Keen forms of kinoforms

One of the most intriguing features of LED lighting systems is their ability to create virtually any lighting pattern desired, with optical efficiencies in excess of 80%. This unique characteristic results from the combination of efficient collimating optics and tailored microstructures. In many cases, LED lighting systems can use approximately 50% of the light of conventional systems and yet do the same lighting job, a fact frequently overlooked in discussions of the cost effectiveness of LED lighting systems. Because LEDs are typically less than 1 mm on a side and do not have a large glass envelope surrounding their die, it is straightforward to create a non-imaging optic approximately 1.5 in. in diameter that will highly collimate virtually 100% of the light leaving the LED. Once light is transformed into a tight collimated beam, secondary optical elements can be integrated into the lighting system to generate any output pattern and uniformity that one could desire.

Invented by IBM in 1969, a kinoform is a computer-generated wavefront that, like a hologram, generates a 3-D image.¹ Kinoforms operate only on the phase of an incident wave, being based on the assumption that only the phase information in a scattered wavefront is required for the construction of an image of the scattering object.

A number of industrial researchers during the 1970s, led by H. John Caulfield, showed that kinoform diffusers could be either volume- or surface-relief structures and recorded using only optical means, eliminating the IBM computer-generated wavefront.^2-10 Caulfield also showed that kinoform diffusers were highly efficient in either transmission or reflection and that they could be designed to completely tailor the angular distribution of their output spectrum.⁴ This meant that it was now possible to take a collimated beam of light and, by passing it through a kinoform diffuser, create any pattern desired, including letters, or even company logos.

The first use of these kinoform diffusers for lighting applications concerned uniformly lighting an area with a light source. This homogenization of the output beam is particularly useful for LED lighting systems.

In general, kinoform diffusers provide very efficient and achromatic forward scattering of light. They exhibit very low backscatter and absorption. The angular scattering pattern can range from perfectly symmetric to highly asymmetric.

Kinoform diffusers achieve forward scattering of 88 to 92% for surface relief designs and 92% for volume designs (without antireflection coatings). In comparison, conventional diffusers offer forward scattering efficiencies in the range of 40 to 80%, depending upon the particular type of conventional diffuser and the fabrication technique. This efficiency constitutes perhaps the most significant advantage of kinoform diffusers.

Kinoform diffusers can be made to scatter in various aspect ratios.

Designers can match the angular extent of the forward-scattering pattern to the specific application. Typically, kinoform diffusers can scatter incident light into cone angles exceeding 90° or less than 3°; furthermore, the scattering lobe remains stationary relative to the kinoform diffuser for all angles of incidence that lie within the scattering lobe. This is very different from the operation of a conventional diffuser in which the output lobe changes direction in direct correspondence to the change in angle of the incident beam.

The angular extent of the forward-scattering lobe can be made circular, as in conventional diffusers, or highly asymmetric (see figure). One application that uses a scattering lobe of high aspect ratio (approximately 20° vertical x 90° horizontal) is a highly efficient TV projection screen. In such applications, designers typically want more horizontal than vertical diffusion of the projected light in order to more closely match the field of view of the audience.

In addition to controlling the size and aspect ratio of the forward-scattering lobe, we can position it away from the surface normal for incoming normal incidence light. This attribute is particularly crucial, for example, to maximize viewing brightness in certain automotive or cockpit displays for which the usual viewing position is significantly off normal. Kinoform diffusers can also scatter incident light into more than one preferred direction.

The kinoform diffusers discussed above all operate in transmission but reflective kinoform diffusers exist. Kinoform diffusers exhibit highly achromatic performance, making them suitable for very broadband light sources such as fluorescent lamps for LCD backlighting. They can also be used directly with white-light LEDs, since they do not cause color breakup in the transmitted beam. oe

References

1. L. Lesem, P. Hirsch, et al., IBM J. Res. Develop., March (1969).

2. H. Caulfield, SPIE 25, Development in Holography (1971).

3. "Uniform Area Illumination Using Kinoforms," IBM Technical Disclosure Bulletin, August (1971).

4. H. Caulfield, Opt. Comm., 4[3], (1971).

5. K. Biedermann, F. Bestenreiner, "Method of Making a Diffusing Layer," U.S. Pat.# 3,619,021 (1971).

6. F. Bestenreiner, W. Weierhausen, et al., "Method and Arrangement for Manufacturing Diffusion Lenses of Arbitrary Indicatrix," U.S. Pat.# 3,698,810 (1972).

7. D. Meyerhofer, Appl. Opt., 12, 2180-2184 (1973).

8. P.F. Gray, Optica Acta, 25, 765-775 (1978).

9. "Light Diffuser with Controlled Divergence," IBM Technical Disclosure Bulletin, 276-279 (1986).

10. J. Tedesco, L. Brady, et al., 29-32, SID Digest (1993).