Optical apparatus includes a primary radiation source, which emits first optical radiation along a first optical axis. A DOE includes at least an entrance surface, upon which the first optical radiation from the primary radiation source is incident, and an exit surface, through which one or more primary diffraction orders of the first optical radiation are emitted from the DOE. At least one secondary radiation source is configured to direct second optical radiation to impinge on the DOE along a second optical axis, which is non-parallel to the first optical axis, causing at least a part of the second optical radiation to be diffracted by the DOE such that one or more secondary diffraction orders of the second optical radiation are emitted through the entrance face of the DOE. At least one detector is configured to sense at least one of the secondary diffraction orders of the second optical radiation.