Apparatus for correcting unevenness of light quantity in an optical reproduction system

A picture reproduction apparatus for recording reproduced picture images of an original picture in which picture information on the original picture is projected onto a photoelectric converting element(s) such as a CCD line sensor(s) etc. through a focusing optical system, and according to an output signal therefrom unevenness of a light quantity of a light flux projected to the photoelectric converting element(s) is corrected to obtain a reproduction picture of uniform quality.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical system in a reproduction 
apparatus for recording a reproduced picture image of an original picture 
by projecting picture information on the original picture onto a 
photoelectrical converting elements(s) such as CCD line sensor etc. and 
basing on its (their) output signal(s) through a focusing optical system, 
and particularly relates to an apparatus for obtaining a reproduction 
picture image(s) of uniform picture quality by correcting unevenness in 
light quantity of a light quantity projected onto the photoelectrical 
converting element(s). 
PRIOR ART 
In Japanese Patent Laid-Open Publication No. 57-87277 there is disclosed an 
invention titled "A picture reading apparatus". Japanese patent laid-Open 
Publication No. 57-87277 corresponds to Japanese patent application No. 
55-162980, which is one of three priority applications upon which U.S. 
Pat. No. 4,415,934 (entitled Image Reading Apparatus to M. Konishi issued 
Nov. 15, 1983) relies for its priority claim. The invention relates to a 
reproduction optical system for focusing each of light fluxes which pass 
through a lens and divided into two by a dome shaped reflection mirror 
disposed between the lens and its focusing plane by projecting each of the 
light fluxes onto individually independent line sensors in an apparatus 
for projecting and focusing picture information on an original picture 
onto the CCD line sensors disposed on the projecting plane through the 
lens. 
The apparatus disclosed in the afore-mentioned publication is considered to 
be adapted that, when picture information on relatively small original 
pictures such as a micro film or the like is read out, size of each of 
line sensors is not required to be excessively large, and that adapted to 
obtain high resolving power. 
The apparatus disclosed in the above described publication is itself 
considerably useful, however, when the apparatus is actually applied to a 
picture scanning reproduction apparatus, for example, such as a facsimile 
or the like, there is found a problem. The problem is as follows, that is, 
in the apparatus between a lens and its projecting image focusing plane a 
dome shaped mirror is disposed, so that according to a portion of the 
focusing plane, certain parts of a light flux which transmitted effective 
diameter (effective part) of the focusing lens are not reflected, that is, 
only some parts of the light flux can be utilized, which results in 
generating partial unevenness in light quantity. In consequence there 
occurs a problem that even in a recorded reproduction picture(s) 
unevenness in density is also reproduced. Such unevenness in density is an 
essential fault in reproduction of the photographic original picture of 
proper gradation required for faithfully reproducing gradation of the 
original picture. Solution of the afore-mentioned problem is highly 
desired. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to obtain approximately uniform 
light quantity distribution over the whole projected light focusing plane 
in the afore-mentioned picture scanning reproduction apparatus by 
providing an iris plate having an opening of a size corresponding to the 
light quantity distribution on the focusing plane or an optical filter 
having density distribution corresponding to the above described light 
quantity distribution with which cross section of a light passage in which 
the light quantity of the light flux passes through is lowered. 
According to the present invention, by limiting appropriately unevenness of 
projecting light flux which causes unevenness of light quantity in the 
focusing plane with an iris or an optical filter, almost uniform light 
quantity distribution can be obtained in the projecting light focusing 
plane, and a reproduction picture of high quality having no unevenness can 
be recorded.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 is a schematic view showing a composition of an embodiment of the 
present invention. 
An original picture 1 to be reproduced is emitted linearly by an emitting 
means (not shown), and is fed in the direction vertical to the surface of 
this sheet. A focused image of the emitted portion is projected by a lens 
2 to a pair of CCD (charge coupled device) line sensors 4 and 5 disposed 
at a focusing plane through a dome shaped or triangular roof-shaped mirror 
3. 
In this case areas of the original picture upper than a boarder line basing 
on an (A) point positioned on an optical axis of the lens 2 are projected 
onto the lower sensor 4, and areas of the original picture lower than the 
point (A) are projected onto the upper line sensor 5. The apparatus of the 
afore-mentioned construction has been already disclosed in the prior 
application, Japanese Patent Laid-Open Publication No. 57-87277. 
However, merely basing on the above described apparatus, if a picture 
signal is output from the two sensors 4 and 5, since there exists partial 
unevenness in light quantity in a light flux projected to the line sensor, 
consequently unevenness in density is generated in a reprojection picture. 
The reason of the afore-mentioned will be able to understand by studying 
geometrically optical passages in the optical system of the apparatus 
shown in FIG. 1. That is, in FIG. 1, considering each of light passages 
passing through which respective images is focused on respective points 
(A), (B), (C) and (D) (the point (A) is positioned on the optical axis) on 
the area of the original picture which is projected and focused on the 
sensor 4 at the lower part, at first, in the image of the point (A) on the 
optical axis only a light flux passing through the lower half part of an 
effective diameter 2' of the lens 2 as shown by solid lines are reflected 
at the lower half part of the dome shaped mirror 3, and converge on a 
point (a) on the line sensor 4. Accordingly, light quantity having passed 
through the effective diameter 2' of the lens 2, a half of the whole light 
quantity, is projected on the point (a). 
Next, in the image of the point (B) somewhat remote from the optical axis 
light axis, fluxes which comprises the flux having passed through the 
lower half part of the effective diameter 2' of the lens 2 and a part of a 
flux having passed through the upper half part of the lens are reflected, 
as shown in dotted lines in FIG. 1, at the lower part of the dome shaped 
mirror 3, and converged to the point (b) on the line sensor 4. 
Accordingly, on the point (b), light quantity smaller than that of having 
passed through the whole effective diameter 2' of the lens 2 but larger 
than that of projected on the point (a) is projected. In addition, in an 
image of the point (c) on the original picture 1 remote from the optical 
axis, as shown by dot and chain lines in FIG. 1, fluxes comprising the 
flux passing through the lower half part of the effective diameter 2' of 
the lens 2 and a flux passing through an area of the upper half which is a 
little larger than that of the case of the point (B) are reflected on the 
lower half part of the dome shaped mirror 3, and converged to a (c) point. 
Accordingly, to the point (c), light quantity smaller than the whole light 
quantity which passes through the effective diameter 2', but larger than 
that of projected onto the point (b) is projected. In an image of the 
point (D) on the original picture 1 which is farther from the optical 
axis, as shown by two dots and chain lines in FIG. 1, a light flux having 
passed through the whole effective diameter 2' of the lens is reflected at 
the lower half part of the dome shaped mirror 3, and converged to a point 
(d) on the line 4. As can be well understood, onto the point (d) the 
largest light quantity is projected, however, at the peripheral portion of 
the picture since there exists falling phenomenon is absolute light 
quantity, practically somewhat smaller light quantity than that of at the 
point (c) is projected. 
The above descriptions have been developed regarding fluxes which are 
reflected on the lower half part of the dome shaped mirror 3 and projected 
on the lower line sensor 4 to be focused. However, quite the same 
descriptions can be applied to the case regarding the upper half part. It 
is needless to say that they are shown as a view which is symmetrical in 
the upper half and the lower half with respect to the optical axis. 
In FIG. 2 there is shown a graph representing distribution of light 
quantity projected on each of positions of the sensor 4. From the graph it 
can be seen that at the point (a) light quantity is small, but it 
increases as position separates from the optical axis, and at the vicinity 
of point (c) it attains the maximum value, then it gradually decreases 
towards the point (d). 
In FIG. 3, each of shapes of sections of the respective fluxes at the 
surface of the dome shaped mirror 3 projected to each of the points (a), 
(b), (c) and (d) is diagrammatically shown. Shapes of these sections 
indicate that because of some parts of them being projected on the other 
reflection surface side of the dome shaped mirror 3, partially broken 
circular shaped light fluxes which should have been essentially complete 
circular shaped sections are generated. 
From afore-mentioned geometrical are view it will be well understood that 
in the optical system shown in FIG. 1, in the images of the original 
picture projected on the line sensors 4 and 5, it is consequently 
impossible to avoid occurrence of unevenness in light quantity. As in the 
present invention it is aimed to correct the afore-mentioned unevenness in 
light quantity and record a reproduction picture(s) of good quality which 
is free from unevenness in density, there is disposed an iris plate of 
desired shape or an optical filter having desired density distribution at 
an appropriate position in the course of a light passage from the surface 
of the original picture to the focusing plane, and light quantity of the 
light flux projected at the vicinity of the above mentioned point (c) or 
(b) is limited so that approximately uniform distribution of light 
quantity may be obtained. 
The iris plate for correcting unevenness of light quantity in the apparatus 
of the present invention can be disposed, if roughly classified, at three 
positions. That is, those three positions are, as shown in FIG. 1, a 
position between the original picture 1 and the lens 2 at which an iris 
plate 6 is provided, a position between the lens 2 and the dome shaped 
mirror 3 at which an iris plate 7 is disposed, or a position between the 
dome shaped mirror 3 and the line sensor 4 at which an iris plate 8 is 
disposed. Each of shapes of the respective sections or sectional areas of 
the fluxes passing through each of those positions is different with one 
another, so that shapes of those iris plates to be used should be selected 
basing on the positions at which they are disposed. 
FIG. 4 shows a shape of an opening of the iris plate 6 or 7. In the both 
iris plates 6 and 7, shapes of sections of the fluxes are symmetrical with 
respect to the optical axis, so that the shapes of openings of the iris 
plates 6 and 7 are, of course, symmetrical. Accordingly, at the part of 
the point (a) or (d) through which a light flux of small light quantity 
projected thereto, distance between opposite sides of the opening is made 
larger, and while at the part of the point (b) or (c) through which a 
light flux of large light quantity projected onto the point, distance 
between the opposite sides of the opening is made smaller. 
In addition, as can be clearly understood from FIG. 1, width of the whole 
light flux (in the upper and down direction in FIG. 1) which passes 
through the position at which the iris plate 6 is disposed and that of the 
whole light flux which passes through another position at which the iris 
plate 7 is disposed are different from each other, accordingly, it is 
needless to say that the iris plate 7 is made smaller than the iris plate 
6 so that the whole size of the opening may be coincided with the width of 
the light flux. Therefore, these openings become similar figures of 
different sizes. 
Further, the iris plate 8 disposed between the dome shaped mirror 3 and the 
line sensor 4 or 5 has an asymmetrical sectional shape resulted from the 
light flux passing through this part having been already divided into 
several light fluxes by the dome shaped mirror 3. It is desirable to have 
a sectional shape as shown in FIG. 5, so that the sectional shape may have 
an opening size which is inversely proportion to light quantity by 
corresponding to the graph shown in FIG. 2 which represents distribution 
of light quantity. 
As described the above, though the iris plate may be disposed at any of the 
three positions, theoretically it is possible to achieve the object of the 
present invention, that is, it is possible to obtain evenness in 
distribution of light quantity projected to the line sensor 4 or 5. 
However, in the case of applying the present invention to a practical 
picture scanning recording apparatus, it is advantageous to dispose the 
iris plate between the dome shaped mirror 3 and the line sensor 4 or 5. 
The reason consists in that, in order to form an opening shape 
corresponding to the distribution of the light quantity on the line sensor 
finally, the iris plate 8 which is disposed at the nearest position of the 
line sensor is advisable. 
Heretofore, detailed descriptions have been developed with respect to the 
embodiment providing the iris plate 6, 7 or 8 having an opening of desired 
shape. However, as afore-mentioned, an optical filter having desired 
density distribution can be applied instead of the iris plate. 
Such optical filter as mentioned the above is formed as follows, that is, a 
light shielding pattern having density distribution corresponding to the 
shape of the opening of the iris plate shown in FIGS. 4 or 5 is formed on 
a transparent base. The light shielding pattern is composed of a pattern 
which represents high density at positions where distance between each of 
their opposite sides is smaller, but represents low density at positions 
where distance between each of their opposite sides is large. By disposing 
this optical filter in the light passage instead of the iris plate 6, 7 or 
8, the same light flux limiting effect as each of those cases in which the 
iris plate 6, 7 or 8 can be generated. 
The most practical means for manufacturing the above described optical 
filter is to apply a photographical means which utilizes a photographical 
film(s). To realize the above practical means, it is advantageous to 
dispose the optical filter at the front surface shown by a dotted line 9 
in FIG. 1 of the sensor 4 or the 5 (however, the case of the 5 is not 
shown), and hereinafter the means will be described. 
In FIG. 1, instead of the original picture 1 an object of uniform density 
distribution, for example, a sheet of white paper, is set on, an unexposed 
photographic film is disposed at the position 9 opposite to the dome 
shaped mirror 3 at the front plane of the line sensors 4 and 5, and on the 
film an image of the object is exposed through the lens 2 and the dome 
shaped mirror 3. When the exposed film is developed, areas at which 
received light quantity was large turn out to be high density areas, and 
areas at which received light quantity was small are found to be low 
density areas. That is, density distribution thereof is just proportional 
to the distribution condition of the projected light quantity. 
By setting on an exposed and developed film as an optical filter on the 
position 9 which is same position that it was exposed, the original 
picture 1 is projected onto the line sensor 4 or 5 to focus an image of 
the original 1 thereon through the lens 2 and the dome shaped mirror 3. 
Since distribution of the light quantity corresponds to the density 
distribution of the optical filter, unevenness in both of them is offset, 
and light fluxes passing through the optical filter incident upon the line 
sensor with approximately uniform distribution in light quantity. Thus, as 
same as the case of the iris plate being applied, a picture signal(s) of 
excellent quality can be output. However, the above mentioned means for 
manufacturing the optical filter is limited to the only case in which the 
optical filter made of developed photographic film is disposed at the 
position 9, i.e., the position between the dome shaped mirror 3 and the 
line sensor 4 or 5, and an optical filter disposed at the pre-stage of the 
dome shaped mirror 3 can not be manufactured by the afore-mentioned means. 
But by disposing the optical filter at any one of the afore-mentioned 
three positions, the object of the present invention can be achieved, and 
that even the optical filter can be efficiently applied in practical use 
as well as the iris plate, if it is disposed between the dome shaped 
mirror and the line sensor, since it exhibits the most effective function 
when it is disposed at the position. 
The effect and function are as follows: 
(1) in a reproduction optical system adapted to obtain a high resolving 
power with a relatively small line sensor by using the dome shaped mirror, 
unevenness in light quantity which consequently generates in using the 
dome shaped mirror can be surely corrected; 
(2) the present invention can be easily practiced merely by disposing the 
iris plate or the optical filter at an appropriate position in the optical 
passage; and 
(3) the present invention can be easily practiced by the already completed 
reproduction optical system without adding any large scale of modification 
thereto.