Re: ISO 12647-3 measurement (IT8.74 or ECI 2002) Lab data list---black backing and white backing
Re: ISO 12647-3 measurement (IT8.74 or ECI 2002) Lab data list---black backing and white backing
- Subject: Re: ISO 12647-3 measurement (IT8.74 or ECI 2002) Lab data list---black backing and white backing
- From: daniel westcott <email@hidden>
- Date: Thu, 13 Jul 2006 16:44:02 -0400
- Thread-topic: ISO 12647-3 measurement (IT8.74 or ECI 2002) Lab data list---black backing and white backing
Hi Roger,
What I believe you are observing with the black is that the thin stock is
passing some light through to the backing material and that light is being
reflected back by the backer again passing through the stock which acts as a
filter for the light. Thus cast from the backer is being introduced into the
reading. Stacking multiple sheets of the same stock helps insure that the
backing material is of the same cast as the sheet being read, all things
being equal-which they¹re not, and thereby gives the truest reading when
trying to measure the color of a sheet. The assumption here, as was
mentioned earlier in the thread, is that the actual viewing conditions will
be a stack of the sheet as backing, as in a book or magazine.
A reading over air could also take in some amount of ambient light that was
reflected from the surfaces beneath the sheet. The amount would depend on
the opacity of the sheet and the reflective qualities of the surface
beneath. For this reason, I would disagree that this would essentially be
the same as a black backer. While it is true that the light that passes
through would (almost) not come back, in reality the black backer is not a
true light trap and, because of this, some of the light measured will have
been light filtered by the sheet, then reflected off of an almost black
object and again filtered by the sheet. Hence the near neutral reading you
observed.
There are valid uses for all three defs in the standard.
Incidentally a neutral grey backer would suffer form the same reflectance
issues as a black or white backer.
Regards,
Daniel Westcott
On 7/12/06 11:11 PM, "Roger Breton" <email@hidden> wrote:
>> ...3 definitions in ISO standards for backing: white, black, and
>> multiple sheets of the same paper stock.
>
> How, then, could you compare thin paper stocks among themselves, on color?
> If measured on a black backing, they could appear (some #5 coated grades I
> know) very neutral, with a* and b* close to zero. On the other hand, if
> measured on a white backing, like the Spectroscan table, the same paper
> shows a negative b* value, typically around -3... Lately, ... I
> started measuring over "nothing": I place the paper one inch away off the
> surface of my desk so there is no backing behind it, just the space between
> the top of my desk and the floor, about 30" or so. Then I measure, with my
> SpectroEye, holding the paper from underneath between my fingers. It works.
> But it's not an ISO standard :(
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