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

Hi, Roman! Thanks for your question/s. Let's address the reflectivity of 
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.

Now to try to answer your associated question about reflected light from 
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