Re: Spectrophotometer accuracy
Re: Spectrophotometer accuracy
- Subject: Re: Spectrophotometer accuracy
- From: <email@hidden>
- Date: Thu, 25 Aug 2005 14:23:05 -0400
Michael,
The first measurement set (Fuji Crystal) gives a 0.76
DeltaE-94 difference.
The second set (Fuji Crystal) gives a 1.2 DeltaE-94
difference.
The Eye-One is specified for an inter-instrument accuracy
of:
CIE94: 0.4 avg. 1 max
The Spectrolino is specified for an inter-instrument
accuracy of:
CIELAB: 0.3 avg. 0.8 max
Transforming the Spectrolino CIELAB difference in CIE94
gives (by dividing them in two; valid for small "color"
differences lower than one; however, for these neutral
samples, this approximation shows numbers which are
under-evaluated!):
Spectrolino CIE94: 0.15 avg. 0.4 max
By considering the spec of each instrument (note: you
cannot simply add the differences, you should take the
root-of-the-sum-of-the-squares of each), the differences
you measure are acceptable relative to the the maximum
manufacturer tolerances.
If both units have been calibrated within the last two
years and have been well treated, just take the average
value from the two. Then you are back to your sinle watch
example!
Danny Pascale
email@hidden
www.BabelColor.com
On Thu, 25 Aug 2005 09:56:12 -0700
"Michael Fox Photography News Account"
<email@hidden> wrote:
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
------------
Michael Fox Photography - Fine Art Nature Photography
http://www.michaelfoxphoto.com
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