alnovo@inwind.it
Date: 03/26/02-03:06:30 AM Z
> On 26/03/2002 08:59 Judy Seigel wrote:
> Thanks Alberto... I found this quite reassuring... that is, perfect
> measurement is not going to happen outside a lab, and a high-end one at
> that, so I can relax.
Hovever, one can accept a quasi-perfect measurement once some variables have been demonstrated (and not simply argued) to be less significant compared to other ones.
The problems are two, and two words are used to define them:
repeatability, that is the results of a single laboratory (single operator, equipments, and so on) are comparable
reproducibility, that is the results of different laboratories (operators, equipments, environments, and so on) are comparable
In the practice: the measurement of a property (in our case it could be the density of the n-th step of the 21-steps scale, or the difference between the n-th and the m-th steps, or whatever else) under fixed conditions is made several times (say 8-10 times) and the scattering of the measurements is the measure of the repeatability or reproducibility.
I stop here but I could go deeply inside...
> However, your "bet" about the coating confirms my general hunch. Which was
> why, when I tested the bulb spacing I estimated overall *look* of a
> full-size sheet of paper rather than "reading" 21-steps.
>
> There's also the fact that the 21-steps, even if the coating were perfect,
> would have to be the same ones in the same place, since they're different
> one from another, & there are slight differences in light from spot to
> spot ... HOWEVER, speaking of *BETS* I'm not sure but what Sandy King owes
> me -- was it a million dollars?
>
> I remembered from those tests of bulb spacing that the 21-steps LOOKED
> quite different at the different heights -- didn't pay much attention
> since the step tablets were incidental & I figured the 30-second exposure
> was too atypical to mean much. But when Sandy INSISTED that exposure at
> all distances would be pretty much the same, I decided to try a better
> quality test.
I think it is better to measure the reflected density by means of a densitometre.
Alberto
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