Re-instrument calibration.
Re-instrument calibration.
- Subject: Re-instrument calibration.
- From: "Michael Lithgow" <email@hidden>
- Date: Tue, 2 Jul 2002 15:30:16 +1000
C.D. Tobie wrote
>
Try opening the spectro window in your SpectroCam software, reading the
white tile, and saving that as the reference value. Next read several
hundred
>
patches in strip mode, then remeasure the tile, and see if the values have
drifted. If not, read another target or two and try again. If it continues
to
>
hold, you should be in good shape. I have had every SpectroCam I ever
worked with fail to hold a consistant white value eventually, and require
repair or
>
replacement. That procedure currently requires a trip to Europe that takes
a few weeks, meaning that anyone having serious use for the SpectroCam needs
to
>
own at least two of them.
>
I'm told I "must use my SpectroCams a lot" to have them consistantly wear
the light source out within a year. Actually I go days at a time without
using
>
the SpectroCam at all, and build only a few profiles a day when I am
working with it. If that qualifies as higher than expected usage, then that
is a
>
matter of concern with the device's life expectancy.
Sounds very much like a case of sensor temperature compensation failure.
When calibrated a sensor will read correctly but only at that temperature.
Fortunately a formula is applied which corrects the reading at various
temperatures. With use the temperature will change and the corrections will
be applied accordingly, however if the temp. calibration was incorrect then
the result in use is inconsistent readings. I wonder if this was a problem
from day 1 but was only discovered after inconsisitencies were noticed and
then David's test carried out. I have seen devices using colour sensors
where an entire line of product was inconsistent due simply to the
thermometer used as a reference at the manufacturer was incorrect. My
feeling is that it could have been a problem from new and wasn't related to
excessive use.
Spectrocams were developed (not marketed) by the same Dutch company that
produced the excellent Lici Colorstar analyser used in darkrooms around the
world and marketed successfully by Jobo as the Jobo Colorstar. This company
is well aware of temp. compensation and included a simple temp. readout in
some analysers as a way of detecting a sensor fault. It is a shame that the
same idea is not included in the software of the Spectrocam as this would
give an indication of where a potential problem lay. The temp. calibration
is adjusted in fractions of a degree and obviously there does not have to be
a large error to produce an inconsistency. It is in my experience a simple
adjustment of a trimmer and is therefore a pity that it has to be sent to
Holland for repair. However as the device does not provide any temperature
display to indicate its current setting, one has no idea of how much of an
adjustment to make even if the temp. trimmer was marked as such.
As a matter of interest my Spectrocam displays exactly the same fault as
David describes but Holland is a very long way from Australia and I am
reluctant to part with it and must therefore check the consistency of the
readings regularly and recalibrate on the white tile. Perhaps future
Spectrocams could include a temp. check in the software and enable local
technicians to make the required adjustments.
Michael Lithgow
Michael Lithgow Photography & Laboratory
177 East Boundary Road
East Bentleigh
Victoria 3165
Australia
Ph./Fax 61 3 95794088
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