The double-pass technique has been used extensively in physiological optics since first introduced by Flamant in 1955. ^{1} It is based on recording images of a point source projected on the retina after retinal reflection and double-pass through the ocular media. ^{2,3} From these double-pass images, the ocular modulation transfer function is calculated. The modulation transfer function yields the relationship between the contrast of an object and its associated image as a function of spatial frequency. However, in different applications, it is also necessary to have access to the retinal image of the point source, the point-spread function, and the wavefront aberration of the eye. An important advance in the technique was the understanding of the image formation process. When using a symmetric double-pass setup, with equivalent first and second passes, the double-pass image is the autocorrelation of the retinal image.^{4} This means that even in the case where the retinal image has an asymmetric shape (due to odd aberrations, such as coma), the double-pass image is always symmetric. We proposed a simple modification of the technique consisting of asymmetrizing the two passes by using a small aperture in one of them. ^{5 }In this way, the double-pass image keeps the asymmetries present in the retinal image, and the ocular point-spread function (actual retinal image) can be obtained. ^{6} In addition, as the wave aberration is related directly to the point-spread function by an integral equation, it can be computed by phase retrieval techniques.^{7} This is a useful procedure to estimate ocular

# Understanding aberrations using double-pass techniques

### Autors:

P. Artal

### Journal:

J. Refract. Surg.

### Volume:

16

### Pages:

560-562

### Year:

2000

### ISBN:

1081-597X