Articles about LEDs typically report on the improvements in their efficiency (which results in real energy savings), in their color consistency, and the high quality of light they can produce. Less attention has been paid to the optics — the lenses and reflectors — needed to create luminaires based on LEDs. Yet LEDs by their nature require unique optics to perform the way consumers want, so lighting designers must pay special attention to optic design.
Unlike other light sources – such as incandescent, halogen and fluorescent bulbs that emit light in all directions – LEDs naturally emit light in a beam with an angle of 90° to 120°. The angularity of the light can be beneficial, for instance, where task light needs to be directed to a specific area. LED angularity can also be problematic, as when a solid-state lighting (SSL) bulb is used to replace an omni-directional light source, such as an incandescent bulb. Even when the design requires an angular beam, the natural angle of emission of the LED is usually too broad to be used without adjustment.
The basic LED component generally comes with a lens made of a clear optic-quality material that encapsulates and protects it. To apply the LED to a particular task, lighting designers must add secondary optics that will shape the LED’s output to a particular profile. These optics can range from a simple reflector and lens to total internal-reflection (TIR) optic system, in which a center optic with a tightly controlled refractive index directs light to a surrounding TIR mirror. Reflectors tend to provide broad beams with high efficiency, but TIR systems can provide more precise beam control.
The cost of producing custom optics is too high for many lighting applications. Fortunately for designers, secondary optics for LEDs are being mass produced by many manufacturers, including Avago Technologies, Chicago Miniature Lighting, Dialight, Ledil, Lighting Science Group, Panasonic, and TE Connectivity. Designers have many secondary optics to choose from, but need to be aware that each optic is typically designed to work with a particular LED. See Figure 1.
Figure 1: Secondary optics are commercially available for many LEDs. Shown here is a sampling of optics from the Lighting Science Group for the Atlas and Titan light engines.
Designers should test products from several manufacturers to see what meets their needs, because the optics can affect not only the beam pattern, but the lighting efficiency of the entire luminaire. Placement of the optics is also critical, because location can affect beam pattern. Just recently, TE Connectivity introduced the NEVALO lighting system, in which the components are configured to work together. Designers can select from among various LED modules and secondary optics, as well as heat sinks and power sources, from TE Connectivity and know that the components will be well matched. An evaluation tool is also provided to confirm the thermal performance of modules in the fixture design.