Apparatus for the measurement of retinal visual acuity by moiré fringes

Moiré fringes generated by two equal gratings in contact, looked at in Maxwellian view, are well suited for the assessment of retinal visual acuity. High-contrast fringes practically free from spatial harmonics (higher Fourier components) are obtained if, instead of the zero diffraction order, one first-order diffraction spectrum from each of the gratings is isolated by a stop. Thus a pair of coherent light sources is produced, the distance of which can be 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, however, be compensated for by an additional dispersing element near the gratings, e.g., a spectral prism. The device then is an achromatic interferometer, enabling the use of white light. Results do not systematically deviate from those obtained with a monochromatic interferometer using a laser. If desired, fringe contrast can be measurably varied by superimposition of a white light beam, with the intensities of both fields being controlled by polarizers.