GB2212937A - Holographic lens array for use in a line-by-line printing apparatus - Google Patents

Abstract

A holographic lens array (16) and is used in conjunction with an array of light emitting diodes (10) to focus an image in plane (18) from where a second holographic lens array (17) focusses the image onto an image plane (12) of the energised diodes on a line-by-line basis to expose a photosensitive material located at or moving through the image plane. The two lens arrays are preferably first order diffraction patterns to reduce chromatic aberrations. <IMAGE>

Description

HOLOGRAPHIC LENS ARRAY This invention relates to holographic lens arrays particularly, but not exclusively, for use in a line-by-line printing apparatus for imaging light emitting diodes (LEDs) of an LED array onto a photosensitive material, and to a line-by-line printing apparatus including at least one holographic lens array.

In the field of non-contact printing, two competing optical methods are currently vying for market share: (a) point-by-point laser scanning; and (b) line-by-line LED array imaging.

The present invention, as aforestated, relates to the second of these methods. Present line-by-line LED array printing utilises an array 10 of LEDs 11 as shown in Figure 1 of the accompanying drawings. The LED array is of width equal to that of a photosensitive material to be exposed in an image plane 12. Interposed between the array 10 and the image plane 12 is an array 13 of SELFOC (trademark) lenses 14. The lenses 14 focus the light from the LEDs into the image plane 12. It will be appreciated that the SELFOC array is of length equal to that of the array 10 of LEDs and of the designed image width in the plane 12.The LEDs 11 in the array 10 are arranged in diagonal rows so that continuous horizontal line exposure can be obtained, on a moving photosensitive material, by sequential operation of the LEDs of each diagonal row in synchronism with the movement of the photosensitive material. The cost of SELFOC arrays is relatively high and the resolution obtainable therewith does not meet the optimum 600 lines per inch sought by the printing industry.

It is an object of the present invention to provide an improved lens array, a method of making such an array, and a line-by-line printing apparatus incorporating the improved lens array.

According to the present invention, there is provided a holographic lens array comprising an exposed and processed holographic plate having thereon a plurality of interference patterns each serving as a lens to image, in use, a respective point source in a first plane in a second plane and a second similar holographic plate, the interference patterns of which act as lenses to collect the light of the images in the second plane and to focus the same in a third image plane at locations correlated to the locations of the respective point sources.

Also within the scope of the present invention is a line-by-line printing apparatus comprising an array of light emitting diodes acting as point sources, and a holographic lens array having interference pattern lenses for the diodes for imaging each of the point sources of desired spatial locations in an image plane, and means for moving a photosensitive material through the image plane whilst energising selected ones of the light emitting diodes to expose the photosensitive material.

The invention will be described further, by way of example, with reference to the remaining accompanying drawings, in which: Figures 2a and 2b are diagrammatic views of line-by-line printing apparatus, in accordance with the present invention; Figures 3a and 3p are diagrammatic representations of steps in a method of making a holographic lens array, according to the present invention; and Figure 4 is a diagrammatic representation of a preferred lineby-line printing apparatus, in accordance with the present invention.

Referring to Figure 2, a line-by-line printing apparatus comprises a light emitting diode array 10 juxtaposed relative to an image plane 12 wherethrough a photosensitive recording medium (not shown) may be moved by means diagrammatically indicated by reference numeral 15.

Interposed between the array 10 and image plane 12, are two holographic lens arrays 16 and 17. Each of the lens arrays comprises, for each LED of the array 10, an interference pattern lens.

Each such lens is arranged to provide a +1 order diffraction image of a particular LED at a corresponding particular location and the two plates 16 and 17 provide images of the LEDs at respective desired spatial locations in the image plane 12. Two LEDs i and i are indicated giving images at correlated positions il andjl in the image plane 12.

It will be seen from Figure 2 that each of the LEDs may be considered as a point source giving rise, when energised to a diverging beam of monochromatic light impinging on the first plate 16. A lens formed by an interference pattern (in a manner hereinafter described) causes the diverging beam to converge towards a point in an intermediate plane 18 and thereafter to diverge and impinge on the second plate 17 having a similarly formed lens which again causes the beam to converge to a point in the image plane 12.

Referring now to Figures 3a and 3b. the lenses on the plates are formed by illuminating an unexposed photosensitive plate 19 sequentially with a diverging beam 20 of monochromatic light from, for example, a HeNe gas laser and simultaneously illuminating the plate with a converging beam 21 of coherent monochromatic light derived from the same laser light source. The point of convergence of the beam 21 is a desired spatial location at a distance from the plate 19 equal to the distance between the plate 16 and the plane 18 in Figure 2. The monochromatic light is of wavelength substantially equal to the wavelength of light emitted by the light emitting diode.

Interference between the beams 20 and 21 on the photosensitive surface of the plate 19 forms an image thereon which after processing constitutes a lens. The photosensitive plate is stepped a distance less than or equal to the diameter of the previously exposed hologram, thus allowing for a possible degree of close-packing lens overlap. The procedure is repeated until the plate 19 has been exposed to provide an array of holographic lenses for the complete LED array 10. After processing, the plate 18 becomes the lens array 16 (or 17). A second unexposed photosensitive plate is similarly treated to form a second holographic lens array 17 (or 16).

The two plates 16 and 17 are then juxtaposed as shown in Figures 2a and 2b to form, with the LED array 10, a line-by-line printing apparatus.

Each LED of the array 10 forms, with its respective lens in the array 16, a converging beam of light focussed to a point in the plane 18 and thereafter diverging to impinge upon the plate 17 and to be imaged by the respective lens of that plate at a correlated point in the image plane 12.

The holographic lenses of the arrays 16 and 17, being diffractive elements, suffer from chromatic aberration. The light emitted by presently available LEDs is not monochromatic but contains a continuous band of wavelengths of light. In the arrangement of Figures 2a and 2b, the lens array 16 utilises a +1 order diffraction pattern and the lens array 17 utilises a -1 order diffraction pattern to act as a lens and in this case the chromatic aberrations add together to degrade the image formed thereby. It is preferred, therefore to use the arrangement shown in Figure 4 of the accompanying drawings. In this figure, the second plate 17 is positioned so as to use a +1 order diffraction pattern. Chromatic aberration is substantially cancelled.

In the line-by-line printing apparatus shown in Figure 4 like reference numerals are used to indicate similar parts. An energised LED of the array 10 produces a beam of light 20 which is imaged by a respective +1 order diffraction pattern lens of the holographic lens array 16 to form a converging beam 22 focussed at a point in the intermediate plane 18 and thereafter diverging in a beam 23 to impinge upon the second holographic lens array 17. The array 17 includes a respective +1 order diffraction pattern lens for the beam 23 causing it to converge in a beam 24 to a desired spatial location in the image plane 12.

The invention is not confined to the precise details of the foregoing examples and variations may be made thereto. For example, the requirement of the printing industry of a resolution of 600 lines per inch (approximately 250 lines/cm) does not conform to the optimum chip resolution in an LED array where, in fact, a packing density of LEDs twice that high is possible.

The configuration shown for the rows and columns should be maintained but the LEDs can be provided at a density of 1200 per inch whilst permitting adequate intensity of illumination and dispersion of heat.

To provide the required 600 lines per inch resolution, the LED array, with optimum packing density need only be half the length of the required line in the image plane if the holographic lens arrays are prepared with this in mind. The LED array may be a single chip or a plurality of chips at spaced intervals co-linearly along the plane of the array 10. Utilising the substantially lambertian energy distribution of light emitted by the LEDs together with anamorphic lenses formed in the holographic plate, the required 600 lines per inch resolution in the image plane can be obtained. The anamorphic lenses in the arrays 16 and or 17 are formed by the condensed size of the optimum LED array causing off axis illumination of a photosensitive plate 19 whilst maintaining the illumination by the beam 21 directed to a desired resultant spatial location in an intermediate plane 18.

The arrays 16 and 17 need not be exactly at right angles. To avoid the chromatic aberration, it is only necessary that the arrays should each be set to provide the +1 order diffraction pattern.

Should solid state technology improve to the level where truly monochromatic LEDs are available, then the arrays could be used as illustrated in Figures 2 and 2b. It will be appreciated that, in all instances, the arrays should be positioned so that a line joining the lens centres of the first array is parallel to a line joining the lens centres of the second array.

It is conventional to move a photosensitive material through an image plane to expose the same. However, it will be appreciated that the photosensitive material may be held stationary and the array of LEDs and/or the holographic lens array or arrays may be moved relative thereto.

Other variations are possible within the scope of the present invention as defined in the appended claims.

Claims (11)

1. A holographic lens array comprising an exposed and processed holographic plate having thereon a plurality of interference patterns each serving as a lens to image, in use, a respective point source in a first plane in a second plane and a second similar holographic plate, the interference patterns of which act as lenses to collect the light of the images in the second plane and to focus the same in a third image plane at locations correlated to the locations of the respective point sources.

2. A lens array as claimed in claim 1 wherein the interference patterns are substantially identical and serve to provide 1:1 imaging, in use, of point sources at corresponding points in the image plane.

3. A lens array as claimed in claim 1 wherein the interference pattern constitutes an anamorphic lens whereby, in use, the spacing between the point sources is different from the spacing between the images thereof in the image plane.

4. A lens array as claimed in claim 1, 2 or 3 wherein the plane of the second holographic plate is substantially orthogonal to that of the first plate.

5. A holographic lens array substantially as hereinbefore described with reference to and as illustrated in Figure 2 or Figures 3a and 3b or Figure 4 of the accompanying drawings.

6. A line-by-line printing apparatus comprising an array of light emitting diodes acting as point sources, and a holographic lens array having interference pattern lenses for the diodes for imaging each of the point sources of desired spatial locations in an image plane, and means for moving a photosensitive material through the image plane whilst energising selected ones of the light emitting diodes to expose the photosensitive material.

7. An apparatus as claimed in claim 6 wherein the holographic lens array comprises two holographic plates located in series, each plate having an interference pattern lens for each of the point sources and being arranged so that each point source, when energised, is imaged in a desired spatial location in an image plane.

8. An apparatus as claimed in claim 7 wherein the plates are arranged substantially orthogonally.

9. An apparatus as claimed in claim 6 or 7 wherein the array of light emitting diodes comprises a plurality of rows of the diodes extending in a first direction, the diodes also being arranged in rows extending diagonally in a second direction.

10. An apparatus as claimed in any of claims 6 to 9 wherein the interference pattern lens on the plate or plates are anamorphic.

11. A line-by-line printing apparatus substantially as hereinbefore described with reference to Figure 2 or Figure 4 of the accompanying drawing.