Principles of Tscherning aberrometry

Higher-order optical errors of the human eye are often responsible for reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations on the eye involving optical structures. The presented wavefront analyzer is based on Tscherning's aberroscope. A collimated laser beam illuminates a mask with regular matrix pin holes which forms a bundle of thin parallel rays. These rays form a retinal spot pattern on the retina that is more or less distorted according to the optical errors of the eye. This retinal spot pattern is imaged onto the sensor of a low-light CCD camera by indirect ophthalmoscopy. The deviations of all spots from their ideal regular positions are measured by means of a personal computer, and from these values the optical aberrations are computed in the form of Zernike polynomials up to the 8th order.