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Given a point in space and a direction, radiance is defined as the power per unit solid angle per unit area perpendicular to that direction.

When I first saw this question, I thought that the area had decreased to a very small area where the rays were focused, but the power of the ray was the same, so radiance would increase. However, this is the wrong understanding.

Although the rays are being focused/redirected towards a point, the solid angle and area at the origin of the ray are the same.


In this case the radiance stays the same, but the irradiance under the magnifying glass changes. So in the image on the right, the lighter color areas that are being focused by the magnifying glass have increased irradiance while the darker color areas have decreased irradiance since the rays that would have hit those areas have been redirected toward the center.


Since the radiance is conserved, and it can not measure the brightness or energy of the light field, how we change the radiance, and what radiance is very meaningful to measure?