Focus detector as an aid for focusing a laser on a target.

An apparatus is described which assesses the focus of a laser on a surface. An image of the focus is scanned by fiber optics. A rotating slit together with a photodetector behind the fibers detects whether a fiber is illuminated or not. When the focus coincides with the surface of the target, the number of illumiated fibers is a minimum. The apparatus is especially advantageous for adjustment when the radiation is not visible, for example, with neodymium lasers or with frequency doubled radiation from ruby lasers.