It is well known that photoresist undergoes a chemical change on exposure which results in a variation in its refractive index. In the recording of diffraction gratings this index change leads to a detectable phase grating before development the so called latent image. The growth of the grating may be observed in real lime by monitoring the diffraction efficient of the first order beam from this latent image.nBy superposing the diffracted beams from two such latent gratings a moire pattern may be observed, from which the position of the fringes of one grating relative to the other may be deduced. If one of the gratings is moved relative to the other then this pattern may be used as the trigger for an optical exposure, and gratings may be written to cover a length independent of the size of the exposing beams
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