The report describes a new method, which has been developed in the Research Department, of making measurements of the Buchmann-Meyer light pattern of a laterally recorded disk. This pattern is often used to measure the modulation levels of the various frequencies on the disk. The new method is quicker and more simple to use that the traditional method and does not require such a high degree of skill in the operator. The use of the apparatus has revealed the importance of some hitherto unreported optical interference and diffraction phenomena associated with the light patterns. These phenomena are significant in relation to fundamental errors which are inherent in any method of observation of the patterns. The effects are analysed and the report describes how the simple theory of the light pattern measurement must be modified to give a more accurate result. Part I of the report describes the new apparatus and the evaluation of the final results of the measurements. Part II contains a detailed analysis and discussion of the various interference and diffraction phenomena which affect the evaluation of the results described in Part 1.