Apparatus and methods are described, including an apparatus for testing a light beam emitted by a light source. The apparatus includes a transparent substrate, a first face of the substrate being shaped to define a plurality of optical deflectors. At least one optical detector is positioned to face a second face of the substrate that is not opposite the first face. Each one of the deflectors is configured to deflect a portion of the light beam toward the detector, when the light beam is passed through the first face of the substrate. Other applications are also described.