Biomedical Engineering Reference
In-Depth Information
a
b
120
[%]
100
.46
12
Convergence
.53
8
80
60
relative accommodation
relative convergence
.60
Third Purkinje image size
4
.67
40
20
0
.73
0
.5
1.0
Time (sec)
1.5
0
0
100 200 300 400 500 600 700
800
800 900
time [ms]
Fig. 5
Allen's results of measuring accommodation and vergence (
a
), expected course of the
accommodation and vergence (
b
)
accommodation and vergence was M. J. Allen in 1949 [
15
]. His results shows
Fig.
5
a. On the Fig.
5
b there is a theoretically setup curve of accommodation and
vergence. First latency period is the time between the retinal stimulus and the start
of the reaction (accommodation, vergence). Second pre-programmed period is the
Open-loop initiative phase of accommodation or vergence. It is similar to saccadic
movements. The third is the Close-loop phase and it is tune of the reaction conducted
by the nervous feedback.
1.2.1
E.M.AN. - Eye Movement ANalyser (2D stimulation)
E.M.AN. is a videometric device for analysing eye movement and accommodation
it is intended for measuring accommodation vergence synkinesis in small children
[
25
]. The characteristics of the oculomotoric system are ascertained during step
changes in the distance of a fixation images. Instant accommodation and ver-
gence values are obtained by measurements using videometric apparatus and the
subsequent offline software analyses of the image sequences. To detect the eye's
horizontal position the Hirschberg test is used in conjunction with Purkinje images;
the eccentric photorefraction method is used to obtain information about the dioptric
power of the lens. Instant values of both parameters are contained in every measured
image, thus it is possible to subsequently reconstruct the temporal changes of these
parameters during measurement.
1.2.2
Principles and Methods
Purkinje images arise when light falling on the interface of the eye tissues is
reflected. Purkinje described four types of image [
16
](Fig.
6
). The first of these
(1st PI) arises on the interface between the air and the outer surface of the cornea
and it is bright and erect. Thanks to the great difference in refraction indices, it has
the greatest intensity. The second PI is formed by the reflection on the inner surface
