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Reliability of point source approximations in compact LED lens designs

Thøger Kari, Jesper Gadegaard, Thomas Søndergaard, Thomas Garm Pedersen, and Kjeld Pedersen

Open Access Open Access

Abstract

In many applications, compact concentrator lenses are used for collimating light from LEDs into high output beams. When optimizing lens designs, the LED is often approximated as a point source. At small lens-to-LED size ratios this is known to be inaccurate, but the performance compared to optimizations with more realistic models is rarely addressed. This paper examines the reliability of a point source model in compact lens design by comparing with optimisations that use a factory measured LED ray-file. The point source is shown to cause significant, unnecessary efficiency loss even at large lens sizes, while the use of a ray-file allows for a >55% reduction in the footprint area of the lens. The use of point source approximations in compact lens designs is therefore generally discouraged.

©2011 Optical Society of America

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Figures (5)
Fig. 1
Fig. 1 (a) Polar emission profile of the simulated point source (black line) and the factory-measured ray-file used for the LED (gray line). (b) Cross section of a TIR lens, showing the rotational profile that defines the different lens types. Italics signify free optimization variables while bold fonts signify constants. υ and ω are calculated from (R), e and j. Variables with a square marker are only free for some lens types. In addition, k = (R)/2 and θ 2 = 0° for lens type (I).

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Fig. 2
Fig. 2 Selected convergence graphs for type III lenses. ‘PS’ and ‘RF’ signify point source and ray-file optimizations respectively. The ratios in the legend are the lens diameter to the edge length of the LED. Two graphs are shown for each lens/source category, in black and in gray.

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Fig. 3
Fig. 3 Each point on the graphs represents the efficiency of a lens with a given size. ‘PS’ and ‘RF’ indicate source model and ‘I’, ‘II’ or ‘III’ indicate lens type. ‘PS2RF’ graphs in (a) show the efficiency obtained by inserting an RFS into a PS optimized lens at a given size ratio and reversely for ‘RF2PS’ in (b). The first 5 graphs of (b) show the relative efficiency of the RFIII lenses compared with the RF(I), RFII, and PS lenses.

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Fig. 4
Fig. 4 Efficiencies of RF optimized lenses at three FOV angles. ‘WOA’ is the window of application. The ‘Chen’ graphs concern the results listed by Chen and Lin [6], and ‘RF vs. PS2RF’ and ‘RF vs. Chen’ show the relative efficiency of the RF lenses. Each ‘PS2RF’ value represents an average over 5 PS lenses.

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Fig. 5
Fig. 5 Diagram of optimal lens shape as a function of lens-to-LED size ratio. At large ratios, the lens designs approach those found by point source optimizations. The figures are cross sections of the actual optimized 3D lenses.

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