Abstract: The present invention relates to a phase corrected raster scanned light modulator in which information modulated on a carrier is used to form a grating whose spatial frequency along the scanning line varies in accordance with the signal modulation frequency. Thickness variation in the medium on which the grating is formed causes an undesired phase modulation of the light which may be corrected by altering the carrier frequency. A novel oscillator is described which may be stepped discontinuously in frequency while the waveform and its slope remain continuous, which is of sufficient accuracy and agility to provide a real time region by region phase correction of the raster in the light modulator. The variable frequency oscillator is of high stability and can step from one value to another for intervals as short as one microsecond.