|MadSci Network: Physics|
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
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