We talk qualitatively quite a bit about the softness or hardness of light, as it's one of the fundamental qualities and something every photographer and videographer should consider when choosing how to light their subjects. This neat video takes a more scientific approach by asking how we can quantify the softness of light.
Coming to you from Cinematography Database, this fun video examines the idea of measuring just how soft a light source is. The softness of light depends on two things: the distance from the subject (the closer the source, softer it is) and its size (the bigger it is, the softer it is). These two things affect the apparent size of the source relative to the subject, or in more mathematical terms, the solid angle formed by the source. The solid angle is a neat and tidy way of quantifying the apparent size of the source, though we generally still speak of light softness qualitatively. Nonetheless, it's a fun look at what we really mean when we speak about the softness of light and why differently sized light sources placed at correspondingly different distances can still produce the same softness.