From sophisticated deep-UVsteppers to indoor-outdoorlighting applications, passing

through rear-projection televisionscreens and consumer electronic dis-plays, one finds a common opticalcomponent: diffusers.

These devices homogenize and dis-tribute light over a specified region ofspace. Diffusers are an example ofa more general class of componentsknown as beam shapers, whose mainpurpose is to alter and project aninput beam into a useful angularrange with controlled intensity dis-tribution.

The advanced diffusion and beam-shaping capabilities of a new classof diffusers make them suitable formost applications that require a ho-mogeneous light distribution with aspecified intensity profile and spa-tial distribution. These applicationsinclude laser illumination, projec-tion and display systems, and solid-state lighting.

Commonly used diffuser tech-nologies include prismatic glass in-tegrating bars, ground glass, opalglass, holographic diffusers and dif-fractive diffusers. Prismatic glass in-tegrating bars, though sometimesused in high-end systems, are verylimited in capability, are expensiveand occupy a great deal of preciousspace. Ground and opal glass scat-ter light equally in all directions buthave limited light-control capabili-ties. These simple diffusers also oftenoffer poor efficiency.

Reprinted from the June 2004 issue of PHOTONICS SPECTRA © Laurin Publishing

®

Worldwide Coverage: Optics, Lasers, Imaging, Fiber Optics, Electro-Optics, Photonic Component Manufacturing

Engineered microlens arrays provide new control for display and lighting applications.

by Tasso R.M. Sales, Stephen Chakmakjian, G. Michael Morris and Donald J. Schertler, RPC Photonics Inc.

Figure 1. Close-packed arrays of accurately and independently producedmicrolenses offer some advantages forprojection screen applications (top) anddisplay brightness enhancement (bottom).

Light Tamers

A holographic diffuser, a stepahead of ground glass, enables theproduction of simple light distribu-tion patterns. However, it has lim-ited control over the light distribu-tion pattern, generally making onlyround or elliptical patterns withnonuniform intensity variation, typ-ically of a Gaussian nature.

Diffractive elements can, at leastin theory, shape an input beam ar-bitrarily. They are, however, mostlylimited to monochromatic applica-tions with coherent light sources.Fabrication requirements limit dif-fractives to narrow diffusion angles.These elements also can be stronglysensitive to input beam variations,and they present the well-knownproblem of zero order: a bright spotcollinear with the incident beam. In many applications, the zero orderis unacceptable, and the requirementof single-wavelength operation veryrestricting.

New technologyA new diffuser and beam-shaping

technology can challenge some ofthese drawbacks and provide signif-icant performance enhancements forapplications such as lithographicsystems, efficient solid-state light-ing, backlighting, projection screensand displays. This concept, trade-marked as Engineered Diffusers, dif-fers from other technologies in nu-merous ways.

Unlike random diffusers such asground glass, opal glass and holo-graphic elements, these devices con-trol each scatter center, generallywith microlens units. Holographicdiffusers, for instance, can be seenas a random arrangement of lenslets.However, the lenslike features arecreated by the holographic exposureprocess and are controlled only in a statistical sense: Individual mi-crolens units are not individual-ly manipulated, which helps to ex-plain why holographic diffusers can-not control light distribution andprofile.

An Engineered Diffuser individual-ly specifies each microlens unit withrespect to its sag profile and its lo-cation in the array. Furthermore, toensure that the diffuser can handleinput beam variations and does notproduce diffraction artifacts, the

Beam Shapers

Beam Scan

Modulated Laser Exposure

Photoresist~50 to 160 µm Thick

Substrate

Figure 2. To manufacture the diffusers, a modulated laser exposes a thick photoresist point by point in a raster scan mode with a small focused beam.

Figure 3. The measured intensity profile from an Engineered Diffuser illuminatedwith a coherent helium-neon laser beam shows wide, round scatter with a flat intensity profile (top) and square scatter with a center hole (bottom). A source of different wavelength produces the same profile with a slight variation in divergenceangle because of the change in the microlens material’s index of refraction.

distribution of microlenses is ran-domized according to probability dis-tribution functions chosen to imple-ment the desired beam-shaping func-tions. Therefore, this device retainsthe best properties of both randomand deterministic diffusers.

Deep, continuous surfaceMicrolens arrays have provoked

continual interest as fabrication ca-pabilities have matured over theyears. Some of their applications,such as Shack-Hartmann interfer-ometry, laser-to-fiber coupling andoptical switching for telecommuni-cations, have little to do with lightdiffusion. Still, periodic microlensarrays work in illumination systemswith a condenser lens by relying onthe source’s low coherence and con-volution effects.

Periodic microlens arrays are not,strictly speaking, diffusers, and theyprovide limited beam-shaping capa-bilities. Even so, most rear-projec-tion TV screens still use them; e.g.,lenticular (cylindrical) arrays. Toeliminate artifacts from the periodic

nature of the array, screen manu-facturers add volume and surfacediffuser components to homogenizethe scatter and to add vertical gainat the cost of reduced resolution andspeckle.

At the heart of the Engineered Dif-fuser technology is the capability tomanufacture a close-packed array ofaccurately and independently pro-duced microlenses, each designed toa unique lens prescription and spa-tially distributed in an arbitrary fash-ion (Figure 1). The array maintains a100 percent fill factor even with vari-ations in the shape of each lensboundary. Furthermore, the manu-facturing process can control the di-ameter of each microlens unit andthe relative vertical position of eachmicrolens (piston).

Manufacturing these devices in-volves processing photoresist mate-rials about 50 to 160 µm thick, muchthicker than commonly used in li-thography. A laser system1 exposesthick photoresist point by point in araster scan mode with a small fo-cused beam (Figure 2). The system

modulates the laser beam as it tra-verses the coated substrate. Develop-ing the photoresist produces a deep,continuous surface.

Despite the innovative processesused in making the diffusers, thecost per piece is comparable to thatof components that rely on conven-tional photolithographic fabricationtechniques. The main cost item con-stitutes the production of the initialphotoresist master, in which a laserwriting session can take from lessthan an hour to several days, de-pending on the required pattern size.The master can then be transferredto more durable material with meth-ods such as etching, replication ornickel electroforming, as dictated bythe application. From this point,techniques such as embossing or in-jection molding can be used to pro-duce volume quantities at costs com-parable to those of other diffusercomponents on the market.

Displays and lasersSuch a microlens array could work

in projection screens and enhance

Beam Shapers

Figure 4. Diffusers based on new technology perform beam shaping and homogenization of LED light sources.

display brightness. Projection screenapplications are particularly chal-lenging because they require wideangle divergence — 120° full width athalf maximum — in one direction(horizontal) and narrow divergence— 40° FWHM — in the other (verti-cal), with controlled intensity pro-files. Because it is a visual applica-tion, a projection screen cannot in-troduce image or color defects andshould not introduce speckle. Also,the screen geometry must incorpo-rate a black-absorbing material toprevent ambient light from reflect-ing to the viewer, thus maximizingimage contrast. Illumination fromthe light engine transmits throughapertures created by the focusing el-ements of the screen itself.

The Engineered Diffuser enablesthe design of screens2 that meet thedesired intensity profile within thefield of view. Also, it can control thenear-field distribution of focus spotsto maximize the coverage of blackmaterial and to improve contrast.Finally, controlling the size of the mi-crolens units with regard to the lightengine’s coherence properties avoidsspeckle while maintaining high res-olution.

Controlling individual microlenselements and arbitrarily distributingthem in the array enables advancedbeam-shaping and features that arenot possible with diffractives or anyother diffuser technology. For in-stance, these devices perform equallywell under monochromatic or broad-

band illumination with no zero-orderor diffraction artifacts, irrespectiveof the divergence angles.

An application of particular inter-est is the shaping and homogeniza-tion of laser beams. Most diffusertechnologies have little difficulty gen-erating, for example, round diffusionpatterns. However, in most cases,the pattern has nonuniform Gaus-sian intensity profiles (such as thecase of ground glass and holographicdiffusers) or small divergence angleswith some degree of zero order (suchas diffractive diffusers). Periodic mi-crolens arrays cannot generate rounddiffusion, but a hexagonal array canapproximate a circle.

Engineered Diffusers, however, cangenerate round patterns with uni-form intensity distribution over anyangular range, independent of wave-length (Figure 3). They also can gen-erate more-complex scatter patternsthat are usually attainable only withdiffractive elements.

LED effortsAn important emergent applica-

tion for these diffusers is in solid-state lighting. LED-based illumina-tion systems will become more per-vasive in applications includingdisplays, signage, architectural lighting and general illumination.Some of the advantages that LEDsources offer are longer lifetimes, andcost and energy savings.

Engineered Diffusers address thediffusion and control of LED illumi-

nation with efficient distribution ofthe available luminous energy. Intypical cases, the technology can improve the efficiency of solid-statelight utilization by a factor of two ormore (Figure 4).

For example, they can direct mostof the light into specific regions ofspace, thereby increasing the effi-ciency of light usage. These targetedlight distributions will also be rele-vant in reducing light pollution andimproving the quality and effective-ness of displays by creating effectsthat would otherwise be impossibleto achieve with the raw beam aloneor with simple diffusers. Also, thediffusers can mix light from RGB ar-rays, managing the color tempera-ture by controlling the content of thered, blue and green components,while shaping the mixed light distri-bution (Figure 5). G

References1. D.H. Raguin, G. Michael Morris and

P.M. Emmel. Method for making opti-cal microstructures having profileheights exceeding fifteen microns. USPatent No. 6,410,213 B1.

2. Tasso R.M. Sales. High-contrast screenwith random microlens array. USPatent No. 6,700,702.

Meet the authorsTasso R.M. Sales is vice president of

R&D at RPC Photonics Inc. in Rochester,N.Y. Stephen Chakmakjian is chief tech-nical officer, G. Michael Morris is CEOand Donald J. Schertler is senior opticalscientist at the company.

Beam Shapers

Figure 5. An Engineered Diffuser simultaneously mixes an RGB LED array (left) and shapes the light into an oval scatter distribution with wider divergence in one direction (right).