Programmable freeform optics with extended white light sources: possibilities and limitations.

Freeform optics can be used in lighting applications to generate accurate prescribed illumination patterns from compact light sources such as LEDs. When targeting dynamic illumination systems, a time-variable optical functionality is needed. Phase-only spatial light modulators (SLMs) have been used in the past for various dynamic beam shaping applications with monochromatic, zero-étendue illumination under paraxial conditions. Such limitations can no longer hold when considering lighting applications. In this paper, a novel algorithm for the calculation of smooth phase shift patterns is presented in order to generate arbitrary target patterns from arbitrary incident wave fronts for non-paraxial conditions. When applying such phase shift patterns to SLMs, these devices can be considered as programmable freeform optics. The experimental performance of the calculated phase patterns is analyzed on a real SLM, with a maximal phase shift of 6π, for collimated laser beams and white LEDs. The possibilities and limitations of generating accurate prescribed target patterns are critically discussed in terms of the angular extent of the target pattern, the consider spectrum of the light source and the étendue of the incident light beam.

Freeform Fresnel lenses with a low number of discontinuities for tailored illumination applications.

Most work in the field of freeform lens design has been focused on finding design algorithms for continuous freeform lens surfaces which transform an arbitrary ingoing light distribution into an arbitrary outgoing distribution. The shape of the resulting continuous lens surfaces depends fully on the source and target light distribution for which the lenses are tailored. In some cases this results in large, voluminous optical components which depending on the application are not practical. Fresnel lenses can have a much smaller volume, but are not straightforward to design in the case of freeform lenses. This paper demonstrates a new method to design freeform Fresnel lenses based on concentric freeform segments. Such lenses have a much lower number of discontinuities compared to already existing Fresnel-type freeform lenses which are based on an array of facets. Less discontinuities means less stray light due to the unavoidable rounding errors with current manufacturing processes. The new design method is first explained, and then illustrated for a freeform Fresnel lens with a rectangular target distribution in the far-field.

Luminance spreading freeform lens arrays with accurate intensity control.

Glare and visual discomfort are important factors that should be taken into account in illumination design. Conventional freeform lenses offer perfect control over the outgoing intensity distribution, thereby allowing optical radiation patterns with sharp cut-offs in order to optimize the unified glare rating index. However, these freeform lenses do not offer control over the near-field luminance distribution. Observing the emitted light distribution from a high-brightness LED through a freeform lens gives a high peak luminance that can result in glare. To reduce this peak luminance, freeform lenses should be used in conjunction with light diffusing structures. However, this diminishes the control over the outgoing intensity distribution what is the main benefit of a freeform lens. Another approach to reduce the observed peak luminance, is by spreading the emitted light over multiple optical channels via freeform lens arrays. This paper proposes a novel method to design luminance spreading freeform lens arrays that offer perfect control over the resulting intensity pattern. The method is based on a non-invertible mapping of a 2D parameter space. This results in a source-target mapping in which multiple ingoing ray directions are mapped onto every position of the target distribution. The case of continuous and discontinuous mappings are both discussed in this paper. Finally, the example of a discontinuous freeform lens array with 7×7 individual lenses is designed and experimentally demonstrated.

Ray mapping method for off-axis and non-paraxial freeform illumination lens design.

The ray mapping method for freeform illumination design is an easy and flexible method, but only in the paraxial regime does it result in surface normal fields that are directly integrable into continuous freeform surfaces that provide the desired illuminance distribution. A new mapping scheme is proposed to alter an initial source-target mapping via a symplectic flow of an equi-flux parametric coordinate system. The resulting mapping provides integrable surface normal vector fields for complex off-axis and non-paraxial illumination problems, as demonstrated by two freeform lens examples.

Flexible design method for freeform lenses with an arbitrary lens contour.

A method is presented that allows the design of freeform lenses with an arbitrary contour in a flexible and robust manner. The method is based on the generation of two equi-flux grids representing the source and target beams, with two separate curl-free mappings from an equi-spatial rectangular grid. Because the source and target grids are generated independently from one another, one can map arbitrary complex source beams with certain contours onto arbitrary complex target beams within other contours with high convergence probability. The method is illustrated by calculating a triangular freeform lens that reshapes a triangular beam from a Lambertian source into a uniform pentagonal irradiance distribution on a target plane.