Saturday, April 10, 2010

Interpreting Form: Diffuse Transmission

In this blog I will occasionally talk about some of the conceptual models I find useful when trying to understand the form I am painting. This week I thought I'd address a common phenomenon in translucent forms known as diffuse transmission. In diffuse transmission, light enters into an inhomogeneous surface, breaks down by continually loosing parts of the spectrum to increasing odds of absorption and exits with what visually appears as higher chroma and a shifted hue*, yet darker values than the reflected side of the object**. This can be contrasted with specular transmission where the visible spectrum does not suffer absorption in any measure and exits intact though possibly refracted.

*Let's say that a light emits photons with probability waves of Red, Orange, Green and Blue towards a surface. The surface is composed of atoms with 60% of the electrons having a frequency matching Green, 30% matching Blue, 10% matching Orange and none matching Red. At the surface a bunch of Red and Orange photons would diffusely reflect causing what would appear as an Orange-Red local. The saturation of this local would be neutralized slightly by the smaller percentage of Green and Blue photons who, despite having higher odds for absorption, still diffusely reflect at a smaller percentage across the surface. Light that enters the surface can scatter under the surface (a topic better left for another post) or transmit through. The odds of Green making it far into the surface without finding an absorption match is incredibly small, Blue may make it a little farther with Orange coming in second place to Red which transmits through with greater purity. This would leave us with a slight hue shift from Orange-Red on the diffusely reflected surface to something closer to Red on the Diffusely Transmitted side. Since there are fewer photons transmitting than reflecting, the value remains darker on the shadow side. (Keep in mind, the parts of the spectrum I'm using to describe this are just variables to illustrate the logic of this phenomenon and aren't specifically related to their various wavelengths and how they travel through different densities and distances--more topics for later posts)

**It should be noted that the values on the shadow side would be lighter when transmission is taking place than when it is not. This would be a logical conclusion (more photons=lighter value) in addition to being visually obvious. Above, I wanted to point out that with transmission the value gets darker with the chroma going up to contrast it with the chroma model I use on the diffusely reflected side where value and chroma go up proportionally as the form rolls towards the light (essentially seeing more of the object and therefore more of its local).

Two last things I'd like to point out: The computer generated diagram was made by me in Photoshop. It is a little unsteady and isn't the most solid demonstration of this activity. Maybe one day I will get 3D software and be able to build more consistent depictions of this stuff. Lastly, I'd like to add that I'm obviously not a physicist and other than actively reading about this stuff and discussing it with friends I'm unqualified to make absolute statements about what is happening on an atomic level. These models are the most consistent predictors for what I see daily as I paint and so far offer the most thorough explanation for Hue, Value Chroma shifts in form. I always want to get better and am always open to evolving and improving them. If by chance a physicist or anyone that finds fault in my thinking reads this, please leave a comment and publicly admonish me.


  1. Scott, by no means I'm trying to comment in each one of your post :)
    But this one is another topic in which I'm really interested, and is a subject that always was (and is) really hard to grasp for me. Thanks again for posting this kind of stuff. Really useful and informative.

  2. Additionally, I think that never read an explanation so clear concerning the hue shift in the shadow side.

    I read (and observe it) about the drop in value and saturation (chroma) while we advance in the shadow side (also in the edge plane), EXCEPT when the form is in some point translucent (an issue that I understand but again I think never see it so clear as now, with your post), BUT the hue shift, that point I never read anything that explained so convincingly as here.

    My English is not very good, I hope at least something of this comment has some sense, I apology in advance.

    Again, great post Scott.

  3. this is so awesome. i only wish i was a physicist so i could publicly admonish you...oh hell, maybe i don't need such credentials! :)

  4. Wow, that little picture of diffuse reflection and diffused transmission is worth a thousand words! Did you paint that or did you come upon it somewhere?

  5. Thanks everyone for the comments! Ariel, I'm glad you comment on many of the posts--it makes posting these feel worth while. Carol, if you publicly admonish I will reciprocate in full if and when you get your own blog :) Elisabeth, I made the diagram in photoshop. Thanks Camie!