ABSTRACT
The first technique for measuring aberrations in the human eye was described by Tscherning (1894). In a psychophysical method, a grid superimposed on a 5-D spherical lens was projected on the retina and the subject could see a shadow image of the grid on the retina. The aberrations of the eye were inferred from the distortions of the grid reported by the subject. A modification of this technique (the crossed cylinder aberroscope) was proposed by Howland and Howland in 1968, in which the spherical lens was replaced by a crossed cylinder lens of 5 D power. An objective version (using light reflected off the retina) of the Tscherning aberroscope was proposed by Mierdel et al. (1997) and developed for clinical applications. Smirnov (1961) proposed another psychophysical technique (coined aberrometer for the first time) whereby a grid is viewed through the entire eye’s pupil except a single central point, which is viewed through a small aperture that scans the entire pupil sequentially. The spatially resolved refractometer (Webb et al. 1992, He et al. 1998, Burns and Marcos 2000,
Moreno-Barriuso et al. 2001a), based on a similar concept, uses a efficient pupil scanning system, and a rapid feedback from the subject, allowing measurement of the wave aberration in much shorter time scales, despite the psychophysical nature of the technique. Objective techniques, relying on light reflected off the retina, include the laser ray tracing (LRT) (an objective version of the Smirnov’s aberrometer) and the Shack-Hartmann wavefront sensor, initially developed by Shack, based on principles proposed by Hartmann in 1900, to improve the quality of satellite images, and first applied by Liang et al. (1994) to measure the aberrations of the eye in 1994.
