Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 2
Badal system (that changes vergence at the pupil, i.e. thru-focus position, without
affecting the magnification). This system, developed for Purkinje-image based
phakometric/lens tilt and decentration measurements, was used, instead of the LRT2
psychophysical channel to achieve higher target luminances (as the target is subject to
less losses by reflection and transmission through optical elements) and the
availability of dynamic target which allowed full control of the psyschophysical
experiment.
B
A
D
C
Fig. 2. 30. Optical bench used for the VA measurements, adapted from (Rosales and
Marcos, 2006). A) Eye. B) Badal System. C) SVGA Minidisplay. D) Camera for
pupil monitorizacion
We used a four alternative forced choice psychophysical paradigm to measure visual
acuity with high-contrast tumbling Snellen E letters. The thickness of the lines and
gaps of the E letter were one-fifth of their total size. The subject was asked to identify
the orientation of the letter that was displayed on the minidisplay (one out of the four
possible orientations), pressing right, left, up, or down on the keyboard. A QUEST
algorithm, implemented with Psychtoolbox (Brainard, 1997, Pelli, 1997) and Matlab,
based on previous responses of the subject at each trial, was used for letter-size
threshold estimation at each trial, and for suggesting the letter size for next trial. Each
letter was presented for 0.5 seconds. A point fixation stimulus was shown between
letters.
Fig. 2. 31. Detail of the typical evolution of the estimated threshold vs trial number.
The final threshold is represented by a red line. In our experiments we used 30 trials.
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