In digital projector design, when we want to display a still or video image where the digital source is uniform in radiance, we want the corresponding projected image to be uniform in irradiance on the screen. In order to achieve this uniformity of irradiance of the projected image we need to have the spatial light modulator, such as an LCD panel, uniformly illuminated. The uniform illumination at the spatial light modulator plane cannot come directly from the light source because the irradiance profile of the source from the lamp assembly is (typically) a Gaussian type irradiance profile. We must somehow “degauss” this irradiance profile or spatially transform it from nonuniform to uniform irradiance profile.  This can be accomplished with a pair of fly’s eye array spatial light integrators and we will take a look at how these devices work in this article.

What Is A Fly’s Eye Array?

A fly’s eye array is a two dimensional array of individual optical elements assembled or formed into a single optical element and used to spatially transform light from a nonuniform distribution to a uniform irradiance distribution at an illumination plane.  In digital projectors that use fly’s eye arrays they are almost always used with lamp assemblies with a parabolic reflector that provides semi collimated light.  At the present time they are mostly used in LCD digital projector light engines in the illumination section to deliver spatially uniform or homogenized illumination to the spatial light modulator illumination plane. 

The fly’s eye array can be seen in the above figure.  This photograph is provided courtesy of In Vision, www.in-vision.at. Each of the individual optical elements in the array can be square or rectangular in shape.  The surface shape of individual optical elements can be spherical or anamorphic (different optical power in the vertical and horizontal meridians) and the optical power is typically only on one surface of the array,  the second surface being most often plano. 

In terms of modeling these components in ZEMAX, probably the easiest way is to use the Lenslet Array1 object. A Lenslet Array 1 object consists of an array of rectangular volumes, each with a flat front face and a user-definable number of repeating curved surfaces. The array surface may be plane, sphere, conic, or polynomial asphere; or a spherical, conic, or polynomial aspheric toroid. This allows great flexibility in defining, and optimizing, the precise surface shape of the lens elements in the array.

The above graphic shows a single Lenslet Array 1 object, which comprises a 7 x 5 array of rectangular lenses, each of which is a rectangular section of a spherical lens. Other objects which may be useful for this application include the Lenslet Array 2 object and the Hexagonal Lenslet Array object. Note that any object can be replicated and placed on an array easily using Tools -> Replicate Object.

Lens arrays are also supported in sequential optical design, via the user-defined surface capability. Samples are provided for arrays of spherical, conic aspheric, even-aspheric and cylindrical lens arrays.