Patent ID: 8542345

Claim:
A measurement apparatus which measures a numerical aperture NA of an optical system to be measured, the measurement apparatus comprising: a mirror configured to reflect test light which passes through the optical system; an interferometer unit which includes an image sensor and is configured to form an interference fringe on an image sensing plane of the image sensor by reference light and the test light reflected by the mirror; and a controller configured to control the interferometer unit, and to compute the numerical aperture NA of the optical system based on the interference fringe captured by the image sensor, wherein the controller is configured to compute the numerical aperture NA of the optical system by multiplying a quotient ΔNA/ΔR, describing a change ΔNA in the numerical aperture NA of the optical system with respect to a change ΔR in pupil radius R of the optical system in the image sensing plane, by the pupil radius R of the optical system in the image sensing plane, wherein the controller is configured to compute, using a first radius R 1 as the pupil radius R of the optical system in the image sensing plane, a wavefront difference W 1 that is a difference between a first wavefront of the optical system computed based on a first interference fringe formed on the image sensing plane when a position of the mirror relative to the optical system is in a first state and a second wavefront of the optical system computed based on a second interference fringe formed on the image sensing plane when the position of the mirror relative to the optical system is in a second state which is obtained by a movement of a movement amount ΔX from the position of the first state, to compute, using a second radius R 2 that differs from the first radius R 1 as the pupil radius R of the optical system in the image sensing plane, a wavefront difference W 2 that is a difference between a third wavefront of the optical system computed based on the first interference fringe, and a fourth wavefront of the optical system computed based on the second interference fringe, and to compute the quotient ΔNA/ΔR using the following equation: ΔNA/Δ R =( W 1 −W 2)/{( R 1 −R 2)Δ X}.