RE: Instrument Illuminants
RE: Instrument Illuminants
- Subject: RE: Instrument Illuminants
- From: Roger <email@hidden>
- Date: Tue, 06 Jan 2009 07:22:07 -0500
Mike,
I hope the X-Rite of this world will give us more LED-based instruments like
the iSis and Munki if we're going to continue calibrating proofs and presses
with LED-based instruments, like the iSis, *and* overhead luminaires slowly
migrate to LED-based technology, like the JUST-Normlicht. The way it was
explained to me by Tobias Raush from X-Rite, at the last GATF CMS, is that
the bi-directional reflectance distribution function (BDRF) is the one
factor that has the most profound influence on an actual instrument
measuring performance. He said to me (hope I'm quoting him right) that the
measurement should be the same between tungsten-based and LED-based
instruments *if* the surface properties of the substrate being measured
approaches the Lambertian ideal, isotropic, etc. Then it does not matter
which sources is used to make the measurements because the reflectance will
be the same, he explained. But, of course, the further the surface
properties deviate from the ideal (gloss) then the more the various
instruments sources and measurement geometries starts to make a difference.
As far as I know, no instruments is tuned to a specific illuminant like D50.
To my knowledge, they measure light independently of illuminants. Even if
we'd think it would be fair for many of them to assume Illuminant A since
that's the physical source SPD they correspond to.
My two cents ...
Roger
> -----Message d'origine-----
> De : colorsync-users-bounces+graxx=email@hidden
> [mailto:colorsync-users-bounces+graxx=email@hidden] De
> la part de Mike Eddington
> Envoyé : 5 janvier 2009 08:48
> À : 'colorsync-users?lists.apple.com' List
> Objet : Instrument Illuminants
>
> Question on instrumentation illuminants: Do typical
> spectrophotometers (i1 for example) typically assume the illuminants
> spectral output is illuminant A ( and correlate that to D50) or is the
> correction to D50 tuned to the illuminants custom curve (either as a
> family of devices or each specific device having its own custom
> correction). Also wondering if the LED source used in newer
> spectrophotometers (Isis, Munki) are close to illuminant A or has it's
> own spectral output that is then correlated to D50 (I'm guessing the
> latter)? Since there is no true D50 illuminant in a spectrophotometer,
> would anyone have an opinion as to whether LED technology could in the
> future be used to more closely emulate D50 both in the visible and UV
> portions of the spectrum?
>
> Mike
>
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