Using conventional mapping algorithms for the construction of illumination freeform optics’ arbitrary target pattern can be obtained for idealized sources, e.g. collimated light or point sources. Each freeform surface element generates an image point at the target and the light intensity of an image point is corresponding to the area of the freeform surface element who generates the image point.
For sources with a pronounced extension and ray divergence, e.g. an LED with a small source-freeform-distance, the image points are blurred and the blurred patterns might be different between different points. Besides, due to Fresnel losses and vignetting, the relationship between light intensity of image points and area of freeform surface elements becomes complicated. These individual light distributions of each freeform element are taken into account in a mapping algorithm. To this end the method of steepest decent procedures are used to adapt the mapping goal. A structured target pattern for a optics system with an ideal source is computed applying corresponding linear optimization matrices. Special weighting factor and smoothing factor are included in the procedures to achieve certain edge conditions and to ensure the manufacturability of the freefrom surface. The corresponding linear optimization matrices, which are the lighting distribution patterns of each of the freeform surface elements, are gained by conventional raytracing with a realistic source. Nontrivial source geometries, like LED-irregularities due to bonding or source fine structures, and a complex ray divergence behavior can be easily considered. Additionally, Fresnel losses, vignetting and even stray light are taken into account. After optimization iterations, with a realistic source, the initial mapping goal can be achieved by the optics system providing a structured target pattern with an ideal source.
The algorithm is applied to several design examples. A few simple tasks are presented to discussed the ability and limitation of the this mothed. It is also presented that a homogeneous LED-illumination system design, in where, with a strongly tilted incident direction, a homogeneous distribution is achieved with a rather compact optics system and short working distance applying a relatively large LED source. It is shown that the lighting distribution patterns from the freeform surface elements can be significantly different from the others. The generation of a structured target pattern, applying weighting factor and smoothing factor, are discussed. Finally, freeform designs for much more complex sources like clusters of LED-sources are presented.
The concept of multichannel array projection is generalized in order to realize an ultraslim, highly efficient optical system for structured illumination with high lumen output, where additionally the Köhler illumination principle is utilized and source light homogenization occurs. The optical system consists of a multitude of neighboring optical channels. In each channel two optical freeforms generate a real or a virtual spatial light pattern and furthermore, the ray directions are modified to enable Köhler illumination of a subsequent projection lens. The internal light pattern may be additionally influenced by absorbing apertures or slides. The projection lens transfers the resulting light pattern to a target, where the total target distribution is produced by superposition of all individual channel output pattern. The optical system without absorbing apertures can be regarded as a generalization of a fly’s eye condenser for structured illumination. In this case light pattern is exclusively generated by freeform light redistribution. The commonly occurring blurring effect for freeform beamshaping is reduced due to the creation of a virtual object light structure by means of the two freeform surfaces and its imaging towards the target. But, the remaining blurring inhibits very high spatial frequencies at the target. In order to create target features with very high spatial resolution the absorbing apertures can be utilized. In this case the freeform beamshaping can be used for an enhanced light transmission through the absorbing apertures. The freeform surfaces are designed by a generalized approach of Cartesian oval representation.
Faceted freeform reflectors were designed for intelligent street lighting with LED cluster arrays for main traffic roads.
Special attention was paid to achieve highly efficient illumination on both wet and dry road surfaces. CIE reflection
tables W4 and C2 were applied in the simulation for these two conditions, respectively. The reflector design started with
plane facets, then - to avoid artifacts from the images of the individual LEDs - plane facets were replaced with
cylindrical facets. To get even more flexibility for the design and optimization, freeform facets were employed, modeled
by extruding two different conic curves together. Besides of achieving well-proportioned road luminance distribution,
the basic shapes of the reflectors were formed to control stray light caused by multiple reflections within the reflector
and by reflection of light from neighbor clusters within the cluster array. The merit functions include useful transmission
of light to the road as well as overall and lengthwise uniformity according to road illumination standards. Due to the
large amount of variables, the optimization was carried out sequentially facet by facet. The design loops included
compromising with manufacturing limitations for plastics molding and thorough analysis of conformity with DIN EN
13201 standards for ME road lighting classes. The calculated reflector profiles are realized by plastic injection molding.
Freeforms in illumination systems are directly constructed by adapting some ideas of Oliker and co-workers . The
freeform is created by a set of primitive surface elements which are generalized Cartesian ovals including the optical
response of the residual system. Hamiltonian theory of ray optics can be used to determine the family of primitives
which is in particular a simple task if the freeform is the exit surface of the illumination system. For simple optical
systems an analytical description of the primitives is possible. Contrarily, for more complex optics a conventional raytracer
is additionally utilized to determine the required system's information, like the optical path lengths or mixed
characteristics. To this end a discrete set of rays is traced through the residual systems and the required relations are
interpolated to obtain a quasi-analytic representation of the primitives. The potential of this approach is demonstrated by
some examples, e.g. freeform optics including collimating or deflection elements.