A method of optimising at least one measure that is a quadratic function of a wavefunction, such as the spot size of a laser, the method comprising: superimposing a plurality of wavefunctions; determining a relationship between the superimposed wavefunctions and the quadratic measure, and using the determined relationship to identify the superimposed wavefunctions that provide a desired or optimised quadratic measure.

A defect detection system for an extreme ultraviolet lithography mask comprises an extreme ultraviolet light source (1), extreme ultraviolet light transmission parts (2, 3), an extreme ultraviolet lithography mask (4), a photon sieve (6) and a collection (7) and analysis (8) system. Point light source beams emitted by the extreme ultraviolet light source (1) are focused on the extreme ultraviolet lithography mask (4) through the extreme ultraviolet light transmission parts (2, 3); the extreme ultraviolet lithography mask (4) emits scattered light and illuminates the photon sieve (6); and the photon sieve (6) forms a dark field image and transmits the same to the collection (7) and analysis (8) system. The defect detection system for the extreme ultraviolet photolithographic mask uses the photon sieve to replace a Schwarzchild objective, thereby realizing lower cost, relatively small size and high resolution.