Early this morning I added a correction to the calculation of the concentration of sunlight by a lens in the blog a couple of days ago. The formula could be expressed in terms of the F-number of the lens. A term related to the F-number is that of the Numerical Aperture. Both are connected with the relative angular size formed by an aperture in an optical system. If one thinks of the rim of the Sun and that of its image as stops in the optical system then their angular sizes and "F-numbers" relative to the lens would be the same. Consequently their "numerical apertures" would also be the same. One could possibly simplify the formula for the irradiance of the Sun's image using a "numerical aperture". One would have to think in terms of the angular aperture which would admit the light from the Sun and another that would allow the formation of the image. There seems to be a term missing for this kind of virtual aperture.
Supplemental: There are virtual apertures.