General equations are developed which relate the characteristics of moiré fringes to the geometry of two superposed, dissimilar grid systems. Equations are derived geometrically, assuming the rectilinear propagation of light, and the analysis is restricted to a consideration of linear, periodic, mutually inclined systems. Criteria are established for the production of first and second order fringe systems, and some particular cases of grid geometry are examined in detail. The concepts of mixed grids and hybrid grid combinations are introduced, and the principles and possible applications of fringe sharpening, fringe multiplication and dual sensitivity systems are discussed.
