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Re: Accuracy of instruments


Wow, I've gotta get ahold of those glasses you're wearing because you're reading someone else's thought's in my little paragraph. The point of course is that the degree of accuracy in measurement you seek must bear some reasonable relationship to the purpose at hand. It is not simply the case that more and more accuracy/consistency is always useful. When the final measure of color is the human visual system, which has extremely poor repeatability AND interinstrument agreement (should we also talk about viewing conditions and THEIR consistency?), there are limits to the usefulness of accuracy of your spectrophotometers for this application. Perhaps someone could address what those limits really are. We know this is a complicated question, and the answer depends on what colors we're talking about, the degree of variability between observers, ability to control viewing conditions, and so on. But there is inevitably a point at which increasing an instrument's accuracy becomes statistically meaningless. (To give a crude example, you don't need a micrometer to frame a house.) Perhaps this can be approached empirically: Can anyone demonstrate a noticeable and objectionable variability in printed color that can be traced to the performance of any recent model of spectrophotometer that has passed its manufacturer's certification process? This is the key: With all of the variables in play, some with high degrees of randomness, you need to establish that the instrument was a problem before accusing manufacturers of indifference to quality. 
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Message: 8
Date: Thu, 1 Nov 2007 09:27:03 -0400
From: Terry Wyse <email@hidden>
Subject: Re: Accuracy of instruments
To: "'colorsync-users?lists.apple.com' List"
	<email@hidden>
Message-ID: <email@hidden>
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes

Which means...what exactly? That we should let the manufacturers off
the hook for providing better inter-instrument agreement? I  believe
that we have very good repeatability these days but I'm not convinced
accuracy is all that good. I've done a few informal comparisons
between my SpectroEye, EyeOne and iSis and suffice it to say that I'm
NOT impressed with their inter-instrument agreement. I've even been
told that my SpectroEye, via NetProfiler, is only certifying itself to
another SpectroEye, not any kind of absolute standard.

Regards,
Terry


On Oct 31, 2007, at 11:26 AM, Mike Strickler wrote:

This comment does not exactly address Roger's complaint, but it may
bear on its relevance. Even if we could achieve higher accuracy in
spectrophotometers, which is problematic for reasons that Robin and
Tom have outlined, let's remember that the standards we're talking
about here are not in the end spectrally-based, but VISUALLY-based.
So we're harnessing a lot of spectral measuring power to get the
most accurate conversion to L*a*b, LCH, XYZ, etc., all of which are
based on some thing far vaguer: the "average" human visual response.
Does anyone have figures for (deltaE) variability between THOSE
instruments?

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