Imaging optical apparatus

Details Imaging optical apparatus Imaging optical apparatus

2001-01-08

2002-10-22

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

With graded refractive index

C385S034000, C385S119000, C385S124000

Reexamination Certificate

active

06469837

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an imaging optical apparatus that is typically used in an LED printer to allow the light from an LED array to form an image.
FIG. 8
shows the general layout of an LED printer which is generally indicated by
1
A and which has the following components arranged around a photosensitive drum
10
in the clockwise direction indicated by the arrow: a charging device
12
for uniform charging of the entire surface of a photoreceptor coat
11
on the surface of the photosensitive drum
10
; an LED array
13
for exposing the photoreceptor coat
11
to form an electrostatic latent image; a developing device
14
by which toner particles
14
a
are deposited on the electrostatic latent image to form a toner image; a transfer device
16
for transferring the toner image onto a recording sheet
15
; a fixing device
17
for fixing the transferred toner image on the recording sheet
15
; a cleaner
18
for removing the residual toner particles
14
a
on the photoreceptor coat
11
; and an erase lamp
19
for removing any residual charges on the photoreceptor drum
11
.
The LED array
13
consists of LED devices arranged in a two-dimensional pattern that extends along the photosensitive drum
11
in a direction parallel to the width of the recording sheet
15
. In accordance with the characters, figures and other imagery to be printed, the LED devices are selectively fired to emit light L
1
. A rod lens array
20
is provided intermediate between the LED array
13
and the photosensitive drum
10
; this consists of gradient-index rod lenses known as SELFOC lenses (the trade name of Nippon Sheet Glass Co., Ltd.) in the form of cylinders connected side to side. The rod lens array
20
condenses the light L
1
into light L
2
which forms an image on the photoreceptor coat
11
. The image formed by the rod lens array
20
is a correct life-size image so that the image resulting from the firing of the LED devices in the LED array
13
is straightforwardly formed as the latent image.
The LED printer
1
A has the problem of large overall size since not only the imaging optical apparatus consisting of the LED array
13
and the rod lens array
20
but also other image forming elements including the charging device
12
are all arranged outside the photosensitive drum
10
. In order to reduce the printer size, it has been proposed that the LED array
13
and the rod lens array
20
be placed within the photosensitive drum
10
to reduce the printer size as shown in FIG.
9
. In the resulting LED printer generally indicated
1
B, the photosensitive drum
10
comprises a cylinder
21
that is made of a transparent or light transmissible material such as glass and which has a photoreceptor coat
11
formed on the surface. The LED array
13
emits light L
1
which is condensed by the rod lens array
20
into light L
2
which in turn passes through the transparent or light transmitting cylinder
21
to form an image on the photoreceptor coat
11
. The image forming elements of the LED printer
1
B other than the LED array
13
and the rod lens array
20
, as exemplified by the charging device
12
, are identical to those shown in FIG.
8
and omitted from FIG.
9
.
In addition to its smaller size, the LED printer
1
B has the advantage of being free from the fouling of the LED array
13
and the rod lens array
20
due to the scattering of toner particles
14
a
(see FIG.
8
).
Nevertheless, the LED printer
1
B shown in
FIG. 9
has one serious problem: the thicker portion of the transparent or light transmitting cylinder
21
has a circular cross section, so when the light L
1
condensed by the rod lens array
20
passes through this cylinder, it is refracted by the cylinder as if it were a concave lens. As a result, the light L
2
does not form a precisely focused image and the resolution of the latent image is reduced to produce prints having only deteriorated image quality. For further details, see below.
The resolving power of a rod lens array is evaluated in terms of MTF (modulation transfer function) defined by the following equation:
MTF (%)={(i
max
−i
min)
)/(i
max
+i
min
)}×100
where i
max
and i
min
are a maximum and a minimum, respectively, of the light intensity on the image plane for the case where bands of light are launched into the rod lens array. The MTF as defined above is usually determined for two directions, one for the length of the rod lens array and called transverse resolution MTFx and the other for its thickness and called longitudinal resolution MTFy. The resolving power of the rod lens array is evaluated in terms of the two MTF values.
The present inventors designed a gradient-index rod lens array having the following specifications (see FIG.
10
): n
0
=1.627; g=0.5348; h
4
=0.75; h
6
=−1.209; h
8
=1.451 (n
0
is the refractive index at the center of each lens, and g, h
4
, h
6
and h
8
are index gradient coefficients); angular aperture (&agr;)=20°; lens length (T)=6.89 mm; and conjugate length (C)=15.1 mm. A plurality of such lenses were combined in an array and the measurement of their MTFx and MTFy was simulated with end points A and B being assumed to present a light source and an image forming area, respectively, and the results are shown in FIG.
11
. Obviously, MTFx and MTFy assume maxima at an optimum focal position (focus=0), demonstrating the high resolving power of the rod lens array.
The present inventors then modified this optical system as shown in
FIG. 12
by providing a cylindrical lens
40
in the image forming area B, with its longitudinal direction being in alignment with that of the rod lens array
20
and with its concave side facing the latter. The cylindrical lens
40
was assumed to have a refractive index comparable to that of BK7 glass and adapted to have a radius of curvature (R) of 15 mm on the outer circumference, a radius of curvature (r) of 13 mm on the inner circumference, and a thickness (t) of 2 mm. The MTFx and MTFy of this cylindrical lens were also measured by simulation and the results are shown in FIG.
13
. Compared to the case where the cylindrical lens
40
was absent (FIG.
11
), MTFx presents a similar profile but a maximum of MTFy is way off the optimum focal position, creating a difference greater than 100 &mgr;m between the optimum focal positions of MTFx and MTFy.
Most probably, this happened because the light issued from the rod lens array
20
refracted when passing through the cylindrical lens
40
and focused in an untoward point. Since the cylindrical lens
40
corresponds to the transparent or light transmitting cylinder
21
in the LED printer
1
B shown in
FIG. 9
, a similar difference between MTFx and MTFy occurs in an actual LED printer having an imaging optical apparatus within a photosensitive drum and the latent image formed on the photoreceptor coat
11
decreases to cause eventual deterioration in the quality of printed characters and figures.
The image deterioration problem of defocusing is not limited to the LED printer but can occur in all situations where an imaging optical apparatus using a rod lens array has various transparent or light transmitting optical elements inserted into the optical path between the rod lens array and the image forming area.
SUMMARY OF THE INVENTION
The present invention is based on the review of the problems described above and its objective is to suppress the problem of reduced image resolution often encountered in imaging optical apparatus having a rod lens array with transparent or light transmitting optical elements inserted into the optical path between the rod lens array and the image forming area, as exemplified by a compact LED printer in which an imaging optical apparatus comprising an LED array and a rod lens array is placed within a photosensitive drum.
The stated object of the invention can be attained by an imaging optical apparatus comprising a gradient-index rod lens array and two transparent or li

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