Biomedical Engineering Reference
In-Depth Information
Fig. 6
Purkinje images
of the cornea and the aqueous humour. It is bright and erect, though less bright than
the 1st PI often merging with it. The third image (3rd PI) arises on the interface of the
aqueous humour and the anterior surface of the lens. The difference in the refraction
indices is small (about
n
D
0.09), thus the intensity of 3rd PI is very low. Similar
to the others this image is bright and upright. Its size depends on the curvature of the
anterior surface of the lens, thus it can be used to detect the eye's accommodation.
The 4th PI, which is formed on the interface of the posterior surface of the lens
and the vitreous humour, is real and inverted. Several optometric methods were
elaborated on the basis of knowledge about the Purkinje images based on analysis
of their position or size in relation to the stationary point source of the measuring
light. The instant position of the eye is ascertained using the Hirschberg test [
19
].
On the recorded image, the centre of the pupil is determined and compared with
the position of the 1st PI, which is immobile with regards to the stationary light
source. The eyes' position can also be measured from the disparity of 1st PI and
4th PI [
17
,
18
]. An analysis of the position and size of 3rd and 4th PI can be used
to determine the radius of curvature of the anterior and posterior surface of the lens
[
17
,
18
]. The problem with this method, however, is that the automated detection of
3rd PI is difficult. The image arising on the interface with a small difference in the
refraction indices is blurred and it has a low intensity. An alternative to ascertaining
the eye's accommodation using 3rd PI can be the eccentric photorefraction method,
which has been used in these experiments.
The eccentric photorefraction method [
19
] is based on modifying and recording
light that passes through the eye. A special diaphragm is placed on the lens of
the recording equipment (camera, video-camera) (Fig.
7
), which is eccentric to the
measuring light source.
The measuring light passes through the eye's optic system and the part of it
reflected from the retina returns. The light direction and intensity change depending
on the dioptric power of the lens. The modified measuring light falls on the eccentric
diaphragm and part of it enters the lens of the recording equipment. Using this
