Tools of the trade - Vol 18 - Thorlabs - #651

from an Engineered Diffuser™ illuminated with a coherent helium-neon laser beam is shown in Fig. 3. For a source of different wavelength the same profile is obtained with a slight variation in divergence angle due to the change in index of refraction of the microlens material. It is also possible to generate more complex scatter patterns usually attainable only with diffractive elements. Figure 3 also illustrates the measured intensity distribution from an Engineered Diffuser™ that creates a square pattern. This achromatic pattern further illustrates the flexibility provided by the concept of Engineered Diffusers™.

see the raw beam and the beam modified by an Engineered Diffusers™. As illustrated, it is possible to direct most of the light into specific regions of space where the light is actually used, thereby increasing the efficiency of light usage. These targeted light distributions will also be relevant in reducing light pollution and improving the quality and effectiveness of displays by creating effects that would otherwise be impossible to achieve with the raw beam alone or with simple diffusers. Also, Engineered Diffusers™ can be used to mix light from RGB arrays with controlled color temperature by controlling the content of the red, blue, and green components. At the same time the Engineered Diffuser™ shapes the mixed light distribution. An example is shown in Fig. 5 where an RGB array is mixed and shaped into an oval scatter distribution with wider divergence in one direction.

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irrespective of the divergence angles. The technology that allows fabrication of Engineered Diffusers™ requires processing of thick photoresist materials in the range of 50 to 160 microns, much thicker than commonly used in the lithography industry. To produce a master, RPC Photonics has developed a laserwriting system² that exposes thick photoresist point-by-point in a raster scan mode with a small focused beam, as illustrated in Fig. 2. The laser beam is modulated as it traverses the coated substrate. Upon development, a deep continuous surface is obtained such as those shown in Fig. 1.

The advanced diffusion and beam shaping capabilities of Engineered Diffusers™ make these devices suitable for most applications where a homogeneous light distribution is required with a specified intensity profile and spatial distribution. Whether it is laser illumination, projection and display systems, or solid-state lighting, Engineered Diffusers™ greatly extend the very concept of light diffusion and beam shaping by providing capabilities previously unavailable with any other diffusion technique.

An important emergent application where Engineered Diffusers™ should play a pivotal role is that of solid-state lighting. It is projected that in the near future LED-based illumination systems will become more and more pervasive in a wide variety of situations such as displays, signage, architectural lighting, and general illumination. Some of the advantages of LED sources include cost, lifetime, and energy savings, thus the considerable interest in developing these systems both in government and industry. Engineered Diffusers™ address an important aspect of the overall LED development effort, that is, the diffusion and control of LED illumination with the additional advantage of efficient distribution of the available luminous energy. In typical cases an Engineered Diffuser™ can improve the efficiency of solid-state light utilization by a factor of 2 or more.

Examples of beam shaping with LED sources are shown in Fig. 4, where one can

An application of particular interest is the shaping and homogenization of laser beams. Most diffuser technologies have little difficulty generating, for example, diffusion patterns with round shape. However, in most cases the pattern has non-uniform Gaussian intensity profiles (such as the case of ground glass and holographic diffusers) or small divergence angles with some degree of zero order (such as diffractive diffusers). Periodic microlens arrays cannot generate round diffusion and the closest it can come to a circle is by means of a hexagonal array. Engineered Diffusers™ however can not only generate round diffusion with uniform intensity distribution over any angular range but also do it independently of wavelength. An example of a measured intensity profile

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¹ Tasso R. M. Sales, High-contrast screen with random microlens array, US Patent # 6,700,702.

² D. H. Raguin, G. Michael Morris, and P M. Emmel, Method for making optical microstructures having profile heights exceeding fifteen microns, US Patent #6,410,213 B1.

Engineered Difussers™ is the trademark of RPC Photonics

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