Re: Correlating Tappi Brightness
Re: Correlating Tappi Brightness
- Subject: Re: Correlating Tappi Brightness
- From: Klaus Karcher <email@hidden>
- Date: Sat, 08 Aug 2009 16:11:02 +0200
Hi Roger et al.
Roger wrote:
Marco,
That's the thing. Tappi "brightness" is a simple measure of
reflectance at 452nm. *But* the measuring apparatus is very peculiar
in its design. I think the light source, the way it illuminates the
sample, is not coming at plain 45 degrees, if memory serves. And a
few other idiosyncrasies like that. Which makes relating this
measurement to CIELab 0/45 daunting.
Your are absolutely right. Whitness measurement devices usually use
d/8° or d/0° geometry and -- most important -- a /calibrated/ UV level
in the light source.
You can use spectral data (not XYZ or L*a*b* tristimulus values!) of
"ordinary" instruments to /estimate/ whiteness when they have enough
intensity in the UV region to excite the optical brighteners, but you
will not get precise results without the ability to regulate and
calibrate the UV level of the measurement light source.
see also <http://mitglied.lycos.de/whiteness/Instruments/instruments.html>
Klaus
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