2510332 - February 2009
LED behavior changes considerably with junction temperature. The characteristics are different depending on the type of LED construction used for the given color. For example, red colors typically behave quite differently from green or blue. This presents a challenge for achieving or maintaining a desired white point. Thermal characterization and active color output monitoring may be required.
LED’s are typically grouped into performance bins that can have a wide range of performance specifications even within the same bin. The more bins that can be selected from to meet your product needs, the lower the cost of the die will be. But this can further complicate the ability to achieve the desired color balance and output.
LED output may decay differently depending on the construction and material used for a given color. This produces differing decay rates between colors, causing white points to shift over time unless active color monitoring is used with some method to correct relative output.
LED lifetime is strongly related to thermal control of the junction temperature.
LED dominant wavelength will shift with temperature effects.
It is not difficult to characterize a given system using LED’s, but it is a very complicated trade study to optimize the design of one.
5.1.1 Projection Optical System Using LED’s
A typical layout for a projection system using LED’s is shown below in the figure below. The projector architectures used are the same as for lamp-based systems, with the exception of the light source and lack of color wheel. In this case telecentric TIR prism architecture is used, but it is easy to see that any telecentric, nontelecentric, or field lens architecture could be used since the projector is basically the same as a lamp system from the integrator to the screen.
There are many ways to arrange and combine the colors as well, such as cross-cube or plate dichroics, multi-color die in one or more packages with fan or wedge dichroic combiners, and some proprietary methods from other companies. Contact TI for details on these options. The method used typically depends on constraints for size, thermal management, and product packaging layout.
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