Re: dtp70 is dead
Re: dtp70 is dead
- Subject: Re: dtp70 is dead
- From: Graeme Gill <email@hidden>
- Date: Sat, 03 Mar 2007 17:04:25 +1100
Klaus Karcher wrote:
Graeme Gill wrote:
I've been taking a look at a Rev A Eye-One Pro, and notice
one not so nice thing about it's accuracy - it seems to
drift significantly as it gets hot. [...]
I think it's time for a treatment at a health resort in Switzerland ;-)
I saw the same effect with a Spectrolino some time ago and it was gone
after the maintenance.
I have doubts that the basic effect is fixable in this manner. The
effect seems to be inherent in the use of an incandescent lamp
as a light source. It's probably different from instrument to
instrument, and probably changes as the lamp ages etc., but it
seems it's inherent in the physics of these instruments.
There are two effects I observer. One has most relevance when
reading strips - when an incandescent lamp turns on, it's
light output overshoots and then stabilizes at an (almost)
constant level. Since brightness is always connected to
color temperature, the spectral balance is slightly effected too.
The Eye-One has a delay from when the lamp is energized to when
it starts measurement of about 200msec, but the lamp takes longer
than this to actually reach stability (about 1 second is enough).
I've found that if one has access at a sufficiently low level,
it's possible to compensate for this dynamic effect, reducing first
patch to last patch variation from a worst case of about +- 1.5%
to +-0.2% in software.
The second lamp effect mainly affects spot readings, where the lamp
structure beyond the filament warms up, and changes the
temperature the filament reaches, hence it's light output. This
effect is more subtle, and seems to add a worst case repeatability
error of about 0.5 delta E. Again if one is sufficiently determined,
it seems feasible to partially compensate for this effect in
software (at the cost a much longer calibration), reducing
the repeatability errors to a maximum of about 0.2 delta E.
Graeme Gill.
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