LEDs are well suited to meet the requirements of high-luminance directional lighting (HLDL) applications. To optimize performance, manufacturers must consider all aspects of optical design such as LED geometry, lens geometry, secondary optics, and more.
Firstly, is the inclusion of any secondary optical elements to collimate light to control the output pattern and help make the light more uniform. Secondary optics may include different formats such as reflectors, total Internal reflectors (TIRs), and lenses.* Additional considerations include the use of flat optics, chip size, and emitter shape.
Advantages of Flat Optics
An LED designed with flat optical elements enhances the intensity and beam distance of the source, supporting optimal HLDL performance, thus LEDs with a flat design are especially well suited for many directional lighting applications. A flat design means, simply that the profile of the LED chip is flat, thanks to the use of a flat window in place of a dome (as seen below in Figure 1). The flat window provides a smaller light emitting area than the typical dome lens LED design.
The flat design also means that lenses can be closely coupled to the LED, increasing the on-axis intensity of the source. The result is higher lumen density, higher optical efficiency with smaller optics, and an extended beam distance. In terms of the throw, a flat window yields a longer distance than a dome window package LED.
Figure 1 - Comparison of Luminus SST-40-WxS vertical chip with dome lens and Luminus SFT-40-WxS flat-window chip combined with secondary optics. Both products are designed for HLDL applications, but where beam distance is a key criterion, the SFT (“F” for flat) directs a beam 30 meters farther.
Another advantage of the flat-window LED design over domed is reduced color variation over angle, which is important for beam-quality. For example, refer to Figure 2, below, showing a graph of the angular performance of a domed LED (SST-40) designed for at CCT of 6500K. It actually reaches >7000K peak CCT at a 0° angle. As the beam angle increases, however, the CCT drops as low as 4750, a significant color difference of roughly 2300K from center to edge.
Compare this with the angular performance of the flat-window chip (SFT-40) in Figure 2, also designed for 6500K. The graph demonstrates a much smaller variation in angular performance —more uniform—between only 5500K to 6750K, a difference of only 1250K from center to edge.
A flat window design also affects the beam quality, improving spot uniformity, as shown below in Figures 3 and 4. The LEDs have reflectors roughly the same size and output, but the flat window design produces greater intensity (60% higher candela than the domed LED in Figure 3 and 5X more than in Figure 4 ), and a larger area of the beam spot is uniform.
Figure 2 – CCT over angle comparing dome lens LED (SST-40 6500K) with flat window LED (SFT-40 6500K) demonstrating superior color uniformity with a flat window design.
Figure 3 – The beam spot of the flat-window LED (SFT-40-W) is higher quality with less spill light (a smaller halo ring) than the domed LED (SST-40-W). This means the spot is more uniform, and the uniform beam spot is also larger, and produces 1.6X more candela.
Figure 4 – As in Figure 3, the beam spot of the flat window LED (SFT-7X0-W, left) has less spill light than the domed LED (SST-70X-W, right) a larger spot size and better uniformity across the spot.
Advantages of Small LED Size
Smaller LED chips provide improved performance for HLDL applications. Larger LED chips require secondary optics with a larger diameter to achieve the same beam angle as a smaller LED. This limits how compact the HLDL device can be and makes it unsuitable for many small form factor HLDL devices such as flashlights.
LEDs generally emit broad beams that require secondary optics to reshape the beam. For directional lighting, the optics convert a Lambertian-like light into a focused spotlight with a long throw distance. When designing the optical components, the first-order optics are typically based on an assumption that the lights are point-sources (ideal sources). However, LEDs are not dimensionless, and the off-axis light rays are hard to control resulting in spill light outside the intended beam area. Therefore, if the same secondary optics are used, a smaller emitting area is beneficial with a higher ratio of on-axis power and higher center-beam candlepower (CBCP).
Advantages of Round Emitters
Most typical SMD (surface-mounted device) LEDs have a square die, except most COB (chip-on-board) LEDs. For HLDL applications, round LEDs demonstrate superior performance and better light quality than standard square emitters for several reasons. Typical HLDL applications need a round beam spot, which is created via the optical elements that essentially convert the light from a square emitter to a round beam.
If the emitter is round, then less optics are needed, and it is easier to deliver high beam spot quality. The round emitter also provides higher on-axis intensity, beam distance, and higher optical efficiency, all with smaller optics (see Figure 5, below). On-axis intensity (K Factor**) is more than 30% higher with the round emitter, which also produces a narrower beam angle (as shown in Table 1).
Another effect of using a round emitter is the quality of the beam spot. The beam spot of the square emitter (Figure 5, top) has a halo ring with a somewhat square shape, compared to the round emitter (Figure 5, bottom) which is perfectly round and has a smaller halo, meaning there is more illumination within the center beam spot.
Figure 5 – Comparing LEDs with a square (top) and round (bottom) emitting surface using the same lens, the round emitter provides higher on-axis intensity, a longer throw distance, and more perfectly circular beam spot.
Table 1 –Comparison of LEDs with a square vs. round emitting surface shows K Factor performance improvement of 33% with the round LED.
Learn more about how to optimize lighting device design and components to achieve the best HLDL performance in the white paper High-Luminance LEDs for Directional Lighting Applications.
Guidance on selecting the right LED for your HLDL application can be found in the Help Center article, “How Do I Select the Right LED for my Directional Lighting Product?” [LINK TO POST]
* For more information about lenses compatible with Luminus LEDs, refer to https://www.luminus.com/resource/ecosystem/optical
** What do we mean by “K-Factor”? To compare the performance of different HLDL light sources, the shorthand “K Factor” can be used, in units of candela per lumen (cd/lm). K Factor refers to the lens efficiency factor, which characterizes the convergence of the light source. Higher convergence means a narrower, brighter beam better for HLDL. Some industry sources refer to this as “peak intensity”.
Note: K Factor is not the same as the K metric used to characterize light source color, where K is Kelvin (e.g., 3000K).
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