Definition

When people think of reflection, they usually think only of the mirror-like or specular reflections you get from glossy materials. Specular reflection occurs when a surface has some level of shininess which results in a significant portion of the reflected radiation travelling in the specular direction - where the angle of reflection is equal to the angle of incidence mirrored around the surface normal. This occurs on polished surfaces, the tops of some liquids and, of course, in mirrors.

However, completely matte surfaces also reflect radiation, but as diffuse reflections in which the reflected light or radiation is distributed over a wide range of directions rather than just a single direction. This occurs because the surface has small irregularities and is not completely flat when considered at the wavelength scale of the incident radiation, causing it to bounce off at many different angles.

[IMAGE: Bumpy and flat materials + Angle of Indicence vs Angle of Reflection]

It is important to note that specularity is not really a material property, but a surface property. This is because the angular distribution of reflected light or radiation is a function of surface treatment. The same material could be made almost matte with a very rough finish or glossy with a highly polished finish. Also, a material’s glossiness or otherwise does not affect its overall reflectance, just the relative amounts that are diffusely distributed or specularly reflected.

Real Materials

All real world materials lie somewhere between completely matte and totally gloss, with some portion of both diffuse and specular reflection. There are some fine powdery materials that get very close to completely diffuse with virtually no measurable specular component. A good example is plain baking flour. However there are no known materials that get similarly close to being completely specular. Even the most polished surface or most effective mirror has some diffuse reflection component, otherwise it would be virtually invisible in all but the specular direction.

Interactive App: This interactive tool demonstrates the differences between diffuse and specular reflections. Use your mouse or finger to click and drag the blue circle to adjust the angle of incidence of light on the surface. Then use the two sliders to adjust the reflectance and specularity properties of the surface to see the resulting reflection distribution.

For a specular reflection to be noticeable, the amount of light reflected in the specular direction must be significantly greater than that which would be expected in that direction from a diffuse distribution.

This is fundamentally why we perceive black cars to be shinier than white cars. With a polished black car, you can clearly see the reflection of lights and other objects off its surfaces. However, a similarly polished white car will typically have a higher specular component, it is just that the diffuse component is so much greater as well that it masks the clarity of the specularly reflected images.

You can test this by looking at the surfaces of the white car at near to grazing incidence. At close to grazing incidence (where the view angle is close to 90° to the surface normal), diffuse reflections are significantly less due to Lambert's cosine law, so the specular component from the white car becomes less masked and therefore much more clearly defined.


BRDF functions and stuff…

Specularity is the amount of specular reflectivity from the surface of a material. It is a key component in determining the size and brightness of specular highlights.

Specular reflection can be observed when the reflection is stronger in one viewing direction, resulted into a bright spot, called a specular highlight. It is mirror like reflection where the angle of incidence equals the angle of reflection. Specular reflection is readily apparent on shiny surfaces.

Units and Measures

The specularity measures the size and brightness of specular highlights. Smooth and glossy object surface have highlights that are small and bright, where as, rough and dull reflective objects have larger and diffused highlights.


References


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