Re: What is Paper Brightness (was RE: defining media white)
Re: What is Paper Brightness (was RE: defining media white)
- Subject: Re: What is Paper Brightness (was RE: defining media white)
- From: Robin Myers <email@hidden>
- Date: Tue, 02 Oct 2001 13:24:05 -0700
- Organization: Robin Myers Imaging
>
> I've never really seen how far away the reflectance meter is positioned.
>
> However, I do know that most fine-paper manufacturers (like
>
> Hammermill), use
>
> a bluing agent in the brightener/bleacher whose purpose is to augment the
>
> reflected light, so numbers near 100 are possible.
>
>
Curious! ... and more curious is ... if the measurement is an angular
>
reflectance, how do we compare a gloss surface with matte? Surely, they
>
have different angular reflectances(?)
They directly compare if you are measuring appearance. The 45/0 geometry
mimics what humans do when looking at a glossy surface, they tilt the object
until the light source reflection (specular reflection) is minimized at 45
degrees and look at the color from 0 degrees (angles measured from the surface
normal). The color will only contain about 4% contamination from the light
source. A matte finish will diffuse the specular reflection evenly over the
surface (for the perfect reflecting diffuser), thus desaturating the color.
The spectrophotometer will see the color similarly to the way humans do, so
the readings will be a good indicator of the color appearance.
There are a number of colorants and surfaces that will exhibit angular
variation in their appearance. Metalic paints, irridescent and pearlescent
materials are some examples of these colorants. These are measured with a
goniospectrophotometer, a device that will measure the spectra at several
angles with respect to the illumination.
Robin Myers