Qualities and Determinants of Diffuse Reflections
Diffuse reflection involves the reflection of light in a scattered manner such that it leads to an even or average blend or mix of the reflected hues, often into a single solid hue (Yot 2020, 72):
From bib. source
Diffuse reflection occurs when the light being reflected is heavily scattered (diffused) by the reflecting surface, and although this is often attributed to the roughness of the surface in question it is actually a phenomenon that occurs at the atomic level.
The scattering is, as mentioned above, a phenomenon occurring at the atomic or molecular level, and is not necessarily related to rough surface texture. The exact process involves the interaction of photons with electrons in the atom, wherein they are either absorbed (and expressed as heat) or re-emitted as a reflective image by those electrons (Yot 2020, 73):
From bib. source
Photons are discrete electromagnetic particles of pure energy: they are not made of ordinary matter since they have no mass, unlike atoms. When photons come into contact with atoms they interact with the electrons that orbit the atoms, and either they are absorbed – in which case their energy is converted into kinetic energy and heat – or they are re-emitted and so create a reflection.
Consequently, as a general rule one can say that “a black object will absorb all the photons that come into contact with it,¨ “a white object will reflect all the photons and stay at a constant temperature,¨ and “surfaces that are coloured will absorb some photons and reflect others,¨ as we should already know (Ibid).
Pigmentation, tonal value and shadow relativity
This explains the way pigmentation filters hues and the relationship between that filtering and tonal value. Namely, it suggests that shadow or shade should be treated as hue-relative. That is, a given area is only dark relative to the predominant electromagnetic wavelength of light that has been defined for an area or region. This means interactions between lights of different electromagnetic wavelengths can interfere with each other’s perceived shadows, either “destructively¨ (disappearing the shadow or shade) or constructively (providing fill light to the shadow or shade); see 20251003160739-Relative_Hue_and_Shadows. This means that darker tonal values in art express either the total absence of light or the interactions and relationships among lights of different hues. Keep in mind when doing art which your tonal value is trying to express.
The difference a diffuse reflection has from a specular reflection is that it arises when there is a lower index of refraction for a given surface and that it occurs largely for opaque dielectric materials (i.e., materials that can be magnetically polarized by an electric field even though they cannot themselves conduct electricity) (Yot 2020, 80-81). The essence of diffuse reflection can be characterized by the randomization of the direction or angular change (relative to surface) of re-emitted photons (Yot 2020, 73):
From bib. source
On diffuse surfaces, when a photon is absorbed then re-emitted by an electron, its direction of travel will be altered: it will reflect away from the surface, but in a more or less random direction. This scattering is what we know as diffuse reflection.
Ultimately, this is destructive of any image that would have otherwise appeared, leading to “matte surfaces¨ that have “a smooth and even appearance because they mix all the light they receive into an even blend¨ (Ibid). This also means, however, as a tint for a diffuse image, as there is no clear, sharp differentiated image(s) for or in it that can be skewed by or expressed in a given monochrome hue or affected by some “underlay¨ (underlying) hue (see more on hues in specular reflection).
Pigmental hues as a smoothie of a surface image
It is apparent that the reflection of light is not just about hue but about image. A hue understood as a solid color from a pigment can be seen as a blending of all the hues involved into a homogeneous mixture that acts as one, solid color of a hue. This is why a surface seems to “lack a color¨ when its reflection is specular–but this would be a mistake. A specular reflection precisely has a high fidelity projective image of all the hues of its environment. A transparent or translucent surface relatively lacks image at all. It is important to then treat the presence of solid, single hues on a pigment or surface as a function of that surface’s or pigment’s reflective behavior (diffuse v. specular).
Light Variation and Gradation
Light falloff is a phenomena for dielectric surfaces under diffuse reflection, wherein the light sources have a finite reach as well as a gradual dimming via being thinly spread across the surface (Yot 2020, 74). This is unique to diffuse images or dielectric solids. Meanwhile, the surface nearest to the highlight often acts as a hot spot that is a highlight for the given solid (Ibid).
Reasons for uniqueness of light falloff in diffuse reflection
This uniqueness may be due to the implicated scattering of light affecting the sharpness of edges between light and dark.
The falloff effect for light is most noticeable for dielectric objects or reflected images that are closest to us (Ibid). This has an implication for the impact (or, rather, lack thereof) of ambient light (or sunlight / skylight), that being that (Ibid):
From bib. source
[…]: falloff does not occur in sunlight, or in overcast daylight outdoors. However, man-made light sources, as well as windows that let in diffuse natural light, are considerably smaller and nearer, so falloff will be a visible and significant property of such light.
Falloff does not occur in a specular reflection, but instead divisions in the image, if there are any, are visibly distinct and segregated, as there is no singular hue or tone which can have a singular tonal value shift visible across the whole solid or surface, save for an extreme level of rough texture or a large distance to blur the specular image enough:
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
bibliography
Link to original
- “Specular Reflections.” In Light for Visual Artists: Understanding and Using Light in Art & Design, 2nd ed., by Richard Yot. Laurence King Publishing, 2020.
Fall-off is important insofar as it contributes to conveying volume and even some minimal information about the texture of the material the solid is made of, by affecting the transitions between light and dark on solids or surfaces, and thereby the relative harshness of edges (Yot 2020, 76).
Ultimately, diffuse reflection allows for cast shadow from other surfaces to affect or impact its surface and thereby its resultant surface images, unlike specular reflection which simply projects back the cast or form shadows around its environment (Yot 2020, 75):
Circular transclusion detected: Research/annotations/20251202181730-Qualities_and_Determinants_of_Specular_Reflections
Because specular images are the result of the reflection of the solid’s surroundings or environment at every one of its sides or planes, that solid can have no form shadow so long as its environment is fully encompassing and itself reflecting light (Ibid). Diffuse images, on the other hand, do have form shadow (Ibid).
Polish and Reflectivity
As pointed out elsewhere, a diffuse image cannot be made specular by the act of polishing the given solid’s surface, and yet a specular image can be made diffuse by the act of roughening the given solid’s surface. This is because specular and diffuse imagery is a function of the interaction between photons and atoms on a solid’s surface (see 20251004151403-Types_of_Surface_Light_Reflection), and so the effect of roughening or polishing a surface is constrained by the molecular quality of the material comprising the solid’s surface. Consequently, its more accurate to say that roughening the surface productive of a specular image blurs that image than that it diffuses it. This distinction allows us to see that blur can happen to both specular and diffuse images, but is more apparent in the former.
If anything, a blur on a diffuse image creates a “fuzzy, velvety look¨ that (Yot 2020, 77):
From bib. source
[…] is created by a network of small fibres all over the surface, which serve to diffuse the light even more than a standard diffuse material. These fibres catch the light in a more complex way than a smooth surface would, and this creates the characteristic sheen of these materials, The appearance is almost like a very soft gloss, […]
Wetness or humidity can affect the underlying diffuse image as well, by smoothing (analogous to “polishing¨) the surface causing that diffuse image–except not really. Wetness or humidity simply add a transparent or translucent layer that nonetheless can have some degree of specular reflection or shine itself, over the diffuse image.
Eureka
This phenomenon is analogous to the case of solids that have a mix of diffusion and specularity visible on their surface, as addressed here, hence one should expect a similar impact or effect in the case of wet or humid diffuse images: the tonal value having enhanced contrast. In addition, one can expect a faded reflection tinted by the underlying color of the diffuse image.
The total effect is that (Ibid):
From bib. source
Rough, diffuse surfaces will tend to get darker when wet, partly because some light is lost to internal reflection, and partly because specular highlights that would have been spread all over the surface owing to the high roughness are lost thanks to the water smoothing out the surface.
Varnish coating on wood
The darker look of wood while under a “clear coat of varnish¨ that nonetheless retains brighter and sharper highlights exemplifies this effect quite analogously (Ibid).
photon electron electro-magnetism electromagnetism molecule molecules atoms kinesis kinetic_energy physics temperature emission energy electromagnetic_spectrum electro-magnetic_spectrum dielectrics fields electro-magnetic_field electromagnetic_field magnetism magnet magnetic_polarization physics light_falloff cast_shadow form_shadow light_source tonal_values tonal_value varnish smoothing polishing polish varnishing
bibliography
- “Diffuse Reflection.” In Light for Visual Artists: Understanding and Using Light in Art & Design, 2nd ed., by Richard Yot. Laurence King Publishing, 2020.