Abstract: In a grating measuring system of a basic type originated by DeLang employing a concave mirror-lens main optical system introduced by Dyson, improved separation of the light fluxes of input illumination and output motion-modulated signals is achieved. The arrangement depends upon the fact that in such a system wherein the grating is imaged back on itself so that for every point in the half plane containing the grating surface at one side of the optical axis, there is a conjugate point in the half plane at the other side of the axis. Separation by reason of beam separation is attained by illuminating one half plane and deriving output signals from the other. Further separation is attained from the fact that the concave mirror lies in a so-called "complementary plane" or "Fourier plane" to the object-image plane. Each point in the complementary plane is the focus of all rays originating in the object-image plane in a particular direction.