Optical inspection device and lithographic apparatus provided with such a device

An optical inspection device for inspecting a first surface and an oppositely located second surface of an object by means of, respectively, a first and a second inspection beam, the path of each inspection beam extending from a radiation source via a number of optical elements for guiding the relevant inspection beam to, and focusing the beam on, the surface to be inspected, and this path being provided with a scanning element for moving a radiation spot formed by the inspection beam on the surface to be inspected across this surface, and a radiation-sensitive detector being present for each inspection beam, which serves to convert radiation from the inspection beam which is scattered by the surface to be inspected into an electric signal, characterized in that for each inspection beam there is first of all provided, in the path of said scattered radiation, an element which causes this radiation to traverse the same path, yet in the opposite direction, with the same optical elements as the inspection beam as far as the scanning element, the optical element positioned such that radiation from the inspection beam which is reflected by the surface to be inspected is not incident thereon and the reflected radiation is not caused to traverse the same path, yet in the opposite direction, with the same optical elements as the inspection beam, and in that the detector is provided behind this scanning element.