The commercial lighting space, with its variety of architectural spaces and functions, is at the forefront of lighting design. Lighting modules of both traditional and new shapes are emerging using standard light sources in addition to LED and flat panel lighting. These lighting modules are created using base LED components – fully integrated light modules with new lens and light pipe designs, new backlit flat panels, and soon AMOLED panels.
These lighting solutions require design expertise in light direction, thermal management, power management, and color control. These new solutions rely on new electronics systems to provide energy efficiency that is not possible without the integration of smart electronics, sensor control, and advanced power management. A critical point in designing an industrial lighting solution involves selecting the light-guiding components, many of which are available as standard products.
Understanding LED lights
Unlike incandescent and florescent bulbs, both of which are distributed-area lights, LEDs are true point source lights. Most filaments in an incandescent bulb cover an area of inches and florescent bulbs range from inches (CFLs) to feet in length (traditional tubes). LEDs on the other hand are very small in comparison, and the light source has to either be grouped with multiple lights and/or channeled through lenses and reflectors to cover a large area. Figure 1 shows the construction of a high-power LED, including the packaging and the lens.
Figure 1: Cut-away view of LED.
Another major difference between traditional and LED lighting is the LED thermal model. Incandescent lights and high-intensity discharge lights lose more than 90 percent of their heat by radiation and less than five percent from both convection and conduction. Fluorescent lights lose 40 percent by radiation, 40 percent by convection, and 20 percent by conduction. LEDs, on the other hand, are deemed low power, since the heat loss from radiation is less than five percent, as is the heat loss by convection, leaving over 90 percent heat loss by conduction. As a result, the light-producing element itself has to have an integrated heat sink and the full electronic system has to remove the conduction heat of the driver and substrate that holds the LEDs.
These differences require that the distribution of the light and heat from the LEDs be incorporated into the design of the lights and light modules. There are two methods to address the requirements of these light units. One method involves using pre-made complete LED modules. The other method requires starting with either bare LED die or individual LED bulb units and creating a custom module.
There are complete LED modules that require only the application of a heat sink and line power to be functional. These modules, such as Cree’s LMR4 LED module (see Figure 2) are complete units. They include housing, lens system, power electronics, LEDs, thermal heat sinks, enclosure, and LED mounting plates. For commercial downlight applications, build an industrial design, balance the thermal needs, and you’re good to go. For the thermal needs, there are various pre-designed heat sinks that fit on the module depending on the amount of air flow the unit will receive.
Figure 2: Cree LMR4 LED module (Courtesy of Cree, Inc.).
For pre-made modules, the key design issues are airflow, secondary lenses, and power connectivity. Most of the units have direct 120 volt or 230 volt wire connections, so they can either be connected by a power cord or hardwired (in the case of canister lights) to the power source. The major variation in the lights is the center color temperature – typically 2700 K, 3000 K, 3500 K, or 4000 K. All the LMR-series units consume about 12 watts and provide 700 lumens of light for about 35,000 hours.
As shown in Figure 3, there is a large diversity in lamp designs. Some are enclosed and have different thermal requirements for inclusion of the heat sink. These issues are best handled with modeling software followed by design testing with thermocouples to ensure that a high duty cycle does not generate temperatures exceeding the maximum operating temperature of either the control circuitry or the LEDs. The modules support dimming down to five percent with industry-standard Lutron-compatible dimmers.
Figure 3: Cree LED module applications (Courtesy of Cree, Inc.).
The light output from the unit is dependent on the design. If there are reflectors involved, then the light can either be concentrated over a smaller area to appear brighter or it can be diffused over a larger area. In some designs there is a secondary lens or light path. The modules can be used as a central light source with light directed through light pipes, light guides, and lenses; such an arrangement allows for curved path lighting and dimmable architectural edge lighting. While the pricing may seem well above the component cost, the units feature Energy Star, UL, and CA-Title 24 compliance along with multiple international standards, each of which would have to be separately obtained for a custom design.
LED module components
A contemporary LED module has a number of components in addition to the diode itself. There are two methods of LED clustering for lighting applications. One method involves using multiple LEDs in a custom arrangement. The other method is to use surface mount LED die on a mount to create the light pattern (see Figure 4).
Figure 4: Surface mount LED arrangement (Courtesy of Cree, Inc.).
The multi-bulb clusters use a through-hole PCB that has the pattern of the bulbs. One of the keys is to set the proper color mix. Typically, the white LEDs are supplemented with yellow, green, blue, amber or a combination of colors in order to produce an application appropriate light beam. For downlight and highlight applications, the lights are clustered according to the beam angle of the LEDs and side reflectors are used to focus the light. This is the common configuration for flashlights. The reflectors come in a number of sizes and light patterns; most attach to the base units with adhesive. These reflectors are optimized for both LED clusters and LEDs in die form, so the selector guides must be reviewed to ensure the correct use.
Due to the structure of the finished LED bulb clusters, they are generally not guided with light pipes to make curved or remote lighted fixtures. However, the clusters do have a thermal component that must be addressed, just as the modules have. The airflow for the design has to accommodate the power regulator circuit. These clustered technologies tend to be the product of choice for portable and battery operated applications since their cost and usability at lower volumes are lower than surface-mount solutions.
The full-build devices are based on surface-mount solutions. They start with the selection of the LED unit itself. Typically they are single LEDs, such as the Cree XLamp® series, which features the two terminals of the diode, the local phosphor, and a sealed lens. These lights are arranged on thermal conduction boards that have the proper placement for the surface mount lights.
Bergquist makes a number of thermal clad products that are specifically designed for various LEDs and set up for high-performance thermal transport. Figure 5 shows one of these platforms under their IMS product line. The I/III/V unit can be used for a center LED, a three light set, or a five light configuration. Bergquist’s selection guide shows the various surface mount parts that are supported and the power that can be managed by the devices. A number of these units are available as pre-assembled units under the Hotenda/Cree brand, such as the XP series units shown previously in Figure 4.
Figure 5: Bergquist IMS LED thermal module (Courtesy of Bergquist).
These units are then placed in fixtures that have lenses, light pipes, reflectors, and other optical guides to produce the light pattern desired. The lens, available from a number of manufacturers, can produce both focused narrow pattern light and wide area light. The lenses also feature color filtering, which allows for fine tuning of the output light from the LED sources. When used with a multicolor source (red-green-blue or RGB-white), it is best to use a clear lens to enable the multiple string drive to set the correct color balance. In this configuration, the devices can be individually dimmed and mixed, unlike the cluster of LED bulbs that run as an overall light mix environment.
For specialty situations, high contrast and high color lenses (such as red) are used, with HB white LEDs for emergency exit and other “known” color applications. Some of the lens arrangements have individual LEDs placed on the thermal platform, and some are single “blending” lenses with both spot and area focus. Most of these lenses have mounting brackets that can accommodate the use of heat sinks on the back of the unit and reflectors mounted around them.
Some applications do not need wide area lighting. For those uses the LEDs have their output guided with light pipes. These devices are designed for end point or edge lighting applications that may be part of control systems in the commercial facility; or they may be used in curved lighting applications that may not have space for the LED module on the same plane where the light is needed. These allow recessed units to have more airflow and large control circuits, while still supporting architectural thin- or flush-lighted designs at the user interface. Light pipes, like lenses, are available for both single and multiple LED designs. Light pipes are primarily used for accent lighting that is built into the infrastructure (wall highlights, elevator indicator lighting, floor lighting for exit paths, etc). Being purely passive components, light pipes work with both surface-mount and single-bulb LEDs.
The last element in light management is the reflector. These are the constraining items on the industrial design and are generally reserved for general area lighting applications. They appear in two locations – around the entire light unit to set the light pattern outside of the bulb and inside the bulb unit itself. In a typical light bulb you have an enclosed unit with a glass or plastic lens that is the familiar light bulb shape, and inside there is a reflector – generally white rather than silver – that guides the light evenly to this lens system. These bulbs are then screwed into an architectural design lighting unit that has a larger reflector to help disperse the light in a known pattern for standard 120 degree output light bulbs. This two-stage reflector system allows the outside unit to support CFL, incandescent, and LED bulbs, while the inside of the bulb unit can be light-source specific.
The last components are the driver electronics and the heat sink. The heat sinks are generally placed in contact with the LED thermal mounting board and also cover the drive electronics. The shape and thermal characteristics of the heat sink depend on the airflow of the lamp, its duty cycle and power, and the industrial design of the light. The LED power control has to handle the peak condition of all lights “on” and the minimum airflow for the thermal condition. The inside lamp environment can reach high ambient temperatures above 400C in extended operation, which will have an impact on the power management circuits. The ICs that drive the LEDs are designed for these conditions; however, the rest of the circuitry has to be selected accordingly.
Transformers, resistors, and capacitors are all available with different materials and characteristics. As a result, it’s important to choose one that has the correct thermal properties for the design. The important items to consider in voltage regulators are the change in resistance of the external resistors and the stability of the capacitors used. Choosing an electrolytic versus ceramic versus tantalum versus thin-film capacitor can have a large impact on the resulting lifetime and quality of experience of the light being produced by these component-built systems.
The commercial lighting space is one with a large diversity in industrial design and application. As a result, the selection of thermal management, lens, lighting modules, light pipes, and reflectors is both complex and critical. The designs and capabilities of the new smart lights can incorporate sensors and microcontrollers to simplify their use. The launch of new lighting sources and multi-light modules is helping to bring both new designs and new capabilities with reduced energy use to the commercial sector.