Wednesday, June 1, 2011

Reflectivity and viewing angle

Smooth surfaces are more reflective when you look at them at shallow angles.

You can see this subtle phenomenon best by looking at cars in an open parking lot. Note how the foreshortened plane on the left is highly specular, or mirrorlike, reflecting the scene beyond with a wide range of values.

The back window and tailgate do not reflect the sky or the surrounding cars as brightly.

In previous posts, we’ve seen a related phenomenon with smooth water surfaces, which also become more reflective at grazing angles of incidence. In the case of a transparent fluid surface like water, light that is not reflected is refracted beneath the surface.

A smooth solid surface like a painted car or a pool ball or an apple is generally a blend of specular and diffuse reflection.

But even surfaces that we think of as diffuse reflectors, such as ordinary paper, can be remarkably specular at shallow angles. Hold up a piece of paper at an extremely shallow angle to your eye, so that you’re sighting right along the surface of the paper. If you have bold light and dark lines above that surface (such as the mullions on a window), the paper will behave almost like a mirror, reflecting a fairly clear image of the scene beyond. (Good party trick.)
Thanks, Kevin Bjorke, for telling me about this.  

More on such things in "Color and Light: A Guide for the Realist Painter"

Related previous posts on GJ:
Escher’s Three Worlds
Color in Mountain Streams
Transparency of Water
Specular and Diffuse Reflection


italic said...

This is called the Fresnel effect and is extremely handy when working in 3D, especially making realistic water and inorganic mechanical objects, like robots.

kevinwparker said...

Interesting (I hope) trivia: X-rays only reflect at very shallow angles. X-ray telescopes, therefore, have nested mirrors that are pointed almost edge-on at their targets.

Emilio said...

Davvero interessante!

tccin3d said...

I just received "Color and Light" awesome book!,much thicker than I thought. Now the trouble is how do I get it from hands of my 3yrs old son.

craig said...

Thanks! I've been working digital artwork on a black locomotive from the 1950's, as it appeared new. That stuff reflects like a mirror! It's driving me nuts trying to understand it! This helps enormously.