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Monday, May 23, 2016

How are Opacity and Transparency Different?

When light strikes an object, it is reflected (specular), refracted (scattering & non-specular reflection), transmitted and/or absorbed.  Parallel rays of light which are reflected or transmitted as parallel rays are said to be specularly reflected or specularly transmitted.  Light which is scattered before it is reflected or transmitted is said to be diffusely reflected or diffusely transmitted.  In general, incident light that is specularly reflected determines the gloss of an object and that part which is diffusely reflected determines its color (and brightness).  Finally, that part of the incident light which is transmitted determines the opacity and transparency.

Opacity is determined by the total transmitted light while transparency is determined by the amount of light which is transmitted without scattering.  When the light is scattered and diffusely transmitted, the object is translucent. Nearly all papers are translucent because they permit the passage of mainly diffusely transmitted light.
Transmission (without scattering)
Transmission (scattering)

Transparency is important in tracing, reproduction and packaging papers.  A completely transparent paper would be one that transmitted, without scattering, all of the light falling on it, and a clear view of an object would be had when the paper was placed between the object and the viewer, regardless of the distance between the paper and the object.  If, on the other hand, a paper has a tendency to scatter light, its transparency will depend upon the distance between the paper and any object being viewed through it.

Reference: "Properties of Paper: An Introduction" - Scott, W.E & Trosset, S. p. 84 (1989)