Re: VIGC study on spectrophotometers
Re: VIGC study on spectrophotometers
- Subject: Re: VIGC study on spectrophotometers
- From: RaymondCheydleur <email@hidden>
- Date: Wed, 17 Sep 2008 12:36:45 -0400
- Thread-topic: VIGC study on spectrophotometers
Ahh Fear Uncertainty and Doubt.
Having 3 different reports like this surface in the last few months, I felt
moved to comment, I've given it the requisite 2 hour cooling off period in
my drafts folder and I still like it, so here it goes... Don't take this as
the official stance of my employer but as my personal observances.
I'd also like to thank Danny for his input (received during the 2 hour
cooling off period) as it appears to be a reasonably well controlled
experiment, easily understood and thus informative, which is refreshing. I
think it also is still in agreement with the following, understanding that
one data point from each instrument type does not tell us all we would like
to know.
Instruments are clearly not perfect and work is ongoing both from a
manufacturing standpoint and in Standards work to improve them in many ways.
As an example on the standards side, ISO TC130 is meeting next week and
includes proposals that will help instrument data interchangeability (CxF)
and instrument repeatability (Measuring conditions, illuminants/filtration,
M0-M3).
However
This study like many others comes to some conclusions that are pure common
sense: Newer instruments tend to be tighter to each other than older
instruments. Using a visual tolerance (their suggestion is DE2000) the "big"
differences being reported essentially disappear. (I say "big" because 4-5
years ago much larger differences were considered unavoidable system noise,
and this is why visual certification ruled the day.
I'd suggest however that if tight tolerances of your measuring device are
really a concern to you (no matter what your tolerancing preference) a few
basics will almost always improve your results:
- Calibrate - carefully
Make sure your reference is clean, and for some instruments within
expiration dates. Take care in the placement of the instrument to the
calibration reference (if this is a manual process) The foundation of all
your measurements is based on this one reading.
- Recertify annually
If you don't do this you are part of the problem not part of the solution.
- Match the target to the device
Don't make it smaller than the instrument recommends to fit your desires and
then complain about repeatability, please.
- Use a standardized backer that fits CGATS.5 or ISO 13655.
Self backing will generally increase noise in the data - sometimes
dramatically, sometimes only a small amount.
- If your device has a measurement guide, use it.
Generally guides increase repeatability and accuracy of data in a noticeable
way.
RayC
Ray Cheydleur
OEM Project Manager
email@hidden
X-Rite Incorporated
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