What are the characteristics of light?

The list in the Peachpit article is the same one I came up with when I developed a syllabus for a beginner's photography course on my own. I later changed hardness to diffusion, then demoted contrast, explained below.

With regard to light, these are the characteristics I evaluate when making decisions about exposure and composition.

1. Direction - position of the source(s); where all of the light in the scene (incident and reflected) is coming from; the direction, definition, and brightness of shadows

2. Intensity - output level at the source, brightness of illuminated surfaces, relative brightness

3. Color - hue and color temperature, luminance

4. Diffusion - variance of angles of incidence

I don't consider hardness (the rate of transition from light to dark) or contrast (the difference between the darkest and lightest points) to be characteristics of light. They are the perceived effects of light that varies in direction and intensity, so I consider them functions of those characteristics.

Aren't size and distance also characteristics of light?

At first glance these are characteristics of light sources, not light itself.

Size and even "relative size" used to describe how large a source is are useless by themselves. It's more useful to think of the maximum angle at which light arrives at the subject. The greater the angle, the "larger" the source.

But distance is interesting. It's kind of a pseudocharacteristic of light, isn't it? As in "the distance from the source traveled by the light striking the subject", because that segment of the light beam has a unique property: the rate of falloff, defined by the inverse square law.