Abstract: Secondary radiation in a non-linear crystal is produced by irradiating the crystal with a coherent primary radiation, to produce a coherent phase-adapted secondary radiation whose frequency is doubled with respect to the frequency of the primary radiation. In a modification of the method, two different sources of primary radiation are used, in such a way that the secondary radiation in the crystal is equal to the sum or the difference of the frequencies of the primary radiation sources. In apparatus for carrying out the method, the crystal to be irradiated is mounted in a swinging or tiltable carrier. The emerging radiation is partially reflected onto a pair of photoelectric detectors, the output of which controls a servomotor to vary the degree of tilt of the crystal. In a second embodiment, there are two crystals tilting in opposite directions. In a third embodiment of the apparatus, two separate sources of primary radiation serve to irradiate a single crystal.