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Avoiding colour issues in LEDs

Posted: 16 May 2013     Print Version  Bookmark and Share

Keywords:LED  True Color sensors  Temperature drifts 

The LED market is going through phenomenal growth. Many industries have recognised the potential of this market and see opportunities in saving energy, effort and costs. But despite many advantages of the LEDs in general, the lighting industry must tackle many difficulties in this specific field of technology. How can consistent lighting conditions be achieved, even when lamps or LED systems need to be exchanged? Will my newly implemented system have the same colour temperature, even next to lamps of different age and from a different binning? How can ideal CRI values be rendered?

When combining LEDs of different vendors and quality it has been a general procedure to rely on specific binning methods to ensure a stable light colour output. But the utilisation of LED technology is increasing more and more in high quality applications, such as medical or industrial, or architecture design productions. These applications require not only accurate colour value outputs but also need to be stable and consistent in the long-term. By implementing feedback control solutions, it is possible to directly regulate lighting conditions based on colour values on the black body curve to ensure that the colour doesn't only seem to be right, but actually is the same.

Modern solutions nowadays are the application of colour sensors to achieve the feedback control loop. These measure the actual or target colour values and trigger the LED drivers to adequate current output values. The most accurate method to measure spectral values is performed via spectrometer. However, this technology is often too slow and expensive to meet market needs. Therefore, colour sensors are a cost-efficient solution and during initial phases of prototyping and development, spectrometers are used to determine reference values. The colour sensor market knows technologies with various spectral properties. The most common are the traditional RGB, or True Color sensors based on the CIE 1931 standard (human eye perception).

Utilising a feedback loop system
There are known solutions to stabilise the current and voltage of LEDs. There are also solutions that measure the temperature of LEDS (as far as technological possible) and report these values to the LED driver to create a temperature control-loop. These solutions are indirect regulation solutions. An alternative method is the direct regulation via light colour. This concept is shown in figure 1.

Figure 1: Concept of a feedback control-loop system.

The demonstrated solution differs from the (unregulated) control option by implementing a colour sensor, depending on the lighting concept, to send RGB or other colour values to the micro controller to directly regulate the LED light output. The software of the micro controller compares the given and set values and directs these to the output driver. Figure 2 demonstrates an RGB system. This concept works for any LED light source, such as RGB white or RGB amber-white.

The LED driver used in this system can be of simplistic and cost-efficient nature: even a driver without current or LED voltage regulation and without temperature sensor or feedback.

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