The optical disk-memory has very fine grooves.The widely applied Fraunhofer theory fails to describe the diffraction since large diffraction angle should be taken into account and the Fresnel coefficients at the grooves are no longer constant.Under the circumstances

a method has been developed to study this diffraction problem;furt-hermore

some essential factors

such as the influence of the groove form

the contribution of the multiple reflection at the substrate surfaces to the diffraction patterns

particularly when the substrate is not perfectly parallel

and so on

have been discussed.Finally a diffraction technique for testing groove parameters has been established.