The LED – a highly sensitive creature
Without a doubt, the LED is currently revolutionizing the entire lightning technology segment. With the bulb of conventional incandescent and energy saving lamps flickering away into the past, this semiconductor-based light source has paved the way for a virtually unlimited number of designs; inspiring designers and architects alike to conceive new “luminaire” ideas. Besides that, its high efficiency makes the LED the most energy-efficient light source relative to all conventional lighting technologies. If we think about it, a LED is nothing other than a diode – a simple P-N semiconductor junction – that emits light. While a “normal” diode generally uses silicon as an “indirect” semiconductor, the LED diode relies on a gallium compound as a “direct” semiconductor. Without having to explain all the physics in detail, during the transition of the electron from the conduction to the valence band a photon is produced, which is emitted in the form of visible light. By altering the material used and its chemical composition, it is possible to affect the distance between these two bands, i.e. the band gap, and thereby determine the light's frequency and wavelength. Accordingly, it is also possible to have LEDs light up in virtually any color of the visible and even the invisible spectrum (ultraviolet, infrared).
On the downside, however, the LED’s variety of colors, which is literally a product of alchemy, is also the reason for its peculiarly fastidious behavior. Depending on the used chemical compound and the resulting color, there will be changes in the forward voltage, which varies from around 2V for red to up to 3.5V for blue LEDs, whilst also impacting brightness and ageing behavior. While this effect is hardly noticeable on a single signal LED, the side-by-side installation of three monochrome LEDs – normally red, green and blue – in a single housing, i.e. “RGB LED”, can present an almost insurmountable challenge by producing a large number of blended colors caused by the varying brightness of the individual LEDs. It becomes even more complex when these RGB LEDs – mounted in large numbers on copper strips – must meet strict requirements in the vehicle with regard to color fidelity and brightness stability under fluctuating temperatures and over the effective lifetime of the vehicle.