Re: Lightbooth temp
Re: Lightbooth temp
- Subject: Re: Lightbooth temp
- From: Roger Breton <email@hidden>
- Date: Sat, 27 May 2006 21:09:00 -0400
Mark,
> I have found the color temperature measuring capability the eye one to be
> highly suspect.
I saw that post, thank's.
> As I mentioned in another post, I visited GTI with my EyeOne
> and a monitor. My Eyeone gave a color temp reading of their light booths of
> about 5600K, while their Minolta colorimeter showed a temp of 4950K. Not
> that the Minolta is necessarily correct, but after 25 years in the prepress
> business, I have a pretty good idea of what the color of a light booth
> should look like.
It's interesting to note, as you may already know, that general purpose
colorimeters (especially the Minoltas) are calibrated to an illuminant A
type of source (that's what my CS-100 is calibrated to, says Minolta's
calibration report). So what's the performance of a colorimeter on
discontinuous, spiky light sources like fluorescents? Good but, admittedly,
the chromaticities deserve to be qualified. Unless you happen to have some
kind of correction factors which relate the colorimeter measurements to a
spectroradiometer measurements, which does not produce an integrated
measurements like a colorimeter. At least, that's the principle. I still
have a Minolta CL-100 I lug around with me for illuminance measurements but
I don't trust its chromaticities when measuring light sources that are
unlike tungsten when I know that this is an instrument calibrated for
illuminant A.
> I find I have to get my monitor set to about "5800K" (by the EyeOne)
> to match a 5000K light booth. And in all practicality, what a light booth
> looks like is what matters to most prepress users.
Well, I have a different experience with the EyeOnePro. Like you, I also
happen to have one of these units. It's actually my second such one, the
original I bought a few years ago did not have the ambient reading
capabilities then. So, here I am, working on the calibration of my Eizo CG21
monitor. Get this. After calibration, the instrument reports a white point
calibrated color temperature of 5000K. Fine. I took my Minolta CS-100 out
and took a reading of the same white patch on my screen. Got some figures
that, once related to a CS-1000 showed that the actual CCT is 5127K. What
do you think?
> In addition, most light booth manufacturers recommend changing lamps
> about every six months.
GTI specs say 1800 hours or so.
> GTI offers a service to monitor it for you. I know
> from experience that lamps start out very cool - magenta-blue. They should
> be burned in for at least 24 hours before they are even usable. Then, over
> time, they drift toward green-yellow. Over six months of constant use, The
> color temp shift will be about 500 degrees warmer, and if the lamps are
> older than six months, all bets are off.
OK. I once walked into a print shop that had just bought a used GTI booth.
They never bothered to look at the exact model of lamps inside the booth:
they were plain cool-white fluorescents!!
> In addition to that, when the lamps
> are first turned on, they will be cooler than when they are warmed up.
GTI has specs that shows the shift in CCT over time. After 20 minutes they
say, it is stable.
> Also, when I was at GTI, we dimmed the lamps in the booth and tested
> with both the EyeOne and the Minolta color meter.
Aha!
> The Minolta told us the
> both was moving toward a higher color temperature as it was dimmed, but the
> EyeOne said the opposite.
Interesting. My humble irradiance readings of a dimmable GTI booth with an
EyeOnePro showed that they loose color temperature as they are dimmed. Go
figure, Mark.
> Our eyes (Bob McCurdy and myself) agreed that the
> Minolta was correct - higher color temperature as it was dimmed.
Higher as in getting cooler? Any idea which Minolta colorimeter was used?
> Mark Rice
Regards,
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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