Re: Spectrophotometer accuracy
Re: Spectrophotometer accuracy
- Subject: Re: Spectrophotometer accuracy
- From: <email@hidden>
- Date: Thu, 25 Aug 2005 16:34:18 -0400
While the article recommended by Ken is an excellent
pointer for anyone wanting to improve inter-instrument
accuracy, here are a few remarks based on the practical
experience of two persons I know.
Both of them wanted to have an absolute standard, and both
used the BCRA route. After spending thousands of dollars
in material and services (one bought new, the other used,
one of them spent near or over 10K in the project), they
still cannot reach that goal.
The main reason is NIST traceable data is available, from
the tile suppliers or from calibration centers, for a
different spectrometer geometry than the one used by most
graphic professionnals. The Eye-One or Pulse use the 45/0
geometry while, just as an example, Avian Technology (One
supplier of BCRA tiles) use an 8 degrees/hemispheric
geometry. Of course, the measured spectrums are different
with each geometry.
Also, you need a temperature stable setup to optimise the
measurement as the tiles are thermochromic (cyan is a good
choice as it minimizes this effect).
Here is an article which tackles these problems.
http://www3.interscience.wiley.com/cgi-bin/abstract/50248/ABSTRACT
-------------------------
Color Research & Application
Volume 22, Issue 1 , Pages 51 - 60
An abridged technique to diagnose spectrophotometric
errors
Roy S. Berns *, Lisa Reniff
Munsell Color Science Laboratory
Abstract
The effect on colorimetric accuracy of spectrophotometric
errors (±0.5 and ±1% reference white, ±0.25 and ±0.5%
reference black, and ±0.5 and ±1 nm wavelength) was
simulated for the BCRA Series II tile set by using
previously investigated models for these errors. E*94
color differences of up to 5.2 could result. Because the
errors are linearly related to CIELAB differences for each
tile, a technique was derived that enabled L*, a*, and b*
coordinates between a calibrated tile and its measured
value to be transformed into estimates of reference white,
reference black, and wavelength error. The cyan tile was
identified as the most effective tile for this technique.
Universal equations are included for 45/Normal and
Normal/Total geometries traceable to the National
Institute of Standards and Technology (NIST), and
Normal/45, Normal/Total, and Normal/Diffuse geometries
traceable to the National Physical Laboratory (NPL).
Measuring the cyan tile on a regular basis and
transforming its colorimetric coordinates into
spectrophotometric error metrics provides a useful method
to validate the accuracy and reproducibility of a
color-measurement instrument.
---------------------------------
All of this is color science heavy and requires a lot of
time. It does not mean it cannot be done but simply points
to the fact that there is no simple point and click
solution to the problem.
Danny Pascale
email@hidden
www.BabelColor.com
On Thu, 25 Aug 2005 15:26:31 -0400
Ken Fleisher <email@hidden> wrote:
On Aug 25, 2005, at 12:56 PM, Michael Fox Photography
News Account 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.
Here is a link to a paper that describes how you can
improve inter-instrument agreement for your two devices.
If you follow up on many of the entries in the
references at the end, you will also find some great
information on how to determine the precision and
accuracy of your instruments. As already mentioned,
however, you will need BCRA tiles, or similar, to do
this. If you can't purchase them, perhaps there would be
someone who would lend (or rent) them to you.
<http://www.npes.org/standards/cgats/White Paper_Inter-
Instrument Agreement.pdf>
Hope that helps.
Ken Fleisher
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