Is there an “absolute” method to determining
spectrophotometer accuracy? i.e. is there a calibrated standard available
with known LAB values?
And just how accurate should spectrophotometers be (+/- how
much)?
How long is the white calibration tile good for?
Here’s the situation: I have two
spectrophotometers - a GretagMacbeth Spectrolino (with the attached UV-cut
filter) and a GretagMacbeth EyeOne Pro w/ UV-cut.
I measure a paper sample with both. For a given
instrument, sampling different parts of the paper, I get results that are all
within +/- 0.1 for L*, a* and b*. So the measurements for a given device
are highly repeatable. But the measurements between the devices disagree.
The same holds true with other paper samples.
For example, measuring Fuji Crystal Archive Matte, I get the
following:
EyeOne Pro (w/UV-cut) LAB = 92.1, 0.1, -1.3
Spectrolino (w/UV-cut) LAB = 91.5, 0.4, -1.7
For Epson Premium Luster, the numbers vary even more:
EyeOne Pro (w/UV-cut) LAB = 95.5, -1.1, -1.8
Spectrolino (w/UV-cut) LAB = 94.7, -0.6, -2.6
I used MeasureTool with both instruments. As an aside,
I tried the EyeOne Pro with both MeasureTool and i1 Share. Both software
packages report the same numbers for the EyeOne Pro.
It reminds of the old Confucius saying: “Man
with one watch always knows what time it is; man with two watches is never sure”.
Michael