Light emitting diodes are perfect for lighting purposes within cars. But the side effects of LED light is that the wavelength changes with the ambient temperature, the intensity loses its strength over time, and age-related darkening of the lenses can additionally alter colors. But color levels of LED light sources can be maintained through appropriate color sensors and intelligent lighting control.
The usage of LEDs within the display and illumination technology is drastically increasing. The determinant factors for this development are not only the reduced size of the components, but decisively the increased life-cycle, lower energy consumption, increased performance, and broad range of color usage (color gamut).
By combining RGBx-LEDS (x = extension of RGB with additional LEDs colored amber and white) within a light source, it is possible to generate any color or color temperature, which are defined by the limitations of the single LEDS within the color gamut. Therefore it is possible for illumination engineering to simulate daylight, individual ambient lighting, other visual effects, and color temperatures.
Many factors affect light color
The emitted spectrum of a LED depends on various factors. Especially variations within operating temperature, production tolerances, diode currents, life cycle of LEDs as well as tolerances of driver electronics can alter the lighting characteristics drastically. Brightness and colors are affected during operation, hence to environmental influence such as temperature fluctuation.
Active controlling allows maintaining constant LED lighting
How is it possible to maintain constant lighting and color impression for lamps, lights or back lights over a long period of time? This is especially important for a luminary setup of multiple RGBx light sources.
To maintain constant color impression and brightness over a full lifecycle of a RGBW-LED it is necessary to have an active control of the color area and perform a targeted mix of the LED color range. Simple, inexpensive, long-term and temperature stable sensors are essential to measure the color area of specific LED sources. Therefore the sensor may not change its characteristics over the lifetime of the LED. Sensors that lose their characteristics after some hundred hours operating time are unsuitable.
Nominal/actual value comparison provides correction signal
The nominal value of the color area is send from the sensor to the management settings and generates a corrected control pulse after the comparison. This is used to adjust the brightness of the individual LED colors and the resulting color mix will be set to the target color area.
The management of tolerance afflicted components is ideally the action basis of individual units. For LEDs, various diode temperatures influence the color (wavelength ±6 nm) and the brightness (±30%). These can vary during operation from LED to LED and can clearly cause noticeable color area differences.
Management via reference data provides inaccurate results
These color area differences exist for operating temperature fluctuations of 25° to 70°C (77° - 158°F) with up to ?u'v'=0,03 at a factor 6 above the perception limit for color differences of the human eye. It is possible to receive acceptable data by controlling through referential data and tables that are based on thermal sensitivity shift, operating time, etc.—but these methods are bound to average values and are inaccurate, due to their indirect measuring nature.
The characteristics of the human eye must be taken under consideration
An adjustment of the light is only possible if a sensor can measure color and brightness of the light exactly equal to actual value. Furthermore the characteristics of the human eye's color sensation need to be considered within the sensor parameter. The combination of these two factors is the basis for an accurate light color adjustment.
Color measurement at the light source based on special true color sensors at standardized tristimulus value sensitivity are an essential requirement for active color area management with an accuracy of ?u'v'<0.0025, that lays far below the perception limit of the human eye. A further important advantage of this controlling option is that one saves the cost of high-tech electronics to achieve constant LED driver performance above temperature or life cycle. Even inadequateness in electronics can be managed via direct access to the LED, based on fast controlling options.
For the complete article, which looks at a true color sensor design that uses the tristimulus procedure, click here, courtesy of Automotive Designline Europe.
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