Highlights and Specularity, Part 1

This photograph shows three spheres with varying surfaces. The one on the left is matte, the one in the middle is glossy, and the one on the right is highly reflective. 

On the matte sphere there is no highlight. On the glossy sphere in the middle, the highlight is clearly apparent. The mirror-like sphere on the right also has a highlight, which is really a reflection of the light source. The light source is the sun shining through a window.

The right sphere also reflects the scene around the ball, including the white paper background and the dark room behind the camera. In the right ball, you can even see a reflection of the middle ball, with a highlight in the middle of that reflection.

That same pattern of reflections of the paper, room, and neighboring ball is subtly visible in the middle ball as well.

In the middle ball, there is a second, smaller highlight just to the right of the primary highlight. This secondary highlight is the sunlight is reflected three times. The light bounces off the middle ball, bounces back off the right ball, and bounces again off that little highlight on middle ball back to your eye.

So we've arrived at a definition: A highlight is a specular reflection (Latin "speculum"=mirror) of the light source on a shiny surface. The shinier and smoother the surface, the brighter and clearer the highlight.

These three diagrams show what's happening at the surface level. On the matte surface, light arrives from the top left and hits the rough surface. Some light gets absorbed and the rest scatters away in all directions. This is what happens when light hits a matte surface like a sand dune or a sweater.

The glossy surface of the middle ball bounces a a portion of the light at the same relative angle as the incoming light, but some of the light rays hit uneven spots and bounce in random directions. This is like bouncing a golf ball on a country road. It will probably bounce the way you want it to unless the golf ball hits a crack or a pebble.

The mirror-like surface is so smooth that all the light bounces off the surface predictably at the same relative angle, like bouncing a ball off a basketball court. (Edit: a true mirror-like reflection requires a metallic surface, and cannot be achieved by smoothness alone. Thanks, David!)

Since highlights belong to the world of specular reflection, they should be thought of as somewhat separate and distinct from the normal modeling factors (light, halftone, and shadow) of diffuse reflection. Artists in the world of 3D computer graphics can control the form-modeling and the specularity as separate components.

I wrote an article on this subject of "highlights and specularity" for the current issue of International Artist magazine, which should be on the newsstands now. This blog post is just a little piece of it.

The six-page article contains a lot more examples and explanation, and it includes artwork that has never been reproduced in print before. I'll talk more on the blog about highlights in real-world examples tomorrow.
International Artist magazine, Issue 90
This subject is also covered in my book Color and Light: A Guide for the Realist Painter (Amazon), also available signed from my website store.

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