Four-level optical design in LED lighting

Light-emitting diodes are considered the most important light source of the 21st century owing to advantages of energy saving, long life, fast response, high color performance, and environmental benefit. In particular, many applications of LEDs as modern luminaires (lamps) for street lighting have been proposed and demonstrated. Two major obstacles to widespread use of the technology are the efficacy of LED luminaires and their operating cost. Optical design plays a key role in achieving effective LED solid-state lighting (SSL), which in turn makes the technology less expensive. Here, we classify optical design for LED SSL into four levels that precisely describe the light distribution by an LED die to a luminaire and result in a more efficient and user-friendly process.

The zero-level optics for LED SSL aims at increasing the light-extraction efficiency (LEE) or changing the emitting light pattern at the die level. Figure 1 shows an LEE simulation for sapphire and thin gallium nitride LEDs into which we have introduced microstructures. In this case, the LEE enhancement is caused by photon recycling in the LED cavity.^{1, 2} An alternative means of enhancement is lens encapsulation, which involves enlarging the cone of escaping light from the die. When the absorption of the active layer of an LED die is as low as 200cm⁻¹, an appropriate microstructure with concomitant effective photon recycling in the die cavity can lead to an LEE of 90%.³ When the absorption is as strong as 10,000cm⁻¹, quantum photon recycling may be helpful in increasing the efficiency.³ Additionally, specific microstructures can change the lighting pattern of an LED, increasing directionality to 400% that of a standard device (see Figure 2).⁴

Figure 1. Simulation of light-extraction efficiency (LEE) for sapphire-based and thin gallium nitride (GaN) LEDs with photon-recycling structures in strong or weak absorption conditions. LEC: Lens encapsulation.

Figure 2. A simulation of LEDs with photon-recycling structures and the results of directionality enhancement.

Figure 3. Midfield modeling (top) and comparison of the simulated and measured light patterns in the midfield region (bottom). The normalized cross-correlation (NCC) between the simulated light pattern and the corresponding measurement at 1.8, 3.0, and 5.0cm is larger than 99.5%, which is sufficient for precise optical design.

The first-level optics for LED SSL is intended to precisely model the spatial and chromatic distribution of a light source. For spatial distribution we have proposed a modeling scheme based on a so-called midfield model and applied it to a variety of practical LEDs (see Figure 3).^5–7 For chromatic distribution we have demonstrated a phosphor modeling scheme (see Figure 4). The phosphor model accurately predicts the flux ratio of blue and yellow lights to enable precise simulation of the chromatic performance of a phosphor-based LED, including correlated color temperature and color-rendering index.⁸

Figure 4. The modeling algorithm for phosphors.

Figure 5. A butterfly-form lens applied to street lighting with two cluster LEDs.

The second-level optics focuses on designing an appropriate optical component to spread or project emitting lights to the desired target. In contrast to point sources, LEDs are complicated. For example, in practical applications such as street lights, a donut lens is conventionally used for wide-angle illumination along the road. But this solution is not suitable for two or more adjacent LEDs because the light pattern becomes distorted and the illumination on the target will not be acceptable. Rather, based on a precise optical model, we have proposed a butterfly lens to direct the lights by two adjacent cluster LEDs to the designed illuminated area for a street light (see Figure 5).⁹ For another important application, i.e., forward lighting in a vehicle, we have proposed incorporating a reflector into a light pipe and have applied it to a bicycle headlamp.¹⁰ Other designs in second-level optics that we are working on include high-efficiency focal adjustable spotlights, automobile and motorcycle headlamps, and ultra-large-angle street lights.

The objective of third-level optics for LED SSL is to reduce and even prevent glare from LED luminaires. One straightforward approach is to enlarge the effective emitting area of the light source by placing LED arrays at the bottom of an optical cavity covered with a diffuser. We recently reported a study of the theoretical calculation as well as experimental demonstration of the cavity efficiency with one or two diffusers (see Figure 6).¹¹ Surprisingly, we found that, given a proper photon recycling structure in the cavity, the cavity transmittance can be as high as 93%, whereas the one-shot transmittance of the diffuser is only 70% or less. The study shows that a thin cavity with high optical efficiency and uniformity across the exit surface can easily be achieved using the photon-recycling technique.

Figure 6. The calculated cavity transmittance with diffusers of various one-shot transmittance. E: Efficiency. R_bR: The product of the one-shot reflectance of the diffuser and reflectance of the side walls in the cavity. T: Transmittance. T70/T60/T55: One-shot transmittance of 70/60/55% of the diffuser.

In summary, we have effectively applied optical design to LED SSL at four different levels. All the approaches are aimed at precisely determining the spatial and chromatic performance of an LED and increasing the optical utilization factor of an LED luminaire to achieve optimal energy efficiency. In future work, we will study the limit of efficacy of an LED luminaire for various illumination applications.