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The Photon
The Photon
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As you say in comments, in Wikipedia, the luminanceluminance is described as,

the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. [emphasis added]

in which you omitted the portion of the description which I have emphasized.

We can calculate the luminance either as the luminous flux emitted from an element of the surface into a given solid angle, or, equivalently, as the luminous flux falling on an element of a surface from a given solid angle.

In one case we consider the surface area of the source, in the other we consider the surface area of the detector. In either case, the solid angle term encompasses the surface area of the other object (the detector as seen from the source, or the source as seen from the detector) and the distance between them, being defined as $$\Omega = \frac{A}{r^2}$$ where $A$ is the area of the other object and $r$ being the distance.

As you say in comments, in Wikipedia, the luminance is described as,

the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. [emphasis added]

in which you omitted the portion of the description which I have emphasized.

We can calculate the luminance either as the luminous flux emitted from an element of the surface into a given solid angle, or, equivalently, as the luminous flux falling on an element of a surface from a given solid angle.

In one case we consider the surface area of the source, in the other we consider the surface area of the detector. In either case, the solid angle term encompasses the surface area of the other object (the detector as seen from the source, or the source as seen from the detector) and the distance between them, being defined as $$\Omega = \frac{A}{r^2}$$ where $A$ is the area of the other object and $r$ being the distance.

As you say in comments, in Wikipedia, the luminance is described as,

the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. [emphasis added]

in which you omitted the portion of the description which I have emphasized.

We can calculate the luminance either as the luminous flux emitted from an element of the surface into a given solid angle, or, equivalently, as the luminous flux falling on an element of a surface from a given solid angle.

In one case we consider the surface area of the source, in the other we consider the surface area of the detector. In either case, the solid angle term encompasses the surface area of the other object (the detector as seen from the source, or the source as seen from the detector) and the distance between them, being defined as $$\Omega = \frac{A}{r^2}$$ where $A$ is the area of the other object and $r$ being the distance.

Source Link
The Photon
The Photon
  • 29.5k
  • 5
  • 47
  • 79

As you say in comments, in Wikipedia, the luminance is described as,

the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. [emphasis added]

in which you omitted the portion of the description which I have emphasized.

We can calculate the luminance either as the luminous flux emitted from an element of the surface into a given solid angle, or, equivalently, as the luminous flux falling on an element of a surface from a given solid angle.

In one case we consider the surface area of the source, in the other we consider the surface area of the detector. In either case, the solid angle term encompasses the surface area of the other object (the detector as seen from the source, or the source as seen from the detector) and the distance between them, being defined as $$\Omega = \frac{A}{r^2}$$ where $A$ is the area of the other object and $r$ being the distance.