angular aperture

The light crossing this point is confined to a cone of angular aperture 2"α".

In air, the angular aperture of the lens is approximately twice this value (within the paraxial approximation).

Perfectly parallel rays (shown in blue) would be captured, but sunlight, due to its angular aperture, is partially lost.

The angular aperture of a lens is the apparent angle of the lens aperture as seen from the focal point:

Sunlight is not a set of perfectly parallel rays (shown in blue), but it has a given angular aperture "θ", as indicated by the green rays.

The resolution R (here measured as a distance, not to be confused with the angular resolution of a previous subsection) depends on the angular aperture :

In a medium with an index of refraction close to 1, such as air, the angular aperture is approximately equal to twice the numerical aperture of the lens.

All of these reduce the initial acceptance angle and, after they are all factored in, the system must still be able to capture the finite angular aperture of sunlight.

If the acceptance angle of the optic is wide enough, sunlight incident along the optical axis will be captured by the concentrator, as shown in the "angular aperture of sunlight" figure.

Instead, the angular aperture of a photographic lens (or an imaging mirror) is expressed by the f-number, written or N, which is defined as the ratio of the focal length to the diameter of the entrance pupil: