White mischiefRichard WilsonThe white LED is the least understood of all recent LED developments, according toPhilip Logan, business development manager for supplier Marl International.
Consequentially this product is the one most likely to exhibit unexpectedcharacteristics to the unwary designer.
The white LED is produced by a combination of a blue high intensity LED, and a phosphormaterial. The narrow band blue output characteristic is used to excite the phosphor, whichin turn fluoresces, resulting in a wideband response, emitting a bluish-white colour.
"While this would appear to the naked eye to be white, it lacks much of the redspectrum, and has a radically different spectral output when compared with incandescentfilament lamps," says Logan.
LEDs are being used to replace incandescent products in a wide range of retrofitapplications, and in many instances, they are required to operate behind some form ofoptical filter. These optical filters are predominantly colour filters, used to removecertain bands of incandescent output to achieve particular colours.
"Because of the difference in spectral output outlined above, care needs to betaken when using a white LED to replace an incandescent bulb," says Logan.
For example, a red LED behind a red filter will give a better performance than using awhite LED, despite the original light source being a white incandescent bulb. White LEDshave tremendous potential for illumination, rather than indication, and research is inprogress to develop this type of application. www.marl.co.uk