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I was reading this text about radiance and according to the next figure, both leaving and arriving radiances' solid angles are anchored on the surface point?

enter image description here

My logic tells me that the solid angle of the radiance arriving at a surface is supposed to be anchored somewhere in the "air" (say at the light bulb position). Something like this:

enter image description here

This way the radiance formula makes sense too, no? (irradiance per solid angle, meaning when the solid angle gets bigger the radiance at point gets smaller)

Can someone shed some light on this?

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I think that both ways of looking at radiance are valid; it just depends on what you are trying to determine.

Their diagram shows the radiance arriving at a point from a particular direction. If you wanted to determine the total radiance arriving at that point from all directions then you would add up the radiance arriving from all light sources visible from that point.

Your diagram shows the radiance leaving your light source in a particular direction. The total radiance leaving your light source would determine how much light arrives at each of the many points visible from the light source.

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  • \$\begingroup\$ Usually I want to know the flux arriving at a point from the entire hemisphere, and not flux arriving from a set of directions (solid angle). No? \$\endgroup\$ – Pilpel Aug 16 '16 at 8:04
  • \$\begingroup\$ @Pilpel - Yes, you probably want to sum over the entire hemisphere. That hemisphere will be anchored at the surface point. \$\endgroup\$ – Incredulous Monk Aug 17 '16 at 4:33

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