85° meter should be used. Why? Look at the above graph. Samples 1-13 are visually about the same gloss difference apart. That means that the difference, to the human eye, between samples 2 and 3 looks about the same as the difference between samples 6 and 7 or between 12 and 13. However, the gloss curve for the 60° measurement angle drops off considerably when measuring less than 10 GU. Likewise, the curve flattens out when the gloss exceeds 70 GU. Between 10 and 70 GU each sample has about a 15 GU spacing (10, 25, 40, 55 and 70). But below 10 GU the first 5 samples have an uneven spacing and only have 10 GU as the maximum.
If you look at the measurements below 10 GU (at 60°) using the 85° measurement angle the samples once again have close to this 15 GU spacing: 3, 18, 30, 47 and 60 GU. Likewise, once the 60° meter hits about 70 GU the curve flattens out and we lose the 15 GU spacing. Switching to the 20° measurement angle gives us a gloss of about 25 GU (where it was 70 GU at 60°) and proceeds up the scale to 40, 55, 70 and 88 GU. The visual spacing of the gloss panels once again correlates well with the numbers obtained on the meter.
But how would you convert a 60° gloss reading of say, 0.5 GU to an 85° gloss reading for a customer specification. The 85° measurement of that sample could be anywhere from 3 GU to maybe 10 GU. Specifications are generally tighter than that. What would the 20° gloss value be of a sample that measures 10 GU at 85°? How about the 20° gloss value for a sample that measures 45 GU on an 85° meter? I think you can see the problem.
None of these values can be converted. The only way to measure gloss correctly and accurately is to use the appropriate gloss meter.
Jim Roberts is a Technical Support Manager at BYK-Gardner USA, Columbia, MD.