Though they are a relative latecomer to the world of lighting technology, light-emitting diodes (LEDs) are quickly making up for lost time. Since their invention in 1962, LEDs have been applied in a wide range of lighting applications, including traffic signals, automotive panel lighting, aviation lighting, remote controls, DVD players and other household appliances, just to name a few. LEDs’ functionality and longevity have led to a substantial and growing market share. In fact, a recent study presented at the University of Chicago says that LEDs will capture 60 percent of the global lighting market over the next decade. However, in order to reach that market share, LED lighting devices are subject to an important technical challenge – consistency in the brightness and quality of the light emitted from LED bulbs.
To meet that challenge, Labsphere, a company that specializes in light metrology instruments and systems, collaborated with Air Innovations to extend its light-measurement capabilities. The two companies developed a custom system that allows testing of all types of lamps at a wide range of ambient temperatures to examine the effects of temperature on the light output. System testing is currently under way at a facility in Brazil.
The system is being developed to meet the demands of formalized worldwide standards IESNA LM-79 and LM-82 to consistently measure the light output of LEDs. The metrology system consists of a 40-inch-diameter aluminum sphere, coated on the inside with Labsphere’s highly reflective Spectraflect ® material, used in conjunction with a spectrometer to accurately record the light data.
In developing the metrology system, Labsphere needed an environmental control unit (ECU) to provide a consistent environment to perform the testing. Unlike other light sources, LED performance is greatly influenced by the temperature of the device where it is used. As the temperature varies, the light output will vary as well, causing changes in both the total optical power (brightness) and potential shifts in color.
Driving an LED at high power in high ambient temperatures may result in overheating and a shorter usable life of the device. Therefore, the temperature-controlled test environment provides a means for LED and luminaire manufacturers to evaluate the entire performance of their devices prior to installing them in the field.
To develop the ECU, Labsphere turned to Air Innovations, a company in North Syracuse, N.Y., that engineers and manufactures specialty temperature, humidity, filtration and pressure control solutions for process applications across a range of industries, including aerospace, cleanrooms, pharmaceuticals, photonics, nanomachining and more.
Air Innovations derived a prototype ECU from its existing wine cellar cooling system platform, customizing the controls to meet Labsphere’s requirements. The ECU generates 300 to 400 cubic feet of air per minute, with the ability to adjust the temperature from 5° to 50° Celsius (41° to 122° Fahrenheit) in the sphere through a feedback control system that includes a precision proportional–integral–derivative (PID) controller to hold temperature setpoints to +/- 0.25°C. Also, the ECU features external communication capabilities so the customer can change setpoints and monitor performance remotely.
The temperature-controlled air is blown through the sphere to maintain a constant temperature within the sphere. Through computer control, the air is diverted for the few seconds while the optical test is conducted, ensuring reliable results by minimizing air flow around the device that might change the heat transfer from the device to the ambient environment. At the moment the test is completed, air is restored to the sphere to maintain the desired temperature. Hot gas refrigeration control keeps the compressor operating at a reduced cooling capacity when the air is re-circulated back to the ECU during light testing.
To accurately distribute the air within the sphere, computational fluid dynamics (CFD) modeling of the flow and temperature distributions through the sphere was conducted. This allowed the designers to determine the thermal and flow gradients within the sphere and help optimize the flow paths to achieve a uniform velocity and temperature at the center of the sphere where the device under test (DUT) is located.
The heat transfer properties of the sphere proved to be challenging in that the entire sphere had to be insulated to minimize any heat exchange with the surrounding environment. To accomplish this, the sphere was covered with a geodesic pattern of insulating foam. The resulting system provides users with optical parameters such as color temperature, luminous flux, spectral flux, color rendering index, chromaticity coordinates, efficacy, and dominant and peak wavelength characteristics.
Upon completion of the prototype, the metrology system was shipped to the customer in Brazil for testing. The integrating sphere system is being used daily at the client facility, but due to the propriety nature of the testing, no data is available at this time.
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