### Re: What is the reflectivity of standard 8x11.5' printing paper

Date: Mon May 28 04:18:35 2007
Area of science: Physics
ID: 1179598263.Ph
Message:
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print paper first. Optical reflectivity is generally defined as the ratio
of the intensity of the reflected light to the incident light, within the
spectral range of interest. It has a directional component, in that the
angles of incidence and reflection are important to any measurement. At
90 degrees to a plane mirrored surface, we get nearly 100% reflectivity
for visible light. Paper substrates, however, scatter the incident light
at a multitude of angles, depending on the gloss factor. Any measurement
of reflectivity will give a significantly lower value than would a mirror
and will also depend on the spectral absorbance of the paper. There is no such
thing as "standard" print paper : different supplies will have different
spectral properties : some will appear whiter than others, some will be
treated with optical brighteners to fluoresce at short wavelengths to
give bluer reflected light and so appear less yellow. Differences in
gloss (a surface property) will cause more or less scattering and
therefore differences in reflectivity at a given angle, but all
measurements of reflected light will be less dependent on angle of
measurement than with a mirror because of multiple scattering.

a subject exposed to flash, let's start with a thought experiment. Let's
assume a perfect flat mirror, large enough such that at the distance (x)
it is set from the flash, all of the light lands on the mirror. Then, at
the position of the flash, the optical result would be exactly the same
as at a distance 2x in front of the flash, were the mirror absent. With
an imperfect reflector such as a sheet of paper, which scatters the
incident light, you will get a result at the position of the flash the
same as at 2x in front of the flash, but with the reflector replaced by a
scattering, transmissive diffuser. The measured intensity will be less
dependent on angle of measurement (because of multiple scattering) as
mentioned above, but in both cases the inverse square law will apply to
the total distance the light has travelled from source to point of
measurement (i.e. (2x)^2 not - x^4), but of course significantly modified
by the scattering effect.
I hope this helps answer your questions. If you want to get really into
paper reflectivity, it's a complicated subject:
schramm-meyer-pics

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