Multiple lens system for an optical imaging device

An optical imaging device divides a long thin object, such as a line of print, into several sections and arranges the sections into an overlapping image, of area form as compared to the original linear form of the object. A plurality of lenses are used, each lens associated with a particular section of the object. Each lens is tilted relative to the other lenses so that the plane of the lens is perpendicular to the line connecting the center of the related image section and object section, and also displaced in a direction normal to the extent of the line of lenses so that the center of each lens is on the line connecting the center of the related image section and object section. The lenses can extend in a curve to reduce magnification variations from lens to lens, and conveniently the lenses are rectangular and molded as a single unit.

This invention relates to a multiple lens system for an optical imaging 
device and is particularly concerned with such a system to divide a linear 
object into sections and arrange the sections to give an image of area 
form, such as for optical scanners in facsimile and similar printing 
systems. 
In a scanner for copying devices, a page is scanned line by line, and 
signals developed as the line is scanned, indicative of the presence, or 
absence, of light and dark areas. This is achieved by imaging the line on 
some form of detector device made up of a number of elements. For example, 
for high resolution, a device composed of 1728 individual elements in a 
linear array is used. Each element is very small and there results a long 
thin device which is fragile and difficult to manufacture. 
The present invention provides a multiple lens system which effectively 
splits the line which is the object, into sections and repositions the 
sections such that the image is of area form rather than linear form. For 
example, with seven lenses, the line is divided into seven sections 
arranged one above the other, thus requiring a device which is only about 
1/7th as long.

In FIG. 1, a page is indicated at 10 with a line being indicated as being 
between the two lines 11. The line is shown as divided into sections 11a 
to 11g. A plurality of lenses 12a to 12g are provided, in a line, each 
lens relating with a particular section of the line, ie., lens 12a 
relating with section 11a, 12b with 11b and so on. An imaging device is 
indicated at 13, the device having a plurality of detectors arranged in 
parallel rows 14a to 14g. Again each lens is related with a particular 
row, lens 12a with row 14a, 12b with 14b and so on. Thus it will be seen 
that the line 11 is now imaged over an area composed of the seven rows 14a 
to 14g, instead of one long row. 
Considering one particular imager 13, this would consist of seven arrays, 
each array equaling to one of the rows 14a-g. Assuming a 13 .mu.m centre 
to centre spacing of elements in the X direction, a 9.8 X magnification is 
required to give two hundred lines to the inch resolution across the page. 
For 200 lines per inch resolution in the up and down direction of the 
page, the centre-to-centre spacing of the elements in the Y direction is 
also 13 .mu.m. With a chip 0.18 inches long (Y direction), each array 
would be separated by about 0.015 inches. 
The lenses 12a to 12g are arranged so that the centre of each lens lies on 
a line joining the centre of the related array to the centre of the 
appropriate line section and also the plane of each lens is perpendicular 
to this line. Thus the lenses are tilted with respect to each other, their 
centres displaced in a direction normal to the diameter of the linear 
extent of the lenses, and also rotated relative to each other. 
FIG. 2, a side view of the arrangement of FIG. 1, only shows two extreme 
lenses 12a and 12g. Also, these two lenses are shown with greater vertical 
displacement, that is displacement normal to the line of lenses, than is 
actually the case. This is for reasons of clarity only. Assuming a value 
of focal length f.perspectiveto.75mm for each lens, the vertical 
separation of the lens centres is small. Since the centre to centre 
spacing of adjacent arrays is 0.015 inch, with the above selected chip 
size of 0.18 inch in the Y direction, adjacent lens centres are separated 
by .perspectiveto.0.015 inch, that is the centres of lenses 12a and 12g 
are respectively 0.045 inch (1.14mm) above and below the centre of lens 
12d. The vertical tilt angle of these two lenses, 12a and 12g, with 
respect to the centre line, is 0.07.degree.. Thus the seven lenses are 
almost in a horizontal line. 
The horizontal displacement of the lenses is indicated in FIG. 3, which is 
a plan view. The angle between the two end lenses, 12a and 12g, and the 
centre lens (12d), is 5.9.degree. and the displacement of these lens 
centres (12a and 12g) from the centre is 9.2mm. Thus the maximum width of 
each lens is .perspectiveto.3mm. With a single lens, and linear imaging, 
lamp power and imager sensitivity limit the lens to a minimum aperture of 
about F11, corresponding to a 7mm diameter lens with f.perspectiveto.75mm. 
In the present invention, if circular lenses of 3mm diameter are used, the 
power incident on the imager will be less than 20% of that obtained with 
the single lens system. With rectangular lenses, approximately 7mm .times. 
3mm, the power on the imager will be about 50% of that of the single lens 
system and will give an acceptable system. Increasing the light intensity 
on the page, by some means, will improve this value. 
It will also be seen, from FIG. 3, that the total distance from the page 10 
to the imager 13 is greater for lenses 12a and 12g than lens 12d. While 
the depth of field of the long optical path will keep the image in focus 
if each lens has the same (f=75mm) focal length, the magnification will be 
slightly different for each lens and the related line section. Thus, for 
example, in an arrangement in which the distance from the page 10 to the 
lens centre is 810mm, and the mean distance from the lens centre to the 
imager is 89mm, the distance from lens 12d to the imager 13 is 
.perspectiveto.5mm longer than the distance from lens 12d, which 
introduces a 5% increase in magnification, that is, the resolution will be 
about 210 lines per inch instead of 200 lines per inch. This would 
introduce slight distortion into the print copy. It is possible to adjust 
the focal length of each lens to compensate for the longer path. For lens 
12a, f.perspectiveto.75.4mm and the distance from the imager would be 
83.1mm rather than 82.7mm for lens 12d. This is a small variation and can 
be accommodated. As a result, the lenses are rotated relative to each 
other, the lenses in a curve. 
However, the degree to which adjustment is made to the lenses will depend 
to a large extent on the accuracy required. For many purposes minimal 
adjustments will need to be made to the individual lenses to provide an 
acceptable image. No adjustments may be necessary after assembly and the 
lenses can be made as one unit. 
In both FIGS. 2 and 3, only the centre lens of the various optical paths 
are indicated, at 15. 
FIG. 4 illustrates in greater detail one form of lens, taking the above 
adjustments into account. The lens system or array comprises seven lenses 
12a to 12g. The array is approximately 9mm high and 21mm wide, with the 
individual lenses 7mm high X 3mm wide. The axis coincident with, or 
parallel to, the axis of the line on the page is indicated at 18 while the 
axis joining the centres of the lenses is indicated at 19. This indicates 
the vertical displacement on displacement in the direction normal to the 
linear extent of the lenses. The lenses are also on a curve, with the 
centre lens, 12d slightly closer to the page than lenses 12a and 12g, as 
described above. Finally the lenses are tilted relative to each other, as 
indicated by the axes 20 and 21 of lenses 12a and 12g respectively. The 
focal length of the lenses can vary, as described above, with f=75.4mm for 
lenses 12a and 12g and f=75mm for lens 12d, the intermediate lenses 
varying between these two values. Such an array can be made by plastic 
molding. The tilt and vertical displacement are critical in that these are 
responsible for the correct positioning of the image sections. The 
criticality will depend on the size of the detector devices for the image 
but is likely to be quite critical. The rotation, or positioning on an 
arc, is related to the magnification, and may be less critical. Depending 
upon the system it may be possible to ignore this requirement. 
With an arrangement as described, the illuminated area imaged must be 
confined, either by focussing the light on to the page itself, or by 
positioning a slit close to the page to limit the area illuminated. 
The arrangement has other advantages in addition to providing imaging in an 
area form rather than a linear form. It is possible to obtain data from 
two adjacent lines simultaneously. Thus, as an example, each array is 
composed of two lines of elements, each line having, for example 256 
elements. Such arrays serve several purposes. Thus, even with high 
accuracy in manufacture and assembly, it can well be that the sections 14a 
to 14g will not fall exactly on the desired position; there may be some 
vertical displacement. By providing two, or more, lines of elements, a 
section will always be on a line of elements and electronic means can be 
provided for actually scanning the correct line of elements. With two or 
more lines of elements, instead of a line being scanned completely and 
signals transmitted for every element, a second line can be compared to a 
previous line, and only changes detected and transmitted. This can reduce 
transmission time. 
Similarly by providing, for example 256 elements, instead of 247, 
(.perspectiveto.1/7th of 1728) some overlap is provided for lateral 
misalignment. Again electronic means can be provided to avoid overlapping 
transmission once printing. By imaging several lines at one time, it would 
be possible to read a number of lines at one time, reducing the stepping 
of the scan bar. 
The field of view of each lens is considerably reduced so that the image of 
each lens need be flat over, for example only 3mm instead of 25mm. This 
may simplify lens fabrication. The chip size is more compatible with 
conventional processing and avoids complex procedures such as butting 
photo masks for producing a 1728 element linear imager. Processing yields 
will be higher. A particular form of imager is one using charge coupled 
devices (CCD's). 
Although a lens system having seven lenses has been described, it will 
readily be appreciated that other numbers of lenses can be used. The 
minimum number will be decided by the degree to which the shape of the 
chip is to be changed from a long thin one. The number of rows of elements 
in each array will influence this also. Higher numbers of lenses will 
increase the complexity and manufacturing difficulties.