The present invention relates to optical scanners and more particularly to a method for making holographic optical elements with high diffraction efficiencies for use in such scanners.
One of the more significant changes which is occurring in retailing operations in general, and supermarket operations in particular, is the increasing use of optical scanners at customer checkout stations. Such scanners are used to read bar-coded labels which are printed on or affixed to product containers by producers or, in some instances, the store operator.
The bar-coded labels, the best known example of which is the UPC or Universal Product Code label, identify the product and permit the retrieval of product descriptors and prices from a system memory. The product descriptors and prices are used to prepare descriptive customer receipt tapes and to compile transaction totals. The advantages of optical scanners are well known. Since individual containers no longer need be marked, less labor is required to stock and maintain store shelf inventories. Optical scanners can also be used to simplify inventory control, to reduce the chances of operator mis-rings and to improve operator productivity.
Most currently available scanners use rotating mirrored drums or oscillating mirrors to deflect a laser beam onto a stationary set of mirrors which fold the beam along different paths to generate a label-scanning pattern. In recently introduced scanners, the rotating mirrored drums or oscillating mirrors may be replaced by holographic scanner discs. Such discs may include a circumferential array of several holographic optical elements. Each element is a photosensitive film which is a record of an interference pattern originally generated by exposing the film to two overlapping laser beams. Discs made up of circumferential arrays of such elements are potentially much less costly than rotating mirrored drums or oscillating mirror mechanisms, and make it possible to focus scanning beams at different distances.
One available process for making production quantities of multi-element holographic discs requires that a limited number of master discs be prepared. A greater number of copy discs are replicated from each master disc. The master discs are typically made using a silver halide recording material. An interference pattern is recorded in each element of the master disc by interfering a collimated reference beam and a diverging image beam. The angles of the reference beam and image beam are fixed in advance by scanner requirements but are identical to those which are to exist when a copy of the master disc is used in an operating scanner. That is, the angle of the reference beam used in making a master element is the same as the angle of the reconstruction beam which illuminates the copy in an operating scanner. The angle of the image beam used in making a master element is the same angle at which the reconstructed or output beam leaves the corresponding element in a copy in an operating scanner.
When an element in a master disc is formed by interfering the two beams, a series of Bragg surfaces are formed within the recording material. The Bragg surfaces are parallel reflective quasi-planes which extend at an angle generally known as the Bragg angle, to the element surface. The spacing between the planes at the surface of the element is fixed at a distance d in accordance with the grating equation EQU .lambda.=d (Sin .theta.R-Sin .theta.O) Eq. (1)
where
.lambda. is the wavelength of the coherent light beams, PA1 d is the spacing between adjacent Bragg planes measured along the surface of the recording material, PA1 .theta.R is the angle of the reference beam relative to a normal to the recording medium surface, and PA1 .theta.O is the angle of the image beam relative to the normal.
Production quantities of the holographic optical elements are made by placing an unexposed piece of copy material closely adjacent a developed master holographic element and by illuminating the master element with a coherent light beam. When the beam passes through the master element, a part of it is diffracted or bent while the remainder remains undiffracted, passing straight through the element. The diffracted/undiffracted components of the beam interfere in the copy material to form an interference pattern in that material. The interference pattern is fixed or made permanent by processing the copy material. When the processed copy is illuminated with the coherent light beam, the replicated interference pattern is capable of reconstructing the image beam used in generating the master element.
The recording material used in making the master element and the copy elements may be identical. Similarly, the wavelength of the coherent light beams used in making the master elements and copy elements may also be the same. Preferably, however, the master elements are made by illuminating silver halide films with a laser beam having a wavelength in the red range. The interference pattern which is recorded in the silver halide material is fixed by largely conventional photographic processing techniques. The copy material may be a dichromated gelatin material, which is not sensitive to red light. For this reason, the copying is performed with a coherent light beam in the blue or blue-green range. An image is fixed in dichromated gelatin material by washing the material in a series of water and alcohol baths.
One characteristic of dichromated gelatin is that it swells during processing and retains some residual swell normal to the surface after processing and drying. As a result, the recorded Bragg planes become distorted or tilted relative to their orientation at the time of exposure.
If the swelling is ignored and the copy is illuminated with the conjugate of the original reference beam (that is, a beam having the same cross-sectional configuration as the original beam, but being directed in the opposite direction) the copy would still refract part of the beam along the angle .theta.O. However, the diffraction efficiency of the element would be significantly reduced; that is, a greater portion of the beam would pass straight through the element while a lesser portion would be bent along the angle .theta.O. For reasons of scanner performance, it is important that the diffraction efficiency of each copy element be made as high as possible.
Attempts have been made to overcome the problem of reduced diffraction efficiencies due to swell by eliminating the residual gelatin swell through a series of chemical soaking steps. These attempts have only been marginally successful since swelling cannot be completely eliminated. Moreover, the steps are hard to control and the results have been erratic.
Other attempts have been made to solve the problem by changing the angle of the reference beam which is used in making a master to compensate for the Bragg angle tilt resulting from the swelling. Unfortunately, this approach causes distortion of the output beam which is undesirable since it may impact the performance of the scanner.