Moire'' fringes generated by 2 equal gratings in contact, looked at in Maxwelliam view, are well suited for the assessment of human retinal visual acuity. High-contrast fringes practically free from spatial harmonics (Higher Fourier components) are obtained if, instead of the 0 diffraction order, 1 1st-order diffraction spectrum from each of the gratings is isolated by a stop. A pair of coherent light sources is produced; the distance is varied by relative rotation of the gratings. The resulting fringe pattern of variable pitch is characterized by achromatic intensity minima seen against a background of variable color depending on the direction of viewing. These colors can be compensated for by an additional dispersing element near the gratings, e.g., a spectral prism. This device is an achromatic interferometer, enabling the use of white light. No systematic deviation from a monochromatic interferometer using a laser occurs. If desired, fringe contrast can be measurably varied by superimposition of a white light beam, with the intensities of both fields controlled by polarizers.