Image exposing apparatus

An image exposing apparatus comprises a focusing optical system for focusing an image onto a photosensitive member and having a one-line array of a plurality of focusing light transmission media, and light intensity distribution correction means arranged in a light path for forming the image for eliminating an ununiformity of light intensity on the photosensitive member due to the focusing optical system.

BACKGROUND OF THE INVENTION 
The present invention relates to an image exposing apparatus which focuses 
and exposes an image onto a photosensitive member through a converging 
optical system having a plurality of focusing light transmission media 
arranged in a line. 
A converging light transmission medium known by the tradename Selfocs of 
Nippon Sheet Glass is an elongated cylindrical optical element whose 
coefficient of refraction parabolically changes in radial direction around 
a center axis. It is cut to an appropriate length to impart a focusing 
function. An array having a number of such light transmission media is 
applied to an electrophotographic copying machine or a document reader 
using an image sensor, as an optical system for exposing a document image. 
In FIG. 1, light transmission media 1 described above are arranged in a 
line and molded in a support 2 of resin. The light intensity distribution 
of an image on a photosensitive member effected by such an array is the 
sum of the light intensity distributions of images formed by individual 
light transmission media, as shown by a curve E in FIG. 2. As shown, the 
light intensity distribution is highest at an area corresponding to the 
center of each light transmission medium and lowest at an area 
corresponding to the boundary of adjacent light transmission media. The 
distribution is rippled and the distribution period thereof corresponds to 
an arrangement period of the light transmission media. Because of the 
rippled light intensity distribution, an formed image includes a periodic 
stripe pattern. In the past, two layers of such light transmission media 
are stacked in a zig-zag fashion. This requires double expensive light 
transmission media and increases its size. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an image exposing 
apparatus having light transmission media arranged in a line with a 
rippled light intensity districution reduced to an extent that the 
difference between peaks and valleys of the ripple does not remarkably 
appear on an image. 
It is aother object of the present invention to provide an image exposing 
apparatus which can compensate ununiformity of illumination by a light 
source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows an embodiment of the focusing optical system used in an image 
exposing apparatus of the present invention. An opaque light intensity 
distribution correction member 3 is mounted on a converging light 
transmission member 1. The member 3 has a number of light interruption 
projections 4 as shown in FIG. 4 and each projection 4 partially covers a 
corresponding light transmission medium 1. The projections 4 are arranged 
at the same pitch as a pitch of arrangement of the light transmission 
medium 1. Each projection 4 covers a center area of the light transmission 
medium 1 to lower a peak in a ripple of a light intensity distribution. 
The term center area of the light transmission medium is used not only to 
indicate a geographical center of an incident end plane or an exist end 
plane (which is circular) of the light transmission medium but also to 
indicate a line which passes through the geographical center of the above 
end plate (center of circle) and is normal to a direction of arrangement 
of the light transmission media. 
In any case, of the use of the light intensity distribution member 3, an 
image light intensity distribution on a photosensitive plane is 
substantially unified or a difference between a peak and a valley of the 
ripple is reduced to an extent that no non-uniformity in a density of 
image is observed. In a copying machine, a stripe pattern is not observed 
in the image if a light intensity non-uniformity .DELTA.I is less than 
1.5%. The light intensity non-uniformity .DELTA.I is defined by 
EQU .DELTA.I=(I max-I min)/I max 
where I max is a maximum light intensity and I min is a minimum light 
intensity. 
The member 3 is made of plastic film such as polyester or polypropylene or 
metal, which is colored or painted black to prevent light transmission 
therethrough. It is bonded to the array by adhesive material. 
FIG. 5 shows another embodiment of the focusing optical system applied to 
the present apparatus. Ink, Chinese ink or paint is applied in spot form 
at a center of each converging light transmission medium 1 to form a light 
blocking area 4' so that the same effect as that in the previous 
embodiment is obtained. 
In the above embodiment, the light blocking area may be at a light incident 
plane of the array and/or a light exit plane. The light blocking area may 
be formed by a printing technique and may have a perfect light blocking 
property or may have a light attenuating filter function. 
The array 2 described above may be applied to an electrophotographic 
copying machine as shown in FIG. 6. A document sheet 0 is mounted on a 
document table 5 which is moved in a direction of an arrow for scanning 
the document. An image of the document illuminated by a lamp 7 is focused 
and exposed by the array 2 onto an electrophotographic photosensitive 
material 6 which is rotated in the direction of the arrow. 
In the above embodiment, the light blocking area contacts the end plane of 
the light transmission medium. Alternatively, as shown by numeral 9 in 
FIG. 6, a light intensity distribution correction plate having light 
blocking areas each corresponding to each of the light transmission media 
may be arranged in an image forming light flux path 8' facing an image 
space or in an image forming light flux path 8 facing an object space, 
spaced from the array 2. A positional relation between the light 
transmission media 1 and projecting light blocking areas 10 of the light 
intensity distribution correction plate 9 is shown in FIG. 7. Each of the 
light blocking areas 10 blocks a portion of light flux which passes 
through the center of the light transmission medium 1 or would pass 
through the center thereof if the light blocking area 10 were not 
provided. 
In the above embodiment, each of the light blocking areas has a shape 
analogous to the shape of the light transmission medium. A different 
embodiment is now explained. First, a relationship between a degree M of 
overlapping of light fluxes from the light transmission medium on the 
photosensitive member and a non-uniformity .DELTA.I of light intensity is 
explained. The degree M of overlapping is expressed by an effective 
diameter D of the light transmission medium 1 and a radius Xo of a view 
field on the photosensitive member as follows. 
EQU M=Xo/D 
FIG. 8 shows the relationship between the degree M of overlapping on the 
photosensitive member and the non-uniformity .DELTA.I of the light 
intensity. As seen from FIG. 8, the uniformity .DELTA.I of the light 
intensity is low when M=2.75+0.5.chi. (when .chi. is an integer no smaller 
than zero), that is, M.sub.1 =2.75, M.sub.2 =3.25, M.sub.3 =3.75, . . . It 
has been provided, as described above, that a stripe pattern of 
non-uniformity does not appear in the image when the non-uniformity 
.DELTA.I of the light intensity is less than 1.5%. Accordingly, if the 
dogree M of overlapping is set to one of the above values, the stripe 
pattern does not appear, but it is preferably set to M.sub.1 or M.sub.2 
because the resolution power is lowered as the degree M of overlapping 
increases. However, the degree M of overlapping actually varies because of 
manufacturing error even if the degree M of overlapping is set to M.sub.1 
or M.sub.2, assuming that a distance between the document plane and the 
focusing plane is constant. For example, in a line of array having 
M=M.sub.2, the non-uniformity .DELTA.I of the light intensity is 4% at 
maximum if the degree M of overlapping includes a variance of 0.25. If 
such non-uniformity .DELTA.I of light intensity exists, a stripe pattern 
appears in the image of the copying machine and such a copying machine is 
not practically acceptable. 
FIG. 9 shows another embodiment of the focusing optical system used in the 
present apparatus, and FIG. 10 is a perspective view of a linear mask used 
as the light blocking means. In FIG. 9, the linear mask 11 having two 
lines is supported by a support frame 12 in a vicinity of the 
photosensitive material 6 and arranged symmetrically with respect to a 
center line of a light path. Accordingly, a portion of light transmitted 
through the converging light transmission media 1 arranged in a line is 
blocked by the two lines of the linear mask 11. An advantage of the 
present embodiment is explained below. In FIG. 11, D is an effective 
diameter of each of the light transmission media 4, W is a width of the 
linear mask 11 and L is a distance between the center line of the light 
path and the linear mask 11. 
FIGS. 12 and 13 show relationships between the variation of the degree M of 
overlapping and the non-uniformity .DELTA.I of the light intensity when M 
is set to M.sub.2 =3.25, with W being a parameter. FIG. 12 shows the 
relationship when L=4D/10, and FIG. 13 shows the relationship when 
L=6D/10. From FIGS. 12 and 13, allowable ranges for the variation of the 
degree M of overlapping are extracted, and they are shown below. 
TABLE 1 
______________________________________ 
L = 4D/10 
L = 6D/10 
______________________________________ 
W = 0 0.16 0.16 
W = 4D/10 0.27 0.25 
W = 6D/10 0.31 0.32 
W = 8D/10 0.43 -- 
______________________________________ 
As seen from Table 1, the non-uniformity .DELTA.I of the light intensity 
can be sufficiently reduced by properly selecting the width W of the 
linear mask 11 and the arrangement (i.e. the distance L). In the present 
embodiment, the allowable range of variance of the degree M of overlapping 
can be widened more than duble to compare with that without the linear 
mask 6 (W=0). 
When the non-uniformity of the light intensity is to be corrected by using 
the linear light blocking means the width of the linear light blocking 
means may be substantially constant if an object whose image is formed on 
the photosensitive member by the focusing optical system is illuminated 
with a uniform light intensity. However, if the object is illuminated with 
an non-uniform light intensity, the linear light blocking means may have a 
function to correct the uniformity of the illuminating light intensity. 
This may be attained by varying the width of the linear mask along its 
length. One embodiment of such a linear mask is shown in FIG. 14. The 
linear mask 11' shown in FIG. 14 has a larger width at a center of its 
length and a smaller width at ends thereof. The shape of the linear mask 
11' is explained in detail with reference to FIG. 15. 
FIG. 15(A) is a plan view of the linear mask in the present embodiment, and 
FIG. 15(B) is a graph showing a light intensity distribution of a light 
source and a light intensity distribution after transmission through the 
linear mask. 
If a light flux from the light source has a light intensity distribution on 
an object plane shown by a curve 13, the light intensity at a center area 
of the linear mask 11 can be suppressed by selecting a width W.sub.1 at 
the center area wider and a width W.sub.2 at ends narrower so that the 
light intensity (exposure amount) on a focusing plane of the 
photoconductor member can be unified as shown by a curve 14. 
FIG. 16 shows a graph which illustrates a relationship between the width W 
of the linear mask 11 and the exposure amount, where P is the effective 
diameter of each medium of the one-line array 4 shown in FIG. 11 and L is 
the distance between the center line of the light path and the linear mask 
11, and an ordinate represents a ratio of the light intensity of the light 
source to the exposure amount. 
As seen from FIG. 16, the exposure amount essentially depends on the width 
W and decreases by approximately 10% for each increase of 2D/10 of the 
width W. Accordingly, the distribution of the exposure amount can be 
unified as shown by the curve 14 in FIG. 15(B) by setting the arrangement 
and the shape of the linear mask 11 such that L=4D/10, W.sub.1 =8D/10 and 
W.sub.2 =4D/10. The non-uniformity .DELTA.I of the light intensity on the 
photosensitive member is corrected by the linear mask 11 as seen from 
Table 1 and FIG. 12. 
In this manner, the width of the linear mask 11 can be varied along the 
length within a range to correct the non-uniformity of the light intensity 
on the photosensitive member in order to correct the non-uniformity of the 
light intensity distribution due to the light source or other causes. 
The shape and the arrangement of the linear mask may take various 
modifications. 
In FIG. 15, a two-line linear mask 15 is arranged to face the document 
table 5. In this embodiment, it is desirable to incline the linear mask 15 
toward the light source 7 to reduce a loss of the illumination light. 
Thus, the allowable range of the variation of the degree M of overlapping 
can be widened and the non-uniformity .DELTA.I of the light intensity can 
be sufficiently reduced even if the linear mask 15 is arranged in the 
vicinity of the object plane (i.e. document plane). 
FIG. 18 shows a further embodiment of the present invention. In this 
embodiment, a one-line linear mask 16 is fixed on a support frame 12 and 
arranged on the center line of the light path in the vicinity of the 
focusing plane of the photosensitive drum 6. The allowable range of the 
variation of the degree M of overlapping can be widened by properly 
selecting the width W of the linear mask 16. 
FIG. 19 shows yet another embodiment of the present invention. In this 
embodiment, the sectional shape of the linear mask 17 is circular. In the 
linear masks 17 shown in FIGS. 17 to 19, the non-uniformity of the light 
intensity due to the illumination light source can be corrected by 
changing the width along the length. 
In the light blocking mask shown in FIG. 15, the width is largest at the 
center thereof and minimum at the ends, although the shape may be selected 
in accordance with the distribution of the non-uniformity of the light 
intensity. 
The present invention is also applicable to a reader which forms an image 
on a photoelectric conversion image sensor such as a CCD and converts the 
image to an electrical signal. 
A magnification of the focusing optical system in the present invention may 
be unity or large or smaller than unity. 
An object to be imaged by the focusing optical system may be a conventional 
document sheet or a one-line light emitting diode array which emits lights 
in accordance with image information. In the latter case, the light source 
itself contains image information. 
In accordance with the present invention, the difference between the peak 
and the valley in the rippled light intensity distribution on the image 
produced by the one-line array of the plurality of focusing light 
transmission media is reduced by a simple construction so that a stripe 
pattern by the non-uniformity of density in the image is prevented. The 
non-uniformity of the light intensity due to the light source is also 
corrected. Thus, the high quality of image can be reproduced with low cost 
.