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Ánh sáng nổi lên từ mắt là tập trung vào một máy ảnh CCD mỗi lenslet để tạo thành một mô hình điểm. Các mô hình tại chỗ của một đối tượng lý tưởng với một đầu sóng hoàn hảo sẽ được chính xác cùng một khuôn mẫu như lưới tham khảo. | 142 Maeda Figure 9.5 Spot patterns in normal subject. ray of lenslets that consist of a matrix of small lenses 2 6 . The light emerging from the eye is focused on a CCD camera by each lenslet to form a spot pattern. The spot pattern of an ideal subject with a perfect wavefront will be exactly the same pattern as the reference grid. The spot pattern of a subject with a distorted wavefront will create an irregular spot pattern. Displacements of lenslet images from their reference position are used to calculate the shape of the wavefront. Figures 9.5 and 9.6 show examples of spot patterns from a normal and a keratoconic subject with the Topcon Hartmann-Shack sensor. Although the spot pattern in the normal subject is regular the spot pattern in the patient with keratoconus is markedly distorted. As the wavefront of each lenslet is perpendicular to the direction of the ray i.e. displacement Figure 9.6 Spot patterns in keratoconus. Copyright Marcel Dekker Inc. All rights reserved. Marcel Dekker Inc. 270 Madison Avenue New York New York 10016 Wavefront Technology and LASIK Applications 143 Y Z_ R r X n-2m 6 X sin when n-2m 0 cos when n-2m 0 to 1 s m-s n-m-s r Figure 9.7 Equation of Zernike polynomials. of their focusing spots the wavefront of the measured subjects can be reconstructed from these spot patterns. Wavefront aberrations the quantitative measure of wavefront distortions are usually calculated using Zernike polynomials. The wavefront is expanded into sets of Zernike polynomials to extract the characteristic components of the wavefront. The Zernike polynomials are the combination of trigonometric functions and radial functions and the terms of the Zernike polynomials represented as Z Fig. 9.7 are useful to show the wavefront aberrations because of their orthogonality 2 . Examples of Zernike polynomials up to the fourth order are shown in Fig. 9.8. The zero order has one term that represents a constant. The first order represents tilt two terms one for the X axis
