An optical device includes a light source, which is configured to emit a beam of light at a given wavelength, and at least one scanning mirror configured to scan the beam across a target scene. Light collection optics include a collection optic positioned to receive the light from the scene that is reflected from the at least one scanning mirror and to focus the collected light onto a focal plane, and a non-imaging optical element including a solid piece of a material that is transparent at the given wavelength, having a front surface positioned at the focal plane of the collection lens and a rear surface through which the guided light is emitted from the material in proximity to a sensor, so that the collected light is guided through the material and spread over the detection area of the sensor.