Finding a Mate

Ah, that timeless yearning, that need, that burning desire, the age-old struggle to find a mate, the proverbial other half, the complement allowing two to achieve what neither could alone.

No, we're not at the start of some mushy love story, nor a documentary about the animal kingdom. It's about the LED. Like an engine, LEDs spew power (luminous flux) efficiently and effectively. However, it's raw, unadulterated, and undirected power. More often than not, the LED's light beam is not coincident with the requirements of the luminaire.

Consider the Edison base A19 lamp. This legacy product emits light in a beam angle of 360 degrees in the horizontal plane, and somewhat better than 270 degrees in the vertical plane. By comparison, most mid- to high-flux LEDs are hemispherical devices, casting their vertical beams in an arc of 140 degrees or less. So, clearly, the light beam of a single LED will not foot the bill.

Conversely, sometimes it's necessary to focus the light in a much tighter pattern. That would be the case for applications like task lighting or, to take an extreme, feeding a fiber optic cable.

Funneling light into a fiber optic cable is somewhat like adding oil from a five-quart container with a wide, easy-pour spout into a new car engine. Without a funnel, on most cars some of the oil might actually make it into the crankcase, but a large percentage will wind up either dressing the outside of the engine or spilling onto the garage floor. The same is true for the LED and the fiber optic transmission cable. Without a suitable "funnel," most of the light misses the cable.

So, the challenge is to squeeze the LED's flux into the desired direction. That challenge is met by the addition of an optic. Companies like Fraen, Khatod, and LEDiL offer a wide range of off-the-shelf optics for the most popular LEDs from all the large LED manufacturers.

With so many available optics and LEDs, designing luminaires such as task lights is straightforward. We normally start with the product definition (what the customer wants the luminaire to do) in terms of the chromaticity, color rendering, illuminance requirements, illuminance pattern, and the height between the illuminated surface and the light source. The first four constraints define the LED selection. The illuminance pattern and height define the geometry of the beam angle. Upon selecting a suitable LED source that meets the initial constraints, it is time to find its optical mate.

Optics suppliers offer a choice of beam angles for most LEDs. Fraen, for example, offers optics with narrow, medium, wide, wide elliptical, and medium elliptical beam patterns.

Alas, not all LEDs have an appropriate mate. A limitation in physics known as étendue establishes the minimum possible beam angle for any given light source. Some sources, such as chip-on-board LEDs, tend to have broad emitting surfaces and are not well suited to optics. It is just not possible to take an incoherent light beam emanating from a large surface area and focus it into a smaller beam. This is particularly challenging in a fiber optic application. To transmit light effectively through a fiber optic cable of small diameter, it is necessary to select an LED with a small emitting area.