An optical system for detecting light from a 2D area of a sample (36) comprises a collection lens (34) for collecting light from a collection region of the sample. A light detector (44) is positionally fixed with respect to the sample, and a reflector arrangement (61) directs collected light to the detector. The reflector arrangement comprises movable components and the collection lens (34) is movable relative to the sample. The collection lens and the movable components are configurable to define different collection regions, and the movement of the components effects a direction of the light from the collection region to a substantially unchanged area of the light detector (44). This arrangement avoids the need for a bulky detector in order to detect signals from a 2D sample area formed by scanning across the sample.