Abstract:
The optical diffuser mastering of the subject invention includes legacy microstructure surface relief patterns, along with smaller ones, overlaid on the larger ones. The characteristic features produced by the present invention will be found useful to eliminate visible structures in/on optical diffusers, such as those used in movie projection screens (utilizing either coherent (i.e., laser-generated) and non-coherent (e.g., lamp-generated) light), head-up displays (HUDs), laser projection viewing, etc., as the present invention produces much sharper images than those afforded by traditional holographic optical diffusers.

Description:
FIELD OF THE INVENTION 
     The present invention is in the field of optics. In particular, it is in the area of optical diffusers. 
     BACKGROUND OF THE INVENTION 
     The prior art in holographic optical diffusers, which can be found in U.S. Pat. Nos. 6,675,863, 7,700,199, and 8,097,311, all of which are incorporated in their entirety by reference, comprises three-dimensional (3-D) surface relief patterns produced by optically recording laser speckle in thick (10 microns to 60 microns) photoresist material. 
     Referring to  FIG. 1 , an optical bench setup  100  capable of recording prior art holographic optical diffuser(s) in planar format, may be used as the starting point for improvements that comprise the present invention. 
     Referring to  FIG. 2 , an optical bench setup  200  capable of recording prior art holographic optical diffuser(s) in cylindrical (seamed and seamless) format, may be used as the starting point for improvements that comprise the present invention. 
     SUMMARY OF THE INVENTION 
     This invention describes a method to produce high frequency structures on top of classical holographic optical diffusers using modified holographic optical recording setups. 
     This innovation in optical diffuser mastering includes legacy microstructure surface relief patterns, along with smaller ones, overlaid on the larger ones. The characteristic features produced by the present invention will be found useful when viewers do not want to see visible structures in/on optical diffusers, such as those used in movie projection screens (utilizing either coherent (i.e., laser-generated) and non-coherent (e.g., lamp-generated) light), head-up displays (HUDs), liquid crystal displays (LCDs), non-motion picture laser projection viewing, etc., as the present invention produces much sharper images than those afforded by traditional holographic optical diffusers. Additionally, it can reduce glare, reduce Moiré artifacts, and add a specific, controllable amount of haze. As a result, it positively impacts the experience of end users. 
     Therefore, the present invention reduces the visibility of optical artifacts visible with the unaided human eye at working distances relevant to particular application of said optical diffuser. Alternately, a high-fidelity replica of said optical diffuser master may be used in any particular target application with substantially the same benefits as the optical diffuser master of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of prior art equipment setup used to create planar (i.e., flat) optical holographic diffuser masters. 
         FIG. 2  is an illustration of prior art equipment setup used to create cylindrical (seamed and seamless) holographic optical diffuser masters. 
         FIG. 3  is an illustration of one preferred embodiment the present invention, as it would be used in practice, to produce composite holographic optical diffusers in a planar format. 
         FIG. 4  is an illustration of another preferred embodiment the present invention, as it would be used in practice, to produce composite holographic optical diffusers in a planar format. 
         FIG. 5  is an illustration of one preferred embodiment the present invention, as it would be used in practice, to produce composite holographic optical diffusers in a cylindrical (seamed and seamless) format. 
         FIG. 6  is an illustration of another preferred embodiment the present invention, as it would be used in practice, to produce composite holographic optical diffusers in a cylindrical (seamed and seamless) format. 
         FIG. 7  is a chart (plot) of the superposition of the lower-frequencies produced by the base recording and the higher-frequencies produced by the overlay recording. 
         FIG. 8  is a photomicrograph of a holographic optical diffuser with only the lower-frequency surface relief microstructures. 
         FIG. 9  is a photomicrograph of a preferred embodiment of the present invention with higher-frequency surface relief microstructures superimposed upon the lower-frequency surface relief structures shown in  FIG. 7 . 
         FIG. 10  is the photomicrograph of  FIG. 9  without the individual surface relief structure callouts (for visual clarity). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention, a composite holographic optical diffuser and method of fabrication thereof, comprises a composite surface relief structure, which acts as a suppressor to visible optical artifacts—in particular, undesirable ones. 
     The present invention does this by overlaying smaller (higher spatial frequency) microstructures on top of lower-frequency ones by either of the following: 
     Method 1 
     Performing a first exposure with larger microstructure projections (about 5-100 microns) upon a target substrate coated with photoresist recording media followed by a second exposure with smaller microstructures (about 0.5-3 microns) projected upon the same substrate (i.e., on top of the larger microstructures). The photoresist can be either developed or left undeveloped between these two exposures. 
     Method 2 
     Placing a partial mirror between the mid-diffuser and the target substrate coated with photoresist recording media, with the partial mirror close (less than 4 inches and preferably 0.25 inches to 3 inches) to the target substrate, so as to produce a substantial (i.e., recordable) re-reflection of the part of the original recording light that is reflected from the recording media. 
     Referring to  FIG. 3 , the present invention is retrofittable to existing equipment  300  used to fabricate planar holographic optical diffuser masters by introducing a fixture (Method 1) to move the mid-diffuser  305  much closer less than 4 inches (and preferably 0.25 inches to 3 inches, to the target recording media (and substrate) than is done in prior art setups and performing a second exposure. The setup comprises a laser  301 , its raw laser beam  302 , lensing  303 , refracted laser beam  304 , a mid-diffuser  305 , an aperture  306 , a patterned laser beam ( 307  for the first exposure and  314  for the second exposure), a target substrate  308  mounted to a support structure  309  and an x-y translation stage  310  mounted upon an optical bench  311  supported by vibration-isolation elements  312 . 
     Referring to  FIG. 4 , the present invention is retrofittable to existing equipment  400  used to fabricate cylindrical optical holographic optical diffuser masters by introducing a partial mirror  413  between the mid-diffuser and the target substrate coated with photoresist recording media (Method 2), with the partial mirror  413  close (about preferably 3 inches) to the target substrate, so as to produce a substantial (i.e., recordable) re-reflection  415  of the part of the original recording light that is reflected from the recording media. The setup comprises a laser  401  its raw laser beam  402 , lensing  403 , refracted laser beam  404 , a mid-diffuser  405 , an aperture  406 , a patterned laser beam  407  and  414 , a target substrate  408  and an x-y translation stage  410  mounted upon an optical bench  411  supported by vibration-isolation elements  412 . 
     Referring to  FIG. 5 , the present invention is retrofittable to existing equipment  500  used to fabricate seamed and seamless holographic optical diffuser masters by introducing a fixture (Method A) to move the mid-diffuser  505  much closer about 0.25 inches to 4 inches and preferably 3 inches, to the target recording media (and substrate) than is done in prior art setups and performing a second exposure. The setup comprises a laser  501  its raw laser beam  502 , lensing  503 , refracted laser beam  504 , a mid-diffuser  505 , an aperture  506 , a patterned laser beam  507 , a target substrate  508  and an x translation stage with rotation stage  510  mounted upon an optical bench  511  supported by vibration-isolation elements  512 . 
     Referring to  FIG. 6 , the present invention is retrofittable to existing equipment  600  used to fabricate cylindrical optical holographic optical diffuser masters by introducing a partial mirror  613  between the mid-diffuser and the target substrate coated with photoresist recording media (Method 2), with the partial mirror  613  close (less than 4 inches and preferably 3 inches) to the target substrate, so as to produce a substantial (i.e., recordable) re-reflection  615  of the part of the original recording light that is reflected from the recording media. The setup comprises a laser  601  its raw laser beam  602 , lensing  603 , refracted laser beam  604 , a mid-diffuser  605 , an aperture  606 , a patterned laser beam  607  and  614 , a target substrate  608  and a rotating cylinder mounted on a x-axis translation stage  610  mounted upon an optical bench  611  supported by vibration-isolation elements  612 . Mask  616  prevents undesired double exposure. 
     Referring to  FIG. 7 , a plot  700  of the composite structure  701  comprising the overlay of high spatial frequency signals  703  upon lower spatial frequency ones  702  is shown. In this example, the size of the higher optical frequency signal is about 1 micron, and the lower optical frequency signal is about 10 microns. 
     Referring to  FIG. 8 , a photomicrograph of a legacy (prior art) diffuser  800  produced with only the lower frequency structures (approx. 20 microns×10 microns in this example)  801 . 
     Referring to  FIG. 9 , a photomicrograph of the composite diffuser of the present invention, one can see the higher spatial frequency (approx. 5 micron×8 micron) structures  902  superimposed upon the lower ones  901 . 
     Referring to  FIG. 10 , the photomicrograph of  FIG. 9  is shown without the individual microstructures called out (for visual clarity). 
     Method of Production 
     Basic Exposure Process to Produce Primary LSD Structure 
     Equipment 
     
         
         
           
             1) Optical table 
             2) Laser 
             3) Substrate with photo-resist material 
             4) Optical shutter 
             5) Lenses—Spherical, Aspherical and Cylindrical 
             6) Aperture 
             7) Mid Diffuser 
             8) Motion Stage 
             9) Lens holders, posts, post holders, rails, etc.
 
Procedure:
 
             1) Layout the optical elements onto the optical table as shown in  FIG. 1 . Exact layout will depend upon angle desired. 
             2) Measure the spot size at the Substrate. 
             3) Calculate the amount of movement to get uniform angle. 
             4) Calculate exposure (time and laser power) per shot to form the correct angle. 
             5) Program the stage and shutter according to the calculations. 
             6) Run the program and leave the room to reduce noise in the recording. 
             7) When the program is finished, remove the substrate and develop the photo-resist. 
           
         
       
    
     Method for Overlaying Secondary High Frequency Structure 
     Method 1 
     
         
         
           
             1) Expose plate as previously explained.
           a. Method 1 can be applied prior to development step.   b. Method 1 can be applied after development step.   
         
             2) Set up high frequency optical layout. The mid diffuser is set much closer to the recording substrate (0.1″ to 2″), the full width half maximum (fwhm) spot size on the mid diffuser ideally is the same or larger than the distance to the substrate. The diffraction features (minimum size) recorded will be approximately 1.3*wavelength*distance/(spot size on mid diffuser).
           a. If recording on a flat substrate, the mid-diffuser can be flat.   b. If recording on a cylindrical substrate (as with seamless recording), the mid-diffuser can be cylindrical in order reduce the apparent distance to the substrate.   
         
             3) Develop normally. 
           
         
       
    
     Method 2 
     
         
         
           
             1) Set up the optics the same as the basic setup. 
             2) Once the basic set up is done, place an additional optic:
           a. Partial mirror that can also be a flat window (0.1″ to 5″) in front of the recording substrate for flat substrate.   b. Cylindrical window (0.1″ to 5″) concentric to the recording substrate  FIG. 2 .   
         
             3) Follow the exposure process. 
             4) Develop normally. 
           
         
       
    
     This method will produce features that are smaller than 1 micron. The substructure tends to be fractal in nature. Method 2 can only be applied with a single exposure; i.e., optical tiling will not work since the high frequency tends to be blurred out in the process.