Luminit (Torrance, CA; www.luminitco.com) has developed a new master replicating technology for Holographic Optical Elements (HOEs) based on concepts borrowed from the printing industry. The many benefits to be gained by replicating optical elements in a thin sheet are clear, but up till now, the cost and complexity of production have hindered wide spread use. Now, Luminit claims to have developed a novel method for forming the master as well as a new UV-based replication process that will enable cost-effective volume production of HOEs for many applications, including displays.

Art Berman
Insight Media Consultant

HOEs are diffractive components that can function like standard lenses, gratings or mirrors. They can collapse a series of optical elements into a thin lightweight sheet and do not require precise surface machining. Typically, HOEs are created by first making a perfect master hologram, which is then replicated to create additional copies. Until now, this has proven to be a difficult and expensive.

Luminit creates a master HOE that is unique in two ways. First, only one light beam is used to make them compared to the standard holographical recording process, which uses two beams. Second, Lumint’s HOEs have randomized surface structures with a modulation transfer function that is determined by the collection of individual lenslets of varying shapes and sizes rather than by a periodic structure (as in the case of bright-enhancement films such as those produced by 3M). The combination of millions of lenslets determines the output profile and optical properties, leading to important characteristics such as wide band operation (300 to 1500 nm) and high transmission. Such HOEs effectively act as high-efficiency, wide-bandpass optical filters. They exhibit nearly zero chromatic aberration, atypical for holographic elements that usually demonstrate high diffraction efficiency only over a narrow wavelength range.

This master HOE is fabricated with photoresist, a material that has been found to be ideal for making large, seamless masters. Surface patterning is then transferred onto the master through a multistep process. The last step in the process is electroforming and results in the production of a cylindrical master made of nickel or copper.

Creating a HOE master without structural discontinuities is a key to fast replication manufacturing. In the process, an epoxy is coated on to a substrate that is subsequently hardened by UV light as the master rolls over it. By choosing an appropriate web speed and UV dosage, replication from the master can be smooth and with minimal defects. In addition, the process can be highly efficient in producing optical-quality films since the transport speed can be high (up to 500 ft/min) and format large (the roll can be 1500-feet long and more than 48-inches wide). Essentially a printing process, this method is superior to other replication alternatives in terms of:

• Lower machinery cost compared to hot embossing
• Less complexity and robustness of the process with respect to custom products formed with extrusion processes
• Smaller equipment size compared to injection molding

The HOEs produced in this manner have high transmission and are well suited for application in liquid crystal backlights and rear projection displays. Functionality includes scattering light from either a coherent or non-coherent source into a design shape so as to hide the source and redirect the light toward the viewer.

Potential advantages gained from the use of HOEs include the simplification and thus the reduction in cost, weight and size of the backlight. All this while providing the equivalent of a brightness enhancement film with hybrid integrated options.

With further commercialization of these HOEs and the applicability of the printing-based manufacturing process to display and a variety of other markets (such as solar cells), the future will likely see further proliferation of this technology and many more HOEs in production.