I've been thinking about how radiance is invariant along a ray in a vacuum.
This seems pretty intuitive if you have a flashlight producing a cone of light. Surely, as the cone spreads out, the light is still all occupying the same solid angle, so the direction is staying constant. And the light isn't disappearing, so the light per direction must be staying constant.
However, not all flashlights produce cones of light. For example, a laser pointer produces a parallel beam of light. So as distance increases, the solid angle that the beam of light occupies gets smaller and smaller. The amount of light stays constant though. So it seems like we're getting more light per direction.
How is this conundrum resolved? My best guess is that radiance is a property of point sources, and point sources cannot produce parallel beams of light. But that's just a guess.
jezimmer
In fact, lasers do produce cones of light. They're just such small cones that at the distances over which lasers normally travel, the areas attenuated by their solid angles don't change that much.
I've been thinking about how radiance is invariant along a ray in a vacuum.
This seems pretty intuitive if you have a flashlight producing a cone of light. Surely, as the cone spreads out, the light is still all occupying the same solid angle, so the direction is staying constant. And the light isn't disappearing, so the light per direction must be staying constant.
However, not all flashlights produce cones of light. For example, a laser pointer produces a parallel beam of light. So as distance increases, the solid angle that the beam of light occupies gets smaller and smaller. The amount of light stays constant though. So it seems like we're getting more light per direction.
How is this conundrum resolved? My best guess is that radiance is a property of point sources, and point sources cannot produce parallel beams of light. But that's just a guess.
In fact, lasers do produce cones of light. They're just such small cones that at the distances over which lasers normally travel, the areas attenuated by their solid angles don't change that much.