EN 20_070_0009I v1.5
Performance
specification
15
Measurement of wavefront aberrations is based on the technology of a
Shack-Hartmann sensor. Its function is illustrated in a simplified manner in
Fig. 2. An infrared laser beam creates a point of light on the retina of the
eye. The light beam reflected from this point of light passes through the
optical system from the retina until it exits from the eye in the corneal plane,
creating the wavefront there (reverse ophthalmological principle).
Information on the individual optical errors in the eye (lower and higher
order aberrations) is gathered during this process. To ensure correct
measurement, the customer fixes their gaze on a test mark (offered in the
form of a hot air balloon). This ensures the point of light generated by the
measurement laser precisely strikes the fovea centralis, the center point
where eyesight is sharpest.
The wavefront exiting the pupil intersects with the Shack-Hartmann sensor,
which consists of a regular microlens array and video camera (CCD camera).
Each microlens in the array generates a point of light on the chip of the
camera, creating a pattern of light points (Fig. 3). The positions of the points
of light are evaluated using software algorithms. An eye focusing on a
distant point without aberrations (optically perfect) creates an even
wavefront, which is represented as a completely regular grid of light points
on the camera. An eye with aberrations (not optically perfect) creates an
uneven, distorted wavefront, which causes the individual points of light on
the chip of the camera to be shifted. Analyzing these shifts makes it possible
to calculate the wavefront or refraction of the eye within the opening of the
pupil. The results of the measurement are illustrated in the form of a two-
dimensional, color coded wavefront map with the same diameter as the
measured pupil diameter and describing the optical characteristics of the
eye. In addition, the optical aberrations of the eye are characterized and
quantified mathematically by Zernike coefficients.
Fig. 3:
Image of the points of light on the chip of the camera (A) and image of the
reconstructed wavefront map (B) over the entire exit pupil
A
B
