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Hitting the target

Dr. Wu Jiang and Kevin Schneider of LED Engin explain why it is worth evaluating the quality and intensity of light reaching its target, particularly when it comes to directional lighting.

Figure 1: TIR lenses can best be used with compact, high lumen density emitters

For electrical contractors, incandescent lamps have been easy to compare given the similar performances yielded by one 100W lamp and another. The efficiency of lamps from different manufacturers is very similar, products conform to common, long-established physical formats and they emit light in very similar ways. For a given AC grid voltage, incandescent lamps are evaluated by their power rating in Watts and the amount of extra light likely to be produced from a 100W lamp than from its 60W counterpart is fairly well understood.

However, the emergence of LED lamps has made things more complicated, not least because the industry has tried to explain the performance of LED lighting in incandescent terms. Specifications usually include the power consumed in Watts and how that equates to an ‘equivalent’ incandescent lamp. The measure for LED lighting is expressed as luminous efficacy, in lumens per Watt, which is presented as a measure of how efficiently a lamp converts electricity into visible light. The construction of LED lamps is more complex than that of incandescent ones. They have different types of emitters, different substrates upon which the emitters are mounted, different driver electronics, different optical focusing mechanisms, and different housings. All of these factors contribute to staggering differences in performance in real-world applications between products that, at first glance, appear to be very similar.

High-power LED emitters are ever improving light sources for a range of lighting applications. However, for the majority of applications, such as interior spot and downlighting, roadway lighting, architectural lighting and stage lighting, emitters themselves cannot deliver enough light intensity to a target. This is because a point LED light source emits a Lambertian light distribution whereby the apparent brightness to an observer is the same, regardless of that observer’s position. Light is therefore spread far too widely.

Figure 2: The TIR lens produces twice the centre beam lux with negligible glare

In order to direct light onto a target it is necessary to use secondary optics that collimate the light into a controlled beam illuminating the targeted area. Collimated light rays propagate in parallel and the smaller the light source, the more effective the collimating optics can be. Besides collimating light, secondary optics can also be designed to improve colour uniformity and light distribution within the target area.

To describe secondary optics’ ability to collimate a beam, we often refer to viewing angle or FWHM (Full Width at Half Maximum). This is the angular width of the beam whose intensity at the edge is half the maximum (central) intensity. This angle is a useful way to classify optics, but it doesn’t always explain discrepancies between different optical platforms. In practice, depending on the optical design, optics with identical viewing angles can differ quite a lot in the intensity and quality of the beam.

Figure 3: A 3D beam profile clearly shows the smooth illumination profile and central lux performance of the TIR module compared to the lower lux and greater glare from the reflector module

Many lighting applications – in particular high bay, streetlight, and stage lighting – demand high lux at a distance, and that means both a high-power emitter and a highly collimated beam. In an industry with such high standards, it is essential that each emitter be properly matched with appropriate secondary optics. Often, the physical size of the emitter limits the optical options. This is particularly true of certain chip-on-board (COB) or array emitters; they emit from such a large area that the only optical solution is to surround the emitter with a reflective surface.

Reflectors are common with omnidirectional lights such as incandescents, but in LED design they carry a key disadvantage; a majority of light rays originating from the centre of the emitter never hit the reflector. This means that even in a ‘narrow flood’ reflective system where the view angle is in the 20°-25° range, a significant portion of the light strays wide of the target. And it isn’t simply lost; it is very visible as glare and background light, causing distraction and discomfort at the expense of centre-beam lux.

Contrast this with the optical opportunities for a compact, very high lumen density emitter. These emitters are powerful enough to provide the necessary lumens and small enough to be enveloped with a lens that uses total internal reflection (TIR) to guide virtually all of the radiated light toward the target. An inexpensive, efficient, well-designed lens in a low-profile form factor is only possible with these very bright, very compact emitters.

Figure 4: 50% contours are similar but the maps show how much light is wasted outside of this sector by the reflector design

Figure 1 shows examples of such lenses from LED Engin. These produce view angles from 8° to 45° when used with the company’s emitters. They manufacture four compact emitter packages; 1-die up to 5W, 4-die up to 15W, 12-die up to 40W, and 24-die up to 80W. The secondary optics not only direct light to the target but also provide high optical efficiency and colour uniformity while maintaining compact form factors.

When an LED Engin LZC emitter with a 24° TIR lens was compared with a reflector-based module of similar specifications, it was realised that the similar headline specifications of the evaluated modules gave a misleading impression of comparable performance in real-world applications. However, there are three key differences in performance between these modules, as shown in Table 1. The TIR lens system delivers twice the central lux of the reflector module, with double the lux efficacy measured in lux per Watt and visual glare is cut by 80%.

Table 1: The TIR lens delivers twice as much light to the target and 80% less visual glare

Figure 2 shows the measured intensity distribution over viewing angle. It can be seen that FWHM view angle doesn’t tell the whole story. Both the TIR lens and the reflector reached their half-maximum at or near ±12° for a view angle of 24°. But LED Engin’s design produces a smooth, well-controlled slope up to the peak intensity, while the reflector’s intensity distribution flattens out in the wide-angle glare zone. It’s more visually evident in the graph in figure 3, where the profile of the TIR solution shows a smooth gradient toward the centre beam, but the reflector solution shows a ‘spiky’ centre beam with a significant portion of the reflector’s energy coming from outside the peak.

The contour map (figure 4), shows that the 50% contours look identical, but the reflector’s beam widens dramatically on the outside. The visual differences are confirmed numerically in the tables, which reveal that a TIR lens, on a compact emitter, can deliver lumens where they’re needed, without leaking them as unwanted glare.

In real-world applications, comparisons of LED modules and lamps by either power consumption or luminous efficacy may give a completely inaccurate impression of the performance of these light sources.

Table 2: Lumen breakdown by viewing angle

What is needed is a new way of defining luminous performance that takes into account the percentage of lux-on-target that’s delivered, not the total lumens produced by the module. True ‘lux efficacy’ could perhaps be adopted to describe the useful lumens produced by an LED module; light that illuminates the target area. Other factors such as colour uniformity and light distribution still have to be considered, but lux efficacy would be a better measure than anything currently available. The comparison between modules with reflector designs and TIR lenses clearly demonstrates the need for such a measure.

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