An LED is the product of a series of complex manufacturing steps. These steps are tailored to yield LEDs which meet a set of performance targets. However, due to variations in the manufacturing process, the actual performance may vary between LEDs within the same product family. To help customers manage this variability, LEDs are typically divided into different bins according to their performance at a specified operating condition; LEDs within the same bin are guaranteed to have a light output, color point, and forward voltage within a tight set of bounds at that operating condition. This approach improves the predictability and consistency of the LED performance for the end customer.
Traditionally, LEDs are tested and binned with the pad or junction temperature set to 25°C. However, the typical LED junction temperature in a lighting application, such as a MR16 light bulb, is substantially higher. Since most LED performance metrics are temperature dependent, the performance of a LED in an actual application is, in general, different than the nominal performance advertised in the datasheet. Consequently, LED system designers often spend significant resources to predict and verify the performance of their LED applications. For color, in particular, LED system designers often require reams of data in addition to complex algorithms to find suitable color bins such that the application will meet the desired color point at the targeted operating conditions.
In order to reduce complexity and further improve predictability of the performance of a LED at typical application conditions, Lumileds was the first supplier to adopt hot-testing, in combination with a tight elliptical color bin structure, during the production of its high-power LEDs. This approach is enabled through state-of-the art testers and sophisticated algorithms, which carefully match the epi and phosphor materials in the LED to ensure tight color consistency within a product family. In practical terms, the light output, forward voltage and color point of high-power LEDs are now guaranteed within a tight set of bounds at a junction temperature of 85°C, allowing LED system designers to better predict the performance of their application (see Figure 1). This Freedom from Binning reduces engineering efforts, improves time-to-market, and results in more efficient and pleasing lighting solutions. This is especially relevant for light fixtures, which are placed in close proximity of each other, such as wall washers. The color consistency for these fixtures needs to be tightly controlled because the human eye can easily perceive small color differences between neighboring light sources (see Figure 2).
Due to practical considerations during production, hot-testing and binning of mid-power and low-power LEDs is not always feasible. However, in order to facilitate color targeting, Lumileds offers hot-targeting for some low-power and mid-power products. In essence, these products are tested at a junction temperature of 25°C but their color points during production are targeted to meet a nominal ANSI color point at a junction temperature of 65°C or 85°C. With this approach, Lumileds can still offer its customers much-desired Freedom from Color Binning.