A system for measuring the wavefront characteristics of a powerful laser close to an emitting or transmitting surface of the laser. The system includes a beam sampler that has a sampling aperture for sampling radiation from a sampled area along the emitting or transmitting surface. The beam sampler includes a reflector for directing un-sampled radiation onto an absorber, which absorbs un-sampled radiation. Radiation sampled by the beam sampler is sensed using a sensor.