DIFFUSE TRANSMISSION AND TASTING THE RAINBOW

(UPDATE 10/27/11 -For French translation of this post please click "here.")  
 
DIFFUSE TRANSMISSION

I have had a few requests to discuss how "diffuse transmission," which makes up the appearance of translucency, works.

Because I paint a great variety of materials in my still life work I encounter diffuse transmission quite regularly—perhaps in the light passing through milk (Figure1) or the light passing through the thin wall of a shell (Figure 2). However I also encounter it when doing figurative work and this seems to be where many people really take notice of it's effect—perhaps when looking at the intense high chroma reddish, orange seen in the ears of a person who is backlit.

Figure 1.  Black and White 8" x 6" Oil on linen

Figure 2.  Understanding Phi 10" x 16" Oil on Linen

As its name implies diffuse transmission deals with the diffuse component of light. And because this light has passed through the object it is described as "transmission." Diffuse transmission is composed of light that has entered a material, undergone subsurface scattering (losing wavelengths to absorption in the process) and then has been emitted back out of the material on the shadow side of the object. It is actually the same process as "diffuse reflection" except that the light has exited the material on the shadow side, instead of the light side, of the object. For this reason we should expect to observe diffuse transmission mainly on the shadow side of an object. On translucent materials that have some substantial thickness (such as a sphere vs. a sheet of paper) the diffuse transmission will appear just past the terminator or shadow line.

In terms of its affect on the appearance of color, diffuse transmission is usually darker in value than what is seen on the light side of the same object. This is due to the loss of light through absorption as the light passes through the object. However it will be higher in chroma than the color found on the light side of the object. This is because only certain wavelengths were absorbed, in effect filtering out some wavelengths while allowing others to pass through. It has been my experience that there may also be some shift of hue (often very slight) from what the local color appears to be on the light side of the object. Sometimes this may be because as the light travels through the object it encounters different layers of materials (each material absorbing different sets of wavelengths. However, I have often wondered if there are also other factors at work, based more on our perceptions with different relative proportions of wavelengths stimulating the eye differently—however that is something to be contemplated further at another time.

TASTING THE RAINBOW

For a greatly simplified analogy, to understand what has already been stated, lets say I find myself with a huge bowl of Skittles candy that represents a certain quantity of light. Each candy with its own color might be a particular photon with a particular wavelength. I am standing on a stage in a filled auditorium with the audience representing all of the atoms that make up a particular object. In this analogy each person in the audience only likes a particular color of candy but for some reason no one in the room likes green.

A Bowl of Light

I begin to toss out handfuls of Skittles to the first row which they proceed to eat assuming they get the color of candy they like. If they don't like the color or they are currently eating a piece, they can pass it on to someone else. However they can only pass a Skittle on 10 times. If no one eats it in that time they proceed to toss it back up onto the stage. This candy on stage makes up diffuse reflection. There is less candy than what we started with (hence less light) and although some of all of the colors should end up back on stage the proportion of green Skittles should be much greater (giving the appearance of green).

Diffuse Reflection

After all the leftovers are back on stage the audience having just started to work up a sugar high demands an encore! Not wanting to disappoint them I break open some new bags of skittles filling back up the bowl and start the process over again. However realizing that many people in the rows further back never got any candy the first time I allow them to pass on the candy 50 times. This time if no one eats it I ask them to toss the leftovers toward the lobby entrance in the rear of the auditorium. This time the candy left here makes up diffuse transmission. There is a lot less candy than what we started with (hence even greater loss of light) and a very large proportion of green Skittles (giving the appearance of a very high chroma green)

Diffuse Transmission

CONCLUSION

Very often the phenomenon of diffuse transmission causes us to see beautiful glows of high chroma on an object or in spots throughout a scene. Accurately understanding what is occurring helps in capturing these glows with paint ( through "relative" relationships of color or "absolute" color matches). Getting this effect right communicates to the viewer the type of material being represented, giving a "truthful" or "realistic" effect and can also be beautiful in an aesthetic sense at the same time.

Thanks for taking a look!