How to maintain color levels of LED light sources within cars
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 the 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 (wave length ±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 are 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.
True color sensor measurements are based on the tristimulus procedure
MAZeT’s MTCS is such a true color sensor IC, which is offered in various models as sensor IC as well as functional model for numerous applications, including a calibration library. It is a miniaturized color sensor for color measurement, based on the tristimulus method. The difference between usual color sensors with absorption filters and RGB detection is that the MTCS can perform color measurements at a spectral sensitivity based on the accuracy and performance beyond the human eye.
Fig.1: The true color sensor IC MTCS is the world’s only miniaturized color sensor for color measurements based on the tristimulus procedure
Endless combinations of spectral configuration
The standard values X, Y and Z describe the color area that is a composition of endless spectral combinations. Therefore it is possible to generate a standardized white point D65 with a color composition of blue and yellow LEDs or via color composition of RGBx LEDs. At an identical color area differences within the emitted spectrum will remain. The measurement of the color areas is performed by tristimulus function that is based on the color perception of the human eye. Spectral sensitivity is defined within the standard DIN5033 and the international standard CIE1931.
Spectral identical simulation of the tristimulus function
The standard values for the description of the color areas X, Y and Z are a result of the emitted radiation and the spectral tristimulus function. A nearly spectral identical simulation of the tristimulus function is required for precise color measurement of emitted sources. The quality of the viable color measurement depends on the quality of these factors. Depending on the task, the measurement system requires certain accuracy as measurement error that varies from the nominal value, even after a previously performed calibration.
Fig. 2: To manage LED colors and color temperature long-term stable sensors like the MTCS with true color characteristics for all three color channels are required.
This deviation is defined as measurement error delta (for example ∆E for the deviation within the Lab-graph or ∆u’v’ for the color area error within the Lu’v’-graph) and is used for color measurement applications. Certain factors like the measurement procedure, absolute and repeat accuracy of the sensor, lighting, calibration and disturbance sources of the system environment are essential to reach the requested accuracy levels.
The human eye as standard of comparison
At the time the MTCS color sensors are the world’s only miniaturized sensors based on the scaled tristimulus function. The sensor signals X, Y, and Z can directly be evaluated as color areas within the color graph. After calibration and at typical measurement conditions it is possible to achieve values way beyond the perception limit of the human eye (∆E<3; ∆u’v'<0,002). Therefore true color sensors are suitable for any applications where the human eye is used as standard comparison for accuracy.
Three filters from high and low refractive layers
The basic accuracy of the MTCS color sensors uses PIN diode technology with optimized spectral sensitivity for the visible range. The resulting tristimulus sensitivity is a combination of the basic accuracy and specific filter options of the higher level layers. The three filters are explained by specific layer designs consisting of high and low level refractive layers and the combined interference effect for various wavelengths. These interference filter layers are directly patterned onto silicon wafers or glass carriers by a lithographic procedure and therefore are temperature and long-term stable filters.
Fig 3: The filter options of MAZeT’s true color sensors
To adjust the sensitivity to the standard values X, Y, and Z sensor data is scaled to a known white value via various combination colors. Additionally it will be corrected globally or locally via matrixing and can be transformed to any color space at a high accuracy.
About the author: Frank Krumbein is Product Manager Color and Sensor Systems, MAZeT GmbH, 07745 Jena, Germany
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