Split exposure optical system for copying machine

The slit exposure optical system for a copying machine generally includes two mirror-carrying movable tables of which one is driven at half the speed of the other. Normally each table is driven from one and the same end which means that the other end of each table must follow. If such following is not precise, aberrations in the optical system result. The present invention provides means for interconnecting the nondriver ends of the tables in such a manner that the movement of the nondriver ends will precisely follow the movement of the driver ends and aberrations are thereby avoided.

BACKGROUND OF THE INVENTION 
The present invention relates to an improvement in a slit exposure optical 
system which is to be used in a copying machine or the like. 
As is well known, in a copying machine, a slit exposure optical system 
having two movable tables is used in order to constantly maintain an 
optical path length of an original image incident upon a projection lens. 
Specifically, a slit exposure optical system of this kind is equipped with 
a primary movable table having a slit for scanning the surface of an 
original and a primary mirror, and a secondary movable table carrying a 
secondary mirror which receives an incident ray of light from the primary 
mirror, and a motor-driven wire sheave means is suspended between the 
respective one end of said movable tables; the secondary movable table is 
driven by said wire sheave means at a half of the speed of the primary 
movable table. 
In such a slit exposure optical system as described above, it is necessary 
that the mirrors on the primary and the secondary movable tables should 
always be at right angles to the moving direction of the movable tables. 
However, if the primary and the secondary movable tables are driven only 
at one ends thereof, the other ends thereof are delayed in motion in the 
backward direction of the movement thereof, and an aberration is thus 
caused between both ends of the optical axes of the mirror. The above 
phenomenon becomes a problem particularly in large-sized copying machines 
in which the movable table is becoming larger in size and heavier in 
weight, or in high-speed copying machines in which there is a sudden 
acceleration or deceleration of the movable table. To solve this problem, 
in a conventional type of a large-sized copying machines, two pairs of 
wire sheave means are arranged respectively between the both ends of the 
movable tables, and a driving force is applied to the both wire sheave 
means, and the both movable tables are thus driven. However, the 
arrangements of two pairs of wire sheave means for driving use as 
mentioned above complicates the driving system and increases the 
manufacturing costs thereof. 
SUMMARY OF THE INVENTION 
The present invention proposes, taking cognizance of problems of a 
conventional type of a high speed or a large-sized slit exposure optical 
system, a slit exposure optical system in which the optical axes of the 
mirrors is constantly maintained without complicating the driving system 
of the movable tables.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An example of the present invention will be described hereunder, with 
reference to FIG. 1: 
Each end of both of a primary and a secondary movable tables 1 and 2 of a 
slit exposure optical system according to the present invention are guided 
respectively along a guide rod 4 and a guide rail 5 which are positioned 
on either side of an optical system housing 3. The primary movable table 1 
has a slit 7 which scans an original (not shown) illuminated by a lamp 6 
and a primary mirror 9 which reflects an incident image coming from said 
slit 7 upon a secondary mirror 8 being carried by a secondary movable 
table 2. Secondary movable table 2 supports a tertiary mirror 10 which 
meets the secondary mirror 8 at right angles, and the reflected image from 
the secondary mirror 8 impinges upon a projection lens 11 by means of said 
tertiary mirror 10. 
One side of the aforesaid optical system housing 3, is provided with a wire 
sheave means A by which locomotion is applied to either one end 1a and 2a 
of said primary and secondary movable tables 1 and 2. Said wire sheave 
means A is provided with a pair of fixed sheaves 12 and 13 which are 
oppositely arranged to the moving direction of said primary and secondary 
movable tables 1 and 2, a two-part sheave 14 and 15 which is positioned on 
housing 3 between said fixed sheaves 12 and 13, a two-part movable sheave 
16 and 17 which is supported on the end of said secondary movable table 2, 
and a driving drum 18 which is rotated by a motor (not shown). Wire 20 of 
which one end is fixed to a stay 19 of the optical system housing 3, 
passes over the movable sheave 16, and is then fixed to end 1a of the 
primary movable table 1, and is turned about by means of the fixed sheave 
12, and passes over the relay sheave 14, the driving drum 18, the relay 
sheave 15 and the fixed sheave 13, respectively, in that order, and is 
suspended over the movable sheave 17, with the other end thereof fixed to 
the stay 21. Accordingly, as a result of the construction of the wire 
sheave means A on the driving side, it is possible to obtain an 
arrangement that, according to the direction of the rotation of the 
driving drum 18, the secondary movable table 2 moves in the same direction 
as the primary movable table 1, and at a half of the moving speed thereof. 
According to the present invention, on the other side of said optical 
system housing 3, a wire sheave means B is position between the other ends 
1b and 2b of the aforesaid movable tables 1 and 2. Said wire sheave means 
B is provided with a pair of sheaves 22 and 23 fixed on housing 3 at 
opposite ends of the primary and secondary movable tables 1 and 2, and 
with a two piece movable sheave 24 and 25 supported on the other end 2b of 
the secondary movable table 2. A wire 27 of which one end is fixed to a 
stay 26 on housing 3 is then attached to the other end 1b of the primary 
movable table 1 after passing over movable sheave 24 is turned around by a 
fixed sheave 22, and passes over fixed sheave 23 and a movable sheave 25, 
and at the other end thereof is attached to a stay 28 on housing 3. 
A slit exposure optical system of the present invention as described above, 
is similar to those of the conventional ones in that ends 1a and 2a of 
primary and secondary movable tables 1 and 2 are driven by a wire sheave 
means A on the one side, however, because of the wire sheave means B on 
opposite side connected to the other ends 1b and 2b of said primary and 
secondary movable tables 1 and 2, a tensile force is generated by wire 27 
when the other ends 1b and 2b of the primary and secondary movable tables 
1 and 2 are about to be tardier in motion than the motion of opposite ends 
1a and 2a. In this connection, both of the ends of primary and secondary 
movable tables 1 and 2 extend at right angles to the moving directions of 
said tables, therefore, there causes no aberration of the optical axes of 
mirrors 8 and 9. Consequently, in accordance with the present invention, 
it is possible to attain a slit exposure optical system in which the 
relations of the optical axes of the mirrors can be maintained in a 
regular state by the relatively inexpensive arrangement.