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Performance specifications
000000-1520-928_GA_EN_021013
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The wavefront aberration measurement is based on Hartmann-Shack sensor
technology. The mode of functioning is shown in Fig. 2 in a simplified form.
An infrared laser beam generates a light spot on the retina of the eye. The
light pencil reflected from this light spot traverses the optical system from
the point of the retina to exit the eye at the corneal plane, where it
generates the wavefront (reversed ophthalmological principle). Information
on all individual optical defects of the eye (lower and higher order
aberrations) is collected here. To guarantee correct measurement, the
patient fixates on the test mark (in the form of a hot air balloon). This
ensures that the light spot of the measuring laser accurately strikes the fovea
centralis, the point of sharpest vision.
The wavefront emerging from the pupil of the eye strikes the Hartmann-
Shack sensor, consisting of a regular micro-lens array and a video (CCD)
camera. Each micro-lens of the array creates a light spot on the camera chip,
forming a pattern of light spots (Fig. 3). The position of the light spots is
evaluated with the aid of software algorithms. An eye without imaging
errors (optically perfect) adjusted to long range generates an even wavefront
that is displayed on the camera in the form of a completely regular light spot
array. An eye with imaging errors (not optically perfect) generates an
uneven, distorted wavefront, resulting in tilting of the individual light spots
on the camera chip. The wavefront or refraction of the eye within the pupil
opening is calculated from the analysis of these tilts. The results of
measurement are shown in the form of a two-dimensional, color-coded
wavefront map whose diameter corresponds to the measured diameter of
the pupil and describes the optical properties of the eye. In addition, the
optical aberrations of the eye are mathematically characterized and
quantified by Zernike coefficients.
A
B
Fig. 3 Image of light spots on the camera chip (A) and image of the reconstructed
wavefront map (B) over the total exit pupil.