January 10, 2010 / Vol. 8, No. 1 / CHINESE OPTICS LETTERS 103
Development of an optical probe to measure the flattened
area of ocular cornea
Xueyong Zhang (ÜÜÜÆÆÆ]]])
1,2∗
, Jianguo Ma (êêêïïïIII)
3
, Rongsheng Lu (©©©JJJ)
1
,
Yibao Li (oooÂÂÂ)
2
, Zhen Tang (/// )
2
, and Guohong Liu (444JJJùùù)
2
1
Scho ol of Instrument Science and Opto-Electronic Engineering,
Hefei University of Technology, Hefei 230009, China
2
Department of Mathematics and Physics, Anhui University of Architecture, Hefei 230601, China
3
Department of Physics, Huainan Normal University, Huainan 232001, China
∗
E-mail: xyzhang@aiai.edu.cn
Received March 23, 2009
The optical probe in tonometers is a key component in measuring the flattened diameter or area of the
o cular cornea. A new kind of optical probe for the direct measurement of the flattened area of the ocular
cornea is presented. The optical probe uses the cone prism with a modulating flake of light intensity
as its measuring body. The test results on simulated eyeballs with different radii of curvature of the
o cular cornea show that there is a linear relation between the flattened area of the ocular cornea and the
normalized current. The optical probe, which is more compact compared with existing optical probes and
easily configured with its coaxiality of optical parts, may be an excellent probe for constructing a low-cost,
miniaturized applanation tonometer.
OCIS co des: 220.0220, 080.0080, 230.0230, 120.0120.
doi: 10.3788/COL20100801.0103.
Glaucoma is a group of eye diseases characterized by
an elevated intraocular pressure (IOP) that damages
and destroys the axons of the optic nerve, leading to
complete blindness in the absence of continuous medical
treatment
[1]
. For treating glaucoma, ophthalmologists
always attempt to normalize the IOP, thereby reducing
the danger of progressive visual field loss. The mea-
surement of IOP with so-called tonometers therefore
plays an important role in the diagnosis and manage-
ment of glaucoma. During the past decades, the cor-
responding inspection technology of IOP measurement
has received a great deal of attention. Thus far, some
available technologies for the measurement of IOP can
be categorized in principle into the following three cate-
gories: indention tonometry, applanation tonometry, and
non-contact tonometry (pneumatonometry)
[2]
. Nowa-
days, many commercial tonometers, such as Schi¨otz in-
dentation tonometer, Goldmann applanation tonometer
(GAT), non-contact tonometer (NCT), and the most re-
cent dynamic contour tonometer (DCT)
[3−5]
, can achieve
effective IOP measurements.
The success of IOP measuring technique is based on
the development of an optical probe, which allows some
tonometers to measure the flattened diameter or area of
the ocular cornea. GAT is regarded as the most clinically
accurate tonometer, and its optical probe embodies an
applanation prism consisting of two semi-cone prisms
[3]
.
A skilled operator can determine the flattened diameter
of the ocular cornea (3.06 mm) by means of a slit lamp
microscope upon seeing the upper and lower semi-circles
on the flattened stamp, separated and tangential to each
other horizontally. Perkins presented a hand-held ap-
planation tonometer that does not require a slit lamp
microscope, but employs the same principle as that of
GAT
[6]
. In this portable tonometer, the optical probe
uses a prism similar to GAT as the measuring body;
however, the illumination of the prism obtained from
four miniature bulbs is complicated in structure
[6]
. In
general, the above-mentioned tonometers only serve to
measure the flattened diameter of the ocular cornea.
These techniques have a few difficulties in terms of ob-
taining the measurement of the flattened diameter of the
ocular cornea because the shape of an astigmatic cornea
is similar to a tire; consequently, one obtains an elliptic
rather than a circular flattened stamp
[7]
. A right angle
prism acting as the measuring body can directly measure
the flattened area of the ocular cornea, but the appla-
nation surface is square
[8]
. So we develop a new kind
of optical probe for direct measurement of the flattened
area of the ocular cornea.
Using the total internal reflection principle, the cone
prism with a taper is designed at a great scale, as shown
in Fig. 1. R
1
and R
2
are the radii of the bottom and
top surfaces of the cone prism, respectively, and R
0
=
(R
1
+R
2
)/2. E
0
is the intensity of incident light. θ
1
and
θ
2
are incidence angles of light rays at the inclined flank
and the bottom surface of the cone prism, respectively.
A beam of parallel rays enters the cone prism and is
totally reflected at the inclined flank and delivered to
the bottom surface of the cone prism. On this surface,
it is again totally reflected and arrives at the opposite
inclined flank on the top of the cone prism. It should be
mentioned that the bottom surface of the cone prism is
also called the applanation surface because it is in con-
tact with the ocular cornea during IOP measurement.
In view of the above, the taper of the cone prism is
designed. As an estimate, we suppose that the refrac-
tive indices of air, glass, and cornea are 1, 1.5, and
1.376, respectively. Using the total reflection critical
angle formula, we know that θ
1
should be greater than
1671-7694/2010/010103-04
c
° 2010 Chinese Optics Letters