The vast majority of lighting applications require white light, and many designers are drawn to Cree LEDs for their efficiency and flexibility. However, white light is not something an LED can generate on its own – the diode junction emits blue light that is changed to white through a phosphor conversion layer.
There are two ways of doing this conversion. The phosphor may be applied directly to the package of a blue LED to create a stand-alone white LED, or the phosphor may cover an entire array of blue LEDs to create a completely uniform light source. Which is better? Well, (shocker) it depends…
What is Phosphor?
A phosphor conversion layer converts one wavelength of blue light to several wavelengths that our eye interprets as white light. This layer inherently reflects some light back into the source where it is absorbed by the die surface, lowering the efficiency of the light source. The domed surface of a standard LED reflects more light back into the package than a typical remote phosphor situation where the phosphor is separated from the die by a greater distance and more reflective surfaces are present between phosphor and die. Some remote phosphor systems claim efficacy gains of over 25% for this reason alone. Phosphor becomes less efficient at high temperatures, so placing the phosphor farther away from the heat-emitting junction can give a more stable color temperature as the fixture heats up.
Remote Phosphor: What’s the Downside?
White LEDs have come down in price quite a bit over the years. Getting 1000lms of light from a small space used to require high-end LEDs that cost a few dollars each. Now, you can achieve 1000lm with a handful of mid-size LEDs that may cost a few dollars total.
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Phosphor discs are still relatively expensive, and royal blue LEDs are no less expensive than white LEDs. The system cost will almost definitely be greater for a remote phosphor application and can incur additional costs in the certification process. Standard LEDs are pre-certified, and fixtures using them can be certified with reduced test time using the LM-80 data of the source LEDs. A remote phosphor fixture must be submitted to testing facilities as a complete unit and undergo full testing in order to get ratings such as ENERGY STAR. This can delay results for up to 18 weeks. The LED industry moves quickly, and a five-month delay in certification can cripple a product in a competitive market.
Remote Phosphor Applications and Costs
As with most designs, the best solution depends on your goal. If your application is cost or time sensitive, remote phosphor is not going to work for you. If you absolutely require a large uniform light surface with minimal color shift then remote phosphor can help you achieve uniformity and efficacy that would be unobtainable with standard LED fixtures. The most common application for remote phosphor is in outdoor retail, like the lights used to illuminate the sales lot of a car dealership. When the perfect lighting can close a several thousand dollar deal several times a day, it is worth a few extra dollars per light fixture.
Chip on Board (COB) LED Modules use several die very close together under a single phosphor layer and may provide a good compromise. COBs have their own design considerations, but they can provide a high-intensity uniform light source from a pre-certified package.
Does this get you thinking about your own design?