CA1210263A - Imaging system utilizing a gradient index lens array - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A gradient index lens array used in an optical imaging system is modified so as to compensate for factors creating a non-uniform image exposure at an image plane. Various parameters of the individual fibers comprising the array, such as index gradient, radii and packing density, are selectively modified so as to obtain a desired exposure profile.

Description

~ ~0~63 IMAGING SYSTEM TJTILIZING A GRADIENT INDEX LENS ARRAY
BACKGROUND AND PRIOR ART STATEMENT
_ The present invention relates to an imaging system wherein an 5 object in an object plane is illuminated by an elongated light source and the reflected image is transmitted to an imaging plane by a gradient index lens array. More particularly, it relates to a gradient index lens array modified so as to provide a spatia~ly uniform exposure level at the image plane.
Image transmitters comprising bundled gradient index optical fibers are known in the art. U. S. Patent 3,658,407 describes a light conducting fiber made of glass or synthetic resin which has a refractive index distribution in a cross section thereof that varies parabolica31y outward from acenter portion thereof. Each fiber acts as a focusing lens to transmit part of an image of an object placed near one end~ An assembly of fibers, in a steggered tw~row alTay, transmit and focus an image of the obj~ct. The fiber lenses are produced under the trade lTa~c "SELFOC"; the mark is registered in Japan and owned by Nippon Sheet Glass Company, Ltd.
Numerous techniques are known in the ar~ for manufacturing glass or plastic fibers with index-of-refraction variations. These are usefully summar;zed in an article entitled "Gradient Index Optics: A Review" by Duncan T. Moore, Applied Optics, 1 April 1980, Volume 19, No. ~, pages 1035 -103~. Relevant optical characteristics of gradient index lens arrays are described in an article entitled "Some Radiometric- Properties of Gradient Index ~iber Lenses", by Jame~ D. Rees and William Lama, Applied Optics, 1 April 1980, Volume 19, No. 7, pages 1065-10690 Gradient index lens arrays have found use in a number of tech-nologies, e. g. in construction of printed type optical circuits as disclosed inU. ~. Patent 3,922,062 and as a replacement Ior conventional optical systems in copiers as disclosed in U. S~ Patents 3,947,106 and 4,193,679.
In copier applications such as those disclosed in the referenced patents, the light source which pr~vides illumination of the document to be copied must be able to provide an illumination band which is quite narrow and intense relative to the i31umination band required for copiers using conven-tional projection lenses. ThLs requirement is necessitated by the inherent operational structure of the gradient index lens array. Another requirement9 ~3 common to ~11 copier optical systems, is that ~iven a uniformly bright object, a uniform level of image exposure be provided at the image plane. A typical cause of non-uniform image plane exposure is the light fall-off at the ends of elongated tubular sources such as fluorescent lamps. This light fall-off 5 produees a non-uniform illumination of the document scan line, the non-uniformity being transmitted through the lens array to create a corresponding non-uniform image exposure.
Yarious te~hniques have been devised to compensate for this non-uniformity. The light source may be modified to produee a uniform level of 10 document illumination by using a longer tube so that only the central portiono~ the tube provides the use~ul illumination. This9 how ver9 is a relatively inefficient expedient which increases the size of the copier Another techllique, borrowed from conventional lens systems, is to introduce a light shapin~ component su~h as a variable density filter or a variable aperture slit 15 in the optical path, generally jus~ before the imaging plane. The slit is appropriately shaped to permit more illumination to pass through the ends than in the center, i. e. the well known "butterfly slit" while a filter is denser in the middle and increasingly transparent a~ the endsO However, because of the much narro-ver ray bulldle which is projec~ed by the gradient index array, 20 these techniques are exceedingly difficult to implemen~ with any degree of accuracy.
The present invention provides a novel and relatively simple way to achieve a uniform level of image plane exposure without reqwring the use of specially designed lamps or additional light shaping components. The compen-25 sation for end fal~-off is achieved by altering the natuPe o~ the gradient lens array in such a way that greater illumination is transmitted at the end portion than at the central portion. This alteration is achieved by varying any one of the lens parameters such as the fiber pac~ing density, individual fiber radius or individual fiber index gradient. Given a particular illumination profile whi~h 30 has end non-uniformity eharacteristics, a lens array can be designed according to the invention whereby an exps sure distribution that is spatially uniform canbe achieved at the image plane.

- 2a -Various aspects of the invention are as follows:
An optical imaging system including a plurality of gradient index optical fibers combined into at least a single row to form a linear lens array, said array positioned between an object plane and a photosensitive image plane so as to -transmit light reflect-ed from an object lying in the object plane onto the image plane, said array characterized by said optical fibers being separated by at least two different spacing distances, said fibers inter-spaced so as to transmit therethrough a uniform level of light resulting in a uniform exposure level at said image plane.
A lens array comprising a plurality of gradient indexed fibers, said lens array being characterized by said fibers being separated by at least two different spacing distances.
Figure 1 is a schematic end view of a prior art gradient index array in an imaging system.
Figure 2 shows a non-uniform irradiance profile produced by a fluorescent lamp at the object plane of Figure 1.

_ 3 _ Figure 3 shows a top perspective view of a prior art double row gradient index lens array.
Figure 4 shows a parti~l 1op view of a first embodiment of the present invention wherein the gradient index fiber spacing is selectively varied.
Figure 5 shows a pnrtitl top Yiew of a second embodiment of the present invention wherein the gradient index fiber radii (R) are selectively varied.
~igure 6 shows a partial top view of a third embodiment of the present invention wherein the gradient index fibers' constant values (A) are selectively varied.
Figure 7 is a schemaffc end view of a prior art reduction/-enlar~ement gradient index array in an imaging system.
DE~CRIP IION
Referring now ~o Figure 1, there is shown, in schematic side view, a prior art opti~al imaging system 2 which includes a gPadient index lens array 4 comprising two staggere~ rows 6, ~ of in~entical gradient index fibers arranged in a bundled con~iguration as is known in the prior art. Pigure 3 shows a top perspee~ive view o~ array 4. Transparent object plane 10 in ~igure 1 is adapted for movemen~ past lens ~ in the indicated direction. Plane 10 has an object 12, ~hich may be a document, supported thereonO Fluorescent lamp 14 provide~ a narrow intense band of illumination through aperture 15 across the Y width of the object plane 10.
~igure 2 shows a ~ypical object plane irradiance profile for a ~luorescent lamp. When energized9 the lamp provides an object plane illumination output profile P a~ a plane 10 parallel to the axis of the lamp. The pro~ile is fairly uniform over a central portion A but f ns off over end portions B and C.
In operaffon and referring to ~igure 1, plane 10 is moved in the X
direction across the illuminated area at a speed synchronous with that of a photosensitive imaging plane lB. A strip OI light is reflected from object 12 and Iocused by lens 4 onto e2~posure strip 18 of plane 16. If the illumination profile at the objeet plane is not compensated ~or, the exposure profile at the image plane will be subject to the same non-uniformity levels.
Turning now to the present invention, it was per~eived by Appli-cant that the exposure spatial distribution OI a gradient index lens array was dependent upon any one of three parameters. These are:

~Z~ 3 1~ The packing density of the individual ~ibers, i. e. how c~sely spaced are the fibers relative to each other;

2) The radius of each individual fiber, and

3) The individual fiber index gradient constant, commonly 5 designated as A.
It was further realized that these parameters could be var ed, either separately, or in combinatior~, to produce an exposure distribution levelthat is spati~ny uniform even if the document illumination were not. Figure 3 shows the typical dis~ribution of the prior art double row lens array. Pigures 10 4-6 show the lens array modified to change fiber packing density, fiber radius and index gradient, rsspectively.
Tu~ing first to Figure 37 there is shown a top view of lens array 4.
Each of the fibers comprising rows 6, 8 are identical to each other9 i. e have the same radius and index gradient. ~urthermore, the fiber pa~king density is 15 uniform from one end of the array to the other.
Figure 4 illustrates a first modific~tion of the lens array which comp~nsates for the non-uniform object plane illumination condition. ~ this embodiment, lens array 24 comprises two rows 26, 28 of gradient index fibers.
Only olle half 0?'! the lens array is shown for ease of description. As seen, 20 fibers in the center portion are mor~ widely spaced relative to the more closely s~aced fibers at the array end. ~ oth~r words, the fiber spaeing progressively decreases with approach to the ends. With this variable spacing more fibers contribute to the exposure of an image poin~ ne?~ the ends of the array than the center and the inherent illumination falloff is compensated ~or 25 by the increased number of ¢ontributing fibers. A tradeof~ with this embodlment ls some modest increase in e2cpo~ure modulatio3l a~ the center ~nd some depth of focus variabilityO
Figure S illustrates R second embodiment of the lens array wherein the fiber radii are selectively changed. Lens array 34 comprises two rows 369 30 38 o~ gradient index fibers. The fibers have a radius equal to one of tr o selected values Rl or R2 with R2 Rl. As shown~ more of the Rl radius fibers ~re used near the center of the array, while a higher percentage of R2 radius fibers are used near the end of the array. The number of R2 fibers are seen to inerease with increasing distance from the center. Since image plane e2cposure 35 is proportional to the cube of the value of the fiber radius (see reference by Rees ~ Lama), the R2 fibers at the ends will eompensate for the geometric - s -exposure fallofI. Since the exposure of any poin~ in the image ~lane is given by the SUITl of the contribution from several fibers, a reasonably uni~orm exposure profile may be achieved through the use of only two fiber radii although more than two fiber radii may be used. As for the variable spacing 5 solution above, some exposure modulation and depth of focus variations ~re introduced.
For convenient handling and assembly, the Rl fibers in rows 36, 38 have been made to have the same physical diameter as the R2 fibers by surrounding the Rl fi~ers with cladding 39~ This permits the fibers to be seated In the convenffonal parallel grooved seating members used to assemble conventional two row staggered lensesO The conventional clos~packed arrangement may also be employed (see Figure 3). If desired, however, the groove spacing on the seating member could be varied to provide uniform spacing between different radii fibers. ~inally, it is possible to employ fibersall of the ~ame radius R2 and use a mask to limit the aperture of those fibers designated Rl Figure 6 illustrates a third embodiment of the lens array wherein the fibers have one of two values of index gradient constlmt A. Lens array 44 comprises two rows 46, 48 OI equQl radius gradient index fibers. The fibers in each row have gradien~ inde~ value of Al or A2 with A2 ~ Al. Since exposure the image plane is proportional to A ~see re~erence by Rees and Lama), the distribution of ~1 and A2 fibers is designed to compensate for exposure fallof~
by placing a plurality OI Al fibers in the center area while the majority of A2 fibers are used near the ends of the array.
Again, only two di~ferent values of A are needed sin~e several fibers contribute to the exposure at any point. A disadvantage to this eompensation te~hniq~le is that the Al and A2 fibers must h&vD slightly difierent lengths to maintain cons~ant total conjugate.
The above examples have been described with relation to ~n optical system which has an illumination souree providing a non-uniform level of illumination. While the most common lighffng source is a fluorescent lamp, other lamps such as sodium vapor lamps are subject to the same problem. And even spe~ial lamps which produce relatively uniform illumination along their length, such as segmented tungsten, are frequently used with a length less than the lens array length so ~hat less re~lected light enters the extreme ends of the array. The present invention is equally applicable to these systemsO

~2~

Furthermore, other illumination systems ma~ be envisioned which provide even more non-uniform document illumination than the previous examples. For instance, a simple in-expensive illuminator may be constructed of a single mini-ature tungsten lamp positioned at the center of a cylin-drical diffuse cavity with an axial aperture. ~his illu-minator would provide highly non-uniform document illumi-nation which could, however, be compensated for by the special gradient index lens arrays described herein.
Even for optical systems in which a uniIorm illu-mination source is provided, however, certain gradient index lens array applications are still subject to expo-sure non-uniformity problems due to a unique construction.
Referring now to Figure 7, there is shown a gradient index lens array 50 which is capable of transmitting reduced or enlarged images according to the principle dis-closed in U. S. Patent No. 47331~380, issued May 25, 1982 and assigned to the same assignee as the present invention.
As shown in Figure 7, fibers 52 are geometrically arranged in a fan-like array with the fibers near the array center being nearly vertical (perpendicular to object plane 54 and image plane 56) while ~he fibers near the end of the array are tilted from the vertic.l. With this configura-tion~ it has been found that~ even if an illumination source provides a uniform level of illumination at the object plane, the image exposure increases at the ends of the array, compared to exposure le~els at the center.
This increased end exposure is due to the combined effects of an increase in the effective size of the entrance and exit pupils of each fiber and the increased field of view which yields yreater overlap of the irradiance profiles from neighboring fibers.
According to the principles of the pxesent inven-tion, lens array 50 can be appropriately modified to com-pensate for the exposure non-uniformity effects by selectlve alteration of the three parameters as discussed above. In this case~ however, the arrangements would be opposite to the configuration shown in Figure 4 - 6 since the desired e~fect would be to increase rather than 5 decrease center exposure.
And, although the above described lens arrays in Figures 4-7 have been disclosed in terms of a double row of fibers, the principles are equally applicable to a single row gradient index lens array. An exemplary array 10 of this type of array is described in U. S. Patent No.

4,373,780, issued February 15, 1983.
In conclusion, it may be seen that there has been disclosed an improved optical imaging system. The exem-plary embodiments described herein are presently prefer-15 red, however, it is contemplated that further variationsand modifications within the purview of those skilled in the art can be made herein. The following claims are intended to cover all such variations and modifications as fall within the spirit and scope of the invention.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An optical imaging system including a plurality of gradient index optical fibers combined into at least a single row to form a linear lens array, said array positioned between an object plane and a photosensitive image plane so as to transmit light reflected from an object lying in the object plane onto the image plane, said array characterized by said optical fibers being separated by at least two different spacing distances, said fibers interspaced so as to transmit therethrough a uniform level of light resulting in a uniform exposure level at said image plane.

2. The optical imaging system of claim 1, wherein the fibers at the center portion of the array have higher interspacing values relative to fibers towards the ends of the array.

3. A lens array comprising a plurality of gradient indexed fibers, said lens array being characterized by said fibers being separated by at least two different spacing distances.