Re: i1Pro weirdness & fix
Re: i1Pro weirdness & fix
- Subject: Re: i1Pro weirdness & fix
- From: Ernst Dinkla <email@hidden>
- Date: Wed, 17 Feb 2016 10:28:30 +0100
Graeme, thank you.
The i1Pro that I have is mainly used for spot measurements. So far I do not
see the deviations you mention but I will give it the exercise it needs
before starting the spot measurements. It all sounds very plausible.
Met vriendelijke groet, Ernst
Dinkla Grafische Techniek
Quad, piëzografie, giclée
www.pigment-print.com
On Wed, Feb 17, 2016 at 1:20 AM, Graeme Gill <email@hidden> wrote:
>
> I've been tending to use my i1Pro Rev E (i.e. i1Pro2) for random spot
> measurements a lot. Recently it's accuracy had been niggling at me.
>
> What I noticed was this: From cold, do a calibrate.
> Measure spots. The errors would rise (literally the L* was rising).
>
> The closer the measurements together, the faster the errors would rise.
> Let it cool, and the errors would drop. Recalibrate hot, and
> the error would increase as it cooled down.
>
> At its worst, I as seeing errors of 2.0 Delta E between cold and hot!
>
> [ It's pretty easy to replicate this experiment - ArgyllCMS spotread
> makes it straightforward, but you could do something similar with
> ColorPort etc., repeatedly measuring the white calibration tile.
>
> I noticed this type of thing before with one of the i1pro Rev A's I've
> got, but it wasn't as extreme, and I put it down to being an old lamp
> in a well used instrument. ]
>
> Then it got weirder. This characteristic didn't seem stable. I started
> looking at how it was affecting strip measurements, and it kept changing.
> Sometimes it would get better. Then it would get worse again.
>
> Then the light went on (so to speak), and I arrived at a theory.
>
> What if the lamp is a miniature Quartz Halogen lamp ?
> This would make sense in terms of how long the lamp should last,
> QH lamps typically last 2-5 times longer than conventional
> incandescent lamps, and would also be helpful in ensuring constant light
> output over the lamp life, something that is rather desirable for
> an instrument light source. I measured a color temperature of
> just over 3000K, which is consistent with a hotter QH lamp.
>
> <https://en.wikipedia.org/wiki/Halogen_lamp>
>
> QH lamps work on the basis of re-circulating evaporated tungsten
> back onto the filament. To do that they have to get hot enough
> to evaporate any that gets deposited on the inside of the glass,
> and even hotter to disassociate the resulting halide back into
> tungsten at the filament.
>
> If the spot measurement gets the bulb hot enough to evaporate
> tungsten from the filament and the glass, but not quite hot enough
> to deposit it back at the same rate onto the filament, then the glass
> will build up a layer of tungsten on it. This reduces light output.
> As it warms up a bit, the tungsten gets evaporated from the glass,
> increasing light output. When it gets cold again the tungsten
> gets deposited back on the glass. Hence my increasing L* error
> as the instrument warms up, and hot/cold behavior.
>
> But if the bulb gets warmer still, say by being used for strip
> readings, then the tungsten gets re-deposited on the filament,
> and the bulb gets "cleaned". So after some extended strip readings
> (i.e. several measurements in a row, made with the button down
> and the lamp on for as long as possible), the instrument no longer
> displays such dramatic error increases with temperature. In fact
> after "cleaning", the worst I could get was 0.08 Delta E, a 20x reduction,
> and back in line with instrument specifications.
>
> If you predominantly use your i1Pro for reading strips, you
> probably haven't noticed this effect at all, because your lamp
> isn't getting dirty.
>
> I've seen similar effects on all my i1Pro's, and also Spectrolino's,
> although none as dramatic as the Rev E was.
>
> So if you do lots of spot measurements with your i1Pro's, and get the
> impression that your accuracy and consistency of measurement is getting
> squirrelly, you may be right. But there is a way of fixing it!
>
> [ Also posted to Luminous Landscape Forum. ]
>
> Graeme Gill.
>
>
>
>
>
>
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