July 10, 2010 / Vol. 8, No. 7 / CHINESE OPTICS LETTERS 689
Optical configuration of fundus camera based on
inner focusing manner
Haishui Ye (°°°YYY)
∗
§§§Zhishan Gao (pppììì)§§§Ting Luo (ÛÛÛ xxx)§§§and Yang Huang ( )
School of Electronic Engineering and Optoelectronic Techniques,
Nanjing University of Science and Technology, Nanjing 210094, China
∗
E-mail: yhs.1986@163.com
Received December 28, 2009
The fundus camera used in imaging retina includes two parts: fundus imaging path with a positive focusing
lens and annular Kohler illuminating path. We present a pupil joint equation and a focus equation based
on the internal focusing manner to analyze its configuration. Using these equations, the configuration
of the camera is given in detail. The design result of the fundus imaging path shows compact initial
configuration at a total length of about 123 mm. The focusing range is –10−10 diopter, and the movement
of the focusing lens is nearly linear at displacement of more than a sub-millimeter per diopter.
OCIS co des: 170.0110, 170.4460, 170.1610, 120.4570.
doi: 10.3788/COL20100807.0689.
The fundus camera is an optical system that can be used
in imaging fundus tissues. It plays an important role in
contemporary ophthalmic diagnosis, such as macular de-
generation. Considering the complexity and speciality of
the structure of the human eye, the fundus camera con-
sists of b oth imaging and illuminating paths. The former
receives ray scattered by fundus tissues and with a spe-
cial path to realize fundus imaging; the latter provides
enough illumination to the imaging path to offset low
luminance resulting from the low scattering
[1]
of fundus
tissues.
The cornea located at the outer part of the human eye
has a high reflectance. It greatly affects fundus tissue
imaging. Therefore, controlling its reflectance is impor-
tant in fundus camera design. According to Ref. [2], a
special annular Kohler illumination manner has been ap-
plied in the fundus camera. The irradiance into fundus
tissue should be controlled below the maximum permissi-
ble safety exposure (MPE) during fundus imaging. Uni-
form illumination
[3]
is required in the illumination path
design to ensure comfort when illuminated by light at
near-infrared wavelength.
Two requirements should be considered during the pro-
cess of imaging path design: it should be pupil-jointed
(i.e., the position of the pupil entrance of the camera is lo-
cated at the eye’s pupil), and it should allow either near-
sight or far-sight focus. Traditional designs consisted of
moving the eyepiece, objective lens or charge-coupled de-
vice (CCD)
[4]
, and the entire imaging group
[5]
. The mov-
ing eyepiece brings various positions onto the conjugate
image, thus limiting Kohler illumination. Additional ad-
justment systems should be introduced into the illumi-
nation path because of the moving black dot
[2,3]
, which
could then remove the ghost image in the eyepiece. The
manner of moving the objective lens or CCD could avoid
the disadvantages mentioned earlier, but it also results in
shifting conjugated positions and swayed images. Moving
the entire lens group shares the speciality of the simple
structure, but it also fails to acquire a stationary image.
A fundus camera using the inner focus method is pro-
posed in this letter, which could guarantee stationary
images during focusing. We will discuss how the illumi-
nation and imaging path should be chosen for the fundus
camera design. The corresponding configuration is also
calculated.
The imaging path consists of eyepiece L
2
, focusing lens
L
3
, and objective lens L
4
, as shown in Fig. 1. The eye-
piece connects the human eye with the imaging system.
The focusing lens is located between the eyepiece and the
imaging lens. Its movement allows the fundus to be re-
focused when illuminated by the beam of near-infrared
wavelength, and reduces the incident height of objective
lenses, such as field lenses. The objective lens located
at the end part of the fundus camera connects with the
CCD and is composed of the image receiver of the whole
fundus camera.
The illuminating path consists of a collecting lens L
6
,
condenser lens L
5
, and eyepiece L
2
. It shares the eye-
piece with the imaging path, as shown in Fig. 2. The an-
nular Kohler illumination method could effectively avoid
stray light from the reflectance of the cornea and improve
signal-to-noise ratio (SNR) due to its higher reflectance
at the center as opposed to the edge of the cornea.
The central part of the mirror with a central hole per-
mits the imaging light to pass through the camera. The
edge of the reflecting mirror allows the illuminating light
to pass through and enter the pupil from the edge of the
cornea, thus successfully realizing the separation of the
illuminating and imaging paths.
Fundus tissues and CCD form a conjugate imaging pair
in the fundus camera, as shown in Fig. 1. The imaging
system, consisting of intraocular optical system, eyepiece
L
2
, focusing lens L
3
, and objective lens L
4
, satisfies
1
(d
34
− c
4
)
−
1
(f
0
2
− d
23
)
=
1
f
0
3
, (1)
c
4
= (1 − β
4
)f
0
4
, (2)
where d
34
denotes the distance between the objective lens
1671-7694/2010/070689-04
c
° 2010 Chinese Optics Letters