Optical waveguides – Optical fiber bundle – Fiber bundle plate
Reexamination Certificate
1999-04-29
2002-09-10
Font, Frank G. (Department: 2877)
Optical waveguides
Optical fiber bundle
Fiber bundle plate
C359S652000, C359S654000
Reexamination Certificate
active
06449414
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a color image reading technique, and particularly to a rod lens array as an equi-magnified imaging device constituting an image reading optical system and a unit therefor, and an image scanner (image reading device) using the rod lens array or the unit. Further, the present invention relates to an improvement to achieve high resolution by reducing chromatic aberration.
BACKGROUND TECHNIQUE
A rod lens array has been widely used as an equi-magnified imaging device constituting an image reading optical system for an original moving type image scanner (containing an image reader in a facsimile machine, a digital copying machine or the like) because it has advantages that the conjugate length thereof is short, the rod lens array itself is light and it can be manufactured at a low cost.
However, the conventional rod lens arrays have a disadvantage that the chromatic aberration thereof is generally large except for some of them in which the conjugate length is long and the numerical aperture thereof is relatively small (so-called dark). Therefore, these conventional rod lens arrays have been mainly used for monochromatic image reading.
However, color image reading has been recently required, and thus the rod lens arrays have been expected to be used for color image reading in order to positively utilize the above advantages of the rod lens arrays. In order to satisfy this requirement, the chromatic aberration of the rod lens arrays must be reduced.
The focusing parameter g of a square-distribution medium such as a rod lens is given according to the following equation (1):
g
2
=2
[n
o
−n
(
r
)]/(
n
o
r
2
) (1)
Here, r represents the distance from the optical axis of the rod lens in the radial direction, n
o
represents the refractive index on the optical axis of the rod lens, and n(r) represents the refractive index at the position which is at the distance r in the radial direction from the optical axis of the rod lens.
The conjugate length TC of an equi-magnified rod lens is given according to the following equation (2):
TC=Z
o
−[2/(
n
o
g
)]tan[(
Z
o
g
)/2] (2)
Here, Z
o
represents the length of the rod lens.
In the equation (1), since both of n
o
and n(r) has wavelength-dependence (wavelength dispersion), g also shows wavelength-dependence. Accordingly, TC calculated according to the equation (2) has wavelength-dependence. Therefore, the imaging position and the magnification are different between light beams which are different in wavelength (color). Accordingly, when color image reading is carried out by using a conventional rod lens array comprising a plurality of rod lenses arranged, the resolution is lowered due to chromatic aberration.
In order to reduce the chromatic aberration, the rod lens has been hitherto manufactured by using materials having low wavelength dispersion. For example, in the case of glass rod lenses formed by an ion exchange method, it has been proposed to reduce the chromatic aberration by suitably selecting the kind and concentration of ions (K. FUJII, MOC [Microoptics Conference]/GRIN'93 KAWASAKI: G13). Further, in the case of plastic rod lenses, the rod lenses are manufactured by using MMA (methyl methacrylate)-based polymer which has relatively small wavelength dispersion of plastics (see Japanese Patent Application Laid-open No. Hei-3-174105).
However, there has not yet been provided any rod lens for which both of reduction in conjugate length and reduction in chromatic aberration (enhancement in resolution) can be performed at the same time and which can be manufactured at low cost.
The resolution can be estimated by measuring MTF (Modulation Transfer Function), for example.
FIG. 31
shows a method of measuring MTF. A rod lens array
101
having a conjugate length TCs at a predetermined wavelength &lgr;s (the arrangement direction of the rod lenses is vertical to the sheet surface) is put on a measuring apparatus, and a standard rectangular grating
102
and a CCD image sensor array (the arrangement direction of photodetecting elements of the sensor is vertical to the sheet surface)
103
are fixed while the distance TCx between the standard rectangular grating
102
and the CCD image sensor
103
is adjusted to be equal to the conjugate length TCs. The spatial frequency of the standard rectangular grating
102
is set to 6[1 p/mm], for example. Monochromatic light obtained by passing light from a light source (not shown in the figure) through a spectroscope
104
is converted to diffused light by a diffusion plate
106
, and then irradiated to the rectangular grating
102
. An image of the rectangular grating
102
is focused onto the image sensor array
103
by the rod lens array
101
. MTF in each wavelength is measured by varying the wavelength of light emitted from the spectroscope
104
.
MTF of each wavelength is obtained by the following equation:
MTF
[%]=[(
I
MAX
−I
MIN
)/(
I
MAX
+I
MIN
)]×100
Here, I
MAX
and I
MIN
represent the maximum light amount and the minimum light amount measured by the image sensor array
103
, respectively.
The definition of MTF and the measuring method as described above are disclosed in Japanese Patent Application Laid-open No. Hei-3-174105.
In general, the refractive index distribution of the rod lens array
101
is not ideal, and thus MTF of an image of the rectangular grating
102
which is focused onto the image sensor array
103
by the rod lens array
101
is not equal to 100%.
FIGS. 32 and 33
show examples of MTF measured by the above method. In both of the examples, the predetermined wavelength &lgr;s is set to 570 nm, and TCx=TCs is set to 9.1 mm.
FIG. 32
shows a measurement result of a glass rod lens array (SLA
20
D produced by Nippon Sheet Glass Co., Ltd.), and
FIG. 33
shows a measurement result of a plastic rod lens array (RA89S produced by Mitsubishi Rayon Co., Ltd.). In all the cases, MTF has the maximum value at the predetermined wavelength &lgr;s, however, the MTF value is greatly varied depending on the wavelength and the chromatic aberration is large.
Further, not only a white-color light source used in combination with a color image sensor array, but also a three primary color light source including a blue light emission LED, a green light emission LED and a red light emission LED which are used in combination with a monochromatic image sensor array are used as a light source usable in the color image scanner. The light emission spectra of these LEDs are shown in FIG.
34
. The light emission peak of the blue light emission LED appears at about 450 nm, the light emission peak of the green light emission LED appears at about 525 nm and the light emission peak of the red light emission LED appears at about 660 nm (the blue light emission LED and the green light emission LED are described in “Applied Physics”, Vol 65. No. 7, 676(1996)). As described above, the wavelength range of the three primary color light source is smaller than the whole range of the visible range, however, there is a 210 nm wavelength interval between the peak wavelength of the blue light emission LED and the red light emission LED. The following table 1 shows MTF values (at 6 [1 p/mm]) of the conventional rod lens arrays having the characteristics shown in
FIGS. 32 and 33
at the peak wavelengths of the three primary color light sources.
TABLE 1
PEAK WAVELENGTH [nm]
ROD LENS ARRAY
450
525
660
SLA20D
13%
78%
53%
RA89S
27%
63%
59%
As is apparent from the table 1, with respect to the conventional rod lens arrays, even when three primary color light sources are used, the MTF values at the wavelength 450 nm are equal to or less than 50% and this indicates that it is insufficient to enhance the resolution of the image scanner.
Therefore, the present invention has been implemented in view of the foregoing situation, and has an object to sufficiently reduce the chromatic aberration of a rod lens array to
Ishimaru Teruta
Sumi Toshinori
Tahara Yasuteru
Uozu Yoshihiro
Font Frank G.
Mitsubishi Rayon Co. Ltd.
Mooney Michael P.
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