Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

METHODS
Badal lens, corresponds to 0.2 D/mm (one dioptre each 5 millimeters). The 0 D
position in the focusing block scale was determined using a non aberrated emetropic
artificial eye. Trial lenses in front of the artificial eye were used to check the
compensation of defocus by the focusing block (Fig. 2. 29). First, we checked by
direct observation the change in the spot diagram dispersion when the focusing block
was displaced. The final check was performed with the defocus term of the aberration
pattern measured.
2.3.2.8. Optical aberrations introduced by the system
In order to discard that geometrical aberrations were being introduced by the system,
we measured the aberrations of the nominally aberration-free artificial eye. For 3 rd and
higher order aberrations we obtained that the RMS departure of the wavefront from
the reference sphere was much less than /14 (Marèchal’s criterion (Born and Wolf,
1993)). For 2 nd order aberrations (defocus and astigmatism), the residual values will be
subtracted to the measured values. Therefore, we can consider the system is
sufficiently corrected for the purposes of measuring human eyes.
Absence of chromatic aberration in LRT1 had been verified by measuring the
aberrations of a phase plate (Navarro et al., 2000) in front of an artificial eye, using
543 nm and 783 nm as test wavelengths. The difference in defocus obtained with both
wavelengths was 0.04D. Similarly, for LRT2 optical aberrations of an artificial eye
were measured using infrared wavelength (785nm), under the same conditions
(defocus correction, artificial eye position) we used for a wavelength of 532 nm. The
difference between the values of defocus for both wavelengths was 0.12 D.
2.3.2.9. Validation of the aberration measurements
Measurements of wave aberrations from the systems LRT2 were validated by: 1)
measuring cylindrical lenses of known power; 2) measuring phase plates of known
aberrations; 3) measurements in plastic artificial eyes of known aberrations (Campbell,
2005), 4) comparing measurements on real eyes also measured with Hartmann-Shack
aberrometers. More information can be found in Lourdes Llorente’s (Llorente, 2009)
and Elena G. de la Cera’s Thesis (García de la Cera, 2008).
2.3.3. Simulations of retinal images
Retinal images were simulated from the results of optical quality (objective)
measurements. Simulated retinal visual acuity charts were normally compared to the
(subjective) measurements of visual perfomance, described in Section 2.4.
Point Spread Functions (PSFs) were obtained from the Zernike polinomial
expansions. We performed the 2-dimensional Fourier Transform of the circular pupil
function containing the wave aberration function (Atchison and Smith, 2000). In
through focus simulations the Zernike defocus coefficient was changed accordingly.
The Stiles Crawford function was not considered. From the PSFs we also calculated
the Strehl ratio. The retinal images of visual acuity charts were simulated by twodimensional
image convolution.
2.4. MEASUREMENT OF VISUAL PERFORMANCE
The optical bench described in (Rosales and Marcos, 2006) was used and adapted to
perform psychophysical measurements of visual acuity (Fig. 2. 30). The stimuli were
presented on high resolution and high brightness minidisplay (LiteEye) through a
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