Harald Ries

Tailored freeform optical surfaces

Freeform optical surfaces embedded in three-dimensional space, without any symmetry, are tailored so as to solve the archetypal problem of illumination design: redistribute the radiation of a given small light source onto a given reference surface, thus achieving a desired irradiance distribution on that surface. The shape of the optical surface is found by solving a set of partial nonlinear differential equations. For most cases, a few topologically distinct solutions exist, given suitable boundary conditions.

Edge-ray principle of nonimaging optics

The edge-ray principle of nonimaging optics states that nonimaging devices can be designed by the mapping of edge rays from the source to the edge of the target. However, in most nonimaging reflectors, including the compound parabolic concentrator (CPC), at least part of the radiation undergoes multiple reflections, some rays even appear to be reflected infinitely many times, and closer examination reveals that some edge rays of the source are not mapped onto the edge of the target even though the CPC is indeed ideal in two dimensions. Using a topological approach, we refine the formulation of the edge-ray principle to ensure its validity for all configurations. We present two different versions of the general principle. The first involves the boundaries of the different zones corresponding to a different number of reflections. The second version is stated in terms of only a single reflection, but it involves the addition of an auxiliary region of phase space. We discuss the use of the edge-ray principle as a design procedure for nonimaging devices. The CPC is used to illustrate all steps of the argument.

Optimized additive mixing of colored light-emitting diode sources

Given a set of available LEDs or other light sources with known specifications including spectrum, total luminous flux (in lumens), and efficacy (in lumens per watts), we show how to select that combination which yields light of the desired (photometric) color and, in addition, maximizes various objectives such as efficacy, luminous flux, and color rendering index.

Tailoring freeform lenses for illumination

The general problem to find the shape of a refractive surface such as to produce a desired brightness distribution on a given target surface from a known point source leads to a boundary value problem with an elliptic partial differential equation of the Monge-Ampere type. This equation has been described and analyzed in the literature. The purpose of our contribution is to present a venue for a numerical solution as well as several solved examples. The essence of our algorithm for a numerical solution appears to be the explicit incorporation of the condition for the existence of a pseudopotential for a normal vector field.

Designing tailored free-form surfaces for general illumination

3-D tailoring is a constructive method for the design of free-form optical elements for illumination. The light of a point source is redirected in a controlled manner to cast a prescribed irradiation pattern on a target surface. Free parameters can be used to control the shape of the surface resulting from the tailoring process. Every change in the parameters may lead to an entirely different design. Hence the choice of parameters is crucial for the technical feasibility and the visual appearance of the luminaire. Examples of free parameters are the chosen caustics, trimming of the surface, the choice between mirror and lens optics, and the mutual orientation of source and optical elements.

Mixing colored LED sources

Given a set of available LEDs or other light sources with known spectrum, total luminous flux (lumen) and efficacy (lumen/Watt), price etc. we show how to select that combination which yields light of desired (photometric) color and, in addition, maximizes efficacy, luminous flux , color rendering index or other objectives.

Double-tailored imaging concentrators

We present a new approach in optical design whereby two- stage axisymmetric reflectors are tailored with a completely imaging strategy, and can closely approach the thermodynamic limit to radiation concentration at near-maximum collection efficiency. Practical virtues include: (1) an inherent large gap between the receiver and the second-stage mirror; (2) an upward-facing receiver; (3) the possibility of compact units (large rim angles), i.e., low ratios of total depth to total width; and (4) no chromatic aberration. We describe how one can tailor both the primary and secondary mirrors so as to insure that spherical aberration is eliminated in all orders, and circular coma is canceled up to first order in the angle subtended by the radiation source. An illustrative solution that attains about 93% of the thermodynamic limit to concentration is presented for a far-field source, as is common in solar energy and infrared detection applications. Double-tailored imaging concentrators are similar in principle to complementary Cassegrain concentrators that comprise a paraboloidal primary mirror and the inner concave surface of a hyperboloid secondary reflector, but have monotonic contours that are substantially different with far superior flux concentration.

Complex 3D-tailored facets for optimal lighting of facades and public places

Due to antiquated technologies (calculation methods, regulations, lighting and luminaire concepts, production techniques) current outdoor lighting causes a lot of problems like light pollution, glare, energy waste etc. New types of luminaires, and in consequence new outdoor lighting concepts, can be created by combining advanced calculation methods for optical surfaces with recent production technologies and novel light sources such as short arc metal halide lamps. Light emitted from this small Etendue light sources can precisely be redirected by 3D-curved surfaces manufactured with injection molding, milling and aluminium metallization. The required optical design may use techniques like complex surface calculations and 3D-Tailoring. An innovative concept based on the latest findings in visual perception research is to focus the light of such short arc light sources onto a facetted secondary mirror which provides the desired illuminance distribution on a facade or a public place. These systems are designed to fulfill lighting requirements as well as providing visual comfort. Thus lamps with improved color rendering, luminous efficacy and increased lifetime are used and glare is minimized by splitting the reflector into many facets (light spot decomposition). A few examples of realized projects will be presented where such complex facetted surfaces are used to reach a special quality of light. Using novel techniques like 3D-Tailoring, each facet can be designed to individually create the desired (e.g. uniform) illuminance distribution on the target surface - in this case, a large facade. For this particular application, we chose to impose a square boundary for each facet, in order to tile the rectangular aperture of the secondary mirror without compromising efficiency.

Double-tailored microstructures

Microstructures can be viewed as nonimaging devices where all edge rays involved do not change direction over the aperture(s) of the device. This is equivalent to saying that the distance to source and target is much larger than the dimension of the device, which justifies the name microstructure for these devices. Consequently the shape of a microstructure device is independent of its size in this limit. The condition that the etendue of source and target must be equal, generally implies that the projected angle and not the angle itself under which source and target are seen must be equal. This requires at least two surfaces (reflective or refractive). We show how the shape of these two surfaces can be simultaneously tailored to the desired 4 directions delimiting a finite source and target.

Street-lighting with LEDs

Applications requiring high lumen packages are traditional the domain of light sources like discharge lamps. Currently, LEDs make their way into such applications. LED street lighting projects, which are regularly covered in the press, provide a case of point. Life time and luminous efficacy are considered as being the main advantages of LEDs. Nonetheless, other current light sources for street lighting have similar performance. Analysing a street-lamp as a complete system, we can show that LED solutions have significant advantages if highly efficient optics are used. We present an example with tailored free-form optics. These make efficient use of the valuable LED light by exactly redistributing it into the desired illuminance pattern.