Qualities and Determinants of Specular Reflections
Specular reflections involves the reflection of light with very little if any diffusion of that light. By diffuse reflection is meant the scattering of rays of light in many different directions such that they seem to blend or mix together into a solid hue (Yot 2020, 80):
From “Specular Reflection¨ in Light for Visual Artists
Direct, or specular, reflection is the kind of reflection associated with mirrors: rays of light hitting the surface are reflected back at the same angle relative to the surface, thereby creating a recognizable image.
The degree to which a material’s surface engages in the process of specular reflection, thereby producing a specular image, is determined by many factors. One of them is refraction (Yot 2020, 81):
From “Specular Reflection¨ in Light for Visual Artists
The amount of specular reflection for any given substance is determined by its index of refraction.
That is to say, the index of refraction is directly related to specular reflection.
In addition, non-dielectric materials, such as metals, have “far greater levels of specular reflection than dielectrics,¨ dielectrics being materials that can be magnetically polarized by an electric field even though they cannot themselves conduct electricity.
Polish and Reflectivity
Nonetheless the polish or roughness applied to a surface’s intrinsic reflective tendencies can have an effect as well (Ibid):
From “Specular Reflection¨ in Light for Visual Artists
[…] it’s important to understand the role that surface roughness can play in diffusing specular reflection. A very rough metal, for example, will have no diffuse reflection at all, but the very strong specular reflection will be diffused by the rough surface.
That is, the rough texture of a surface can lead to ”re-diffuse¨ or otherwise blur what would otherwise exude a specular or sharp image. And yet if a given surface had already been diffuse given its dielectric substance then that:
From “Specular Reflection¨ in Light for Visual Artists
[…] highly polished dielectric will still have a very strong diffuse component to its reflection.
That is, the effects of roughening versus otherwise polishing/smoothing a surface depends also on the material in question, such that one is not necessarily an effect of the other when applied.
Possible explanation for asymmetry in polishing v. roughening of surfaces
Given the “intrinsic¨ reflective tendency of dielectrics is diffusion, while of non-dielectrics (i.e., metals) is specularity, and that specularity is simply the absence of diffusion and a counterpart to refraction, the act of roughening a surface more often has a differential effect on a reflective image than that of polishing it. Or at least, has a greater intensity of expected effect for the same effort or amount, or has a lower threshold to meet than the other for the same intensity in expected effect. This is why a dielectric material’s surface tends to keep its diffusive image to a great extent despite polishing, while a non-dielectric will be blurred or diffused away from a specular image (especially a sharp one).
For non-dielectrics (e.g., metals) in particular, polishing simply has the effect of reversing the effect of roughening, such that polishing and roughening present opposite effects only for the same type of material–i.e., non-dielectric materials (Ibid):
From “Specular Reflection¨ in Light for Visual Artists
Smooth surfaces will have sharper specular reflections than rough surfaces, and smooth surfaces will always reflect bright light sources via their specular reflections.
Notice that this implies that glossy (i.e., shiny reflections with very distinctly defined image) surfaces are necessarily relatively smooth as “surface roughness will blur specular reflections and create a more matte appearance¨ (Yot 2020, 82).
Spectrum from specularity to diffusivity?
Perhaps surface material electrical status, atomic structure or texture can be placed in a spectrum from specular to diffuse reflectivity, instead of being treated as discrete, non-overlapping categories. The degree to which they are is simply an effect of “sorting¨ this spectra of reflectivity by other divisions that are more discrete and mutually exclusive.
It is important to keep in mind that more technically the “blurring¨ effect on the specular image that renders it into a more “diffuse¨ image, derived from increased texture roughness, is different in mechanism from the actual process of diffuse reflection. Diffuse reflection is from the scattering of light due to the behavior of atoms on the surface when interacting with photons, which is distinct from the physical roughness of that surface. The point is that the contents of what otherwise would have been a sharp, specular image are blended together into a more homogeneous visual blob, in both cases (Ibid):
From “Specular Reflection¨ in Light for Visual Artists
Do not confuse this roughness with diffuse reflection, however – the scattering from diffuse reflection occurs at a molecular level and is not caused by the roughness of the surface, but rather by the way in which the atoms in the material reflect light.
All diffuse reflection leads to diffuse image, but not all diffuse image is due to diffuse reflection
One way of putting this is that diffuse images need not only be caused by diffuse reflection, but all diffuse reflection causes a diffuse image.
Projective Shadows and Visibility
A final broad inequality is that non-dielectrics (e.g., metals) or their specular images are not affected by cast shadow from other solids, but dielectrics and their diffuse images are (Yot 2020, 87). In the former case, it is more likely that you’ll see the light source causing cast shadow on other adjacent surfaces via the specular image of the surface itself (Ibid).
Opaque Dielectrics and Combined Reflection Types
Another asymmetry between dielectrics and non-dielectrics such as metal is that (Yot 2020, 81):
From “Specular Reflection¨ in Light for Visual Artists
[w]hile metals are able to reflect only in a specular manner (they have no diffuse reflection at all), all opaque non-metals will combine diffuse reflection and a small amount of specular reflection.
That is, non-transparent or non-translucent (i.e., opaque) dielectrics have some level or degree of diffuse reflection regardless of the amount or intensity of specular reflection it may or may not have. This is opposed to non-dielectrics (e.g., metals’) insofar as non-dielectrics (e.g., metals) lack (intrinsic) diffuse reflection to begin with.
Specular Reflection and Color
We know that for an object or image to have a hue via pigmentation, it must have reflection (see 20250220145026-Ambient_Light_in_Visual_Art). The relationship between color, or more specifically hue, and reflection can also be distinguished in dielectrics versus non-dielectrics–dielectrics lacking tint while non-dielectrics (e.g., metals) having tint for their specular image (Ibid):
From “Specular Reflection¨ in Light for Visual Artists
In non-metals specular reflection is never tinted, it is always neutral in colour (reflecting the colours of the scene); only metals have coloured specular reflections.
For example, non-dielectrics such as (Ibid):
From “Specular Reflection¨ in Light for Visual Artists
A tint as an overpowering diffuse image projected by an object’s surface, cast over specular images
A tint can then be viewed as a diffuse image that all hues in a specular reflection on its shared surface must mix with or otherwise be filtered through or by. It can almost be seen to “cast over¨ the specular image.
Nonetheless, a diffuse image with a specular reflection will still subtly affect its specular image, even if only in the contrast the diffuse image provides for the specular image, so as to affect the specular image’s tonal values. Typically, a dark surface will lead to treating the brighter areas of a specular image as strong highlights for itself, while a light surface will lead to treating the darker areas of a specular image as deeper dark areas for itself (Yot 2020, 86).
Specular Reflection and Solid Forms
Any single specular reflector “may be distorted by concavity or convexity¨ (Yot 2020, 84). Extremely rough surfaces, especially with visible irregular varieties of concave and convex surface, have a tendency to specularly reflect in such a way that the specular image breaks into a multiplicity of self-same images or image fragments with varied distortion and size (Yot 2020, 83).
Another way in which a reflective image can be affected by the form of a solid, especially at its surface, is found in a phenomenon called anisotropic reflection. This phenomenon involves the stretching/spreading of a reflection along a given axis of the solid, with the reflection typically having a “beveled¨ appearance (Yot 2020, 84).
specular_reflection refractive_index refraction_index specular_image metal opacity tints gold light lights lighting electro anisotropic_reflection art visual_art visual_form magnetism magnet electro-magnetic_field electromagnetic_field magnetic_polarization physics cast_shadow light_source varnish varnishing polish polishing smoothing
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- “Specular Reflections.” In Light for Visual Artists: Understanding and Using Light in Art & Design, 2nd ed., by Richard Yot. Laurence King Publishing, 2020.