Preventing color from becoming a problem: Active LED Feedback Control

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By eeNews Europe

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 color output. But the utilization 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 color 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 color values on the black body curve to ensure that the color doesn’t only seem to be right, but actually is the same.

Modern solutions nowadays are the application of color sensors to achieve the feedback control loop. These measure the actual or target color 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, color sensors are a cost-efficient solution and during initial phases of prototyping and development, spectrometers are used to determine reference values. The color 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).

Utilizing a feedback loop system

There are known solutions to stabilize 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 color. This concept is shown in Image 1.

Image 1: Concept of a feedback control-loop system (Source: MAZeT)

The demonstrated solution differs from the (unregulated) control option by implementing a color sensor, depending on the lighting concept, to send RGB or other color 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. Image 1 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.

A main issue of the overall potential savings is the calculation of the total costs as well as future maintenance costs. An additional issue is the question of LED binning. It would be ideal if the light sectors could be directly controlled via the given light output. This would eliminate the requirement of buying from specific bins or just from a single LED manufacturer. Therefore this approach is not only technological beneficial but can also save money during the development planning. It is enough if a LED is within a specific color segment to later on adjust the overall light output. The consistency of the light will be regulated via feedback control.

Tackling the known LED issues

Temperature drifts – Drifts within the color perception and brightness of mixed light are inevitable. Additionally, the user must decide how to calibrate the light source, depending on the environmental influences (such as temperature or pressure) and desired output of light color. Especially in larger projects, that consist of multiple light sources it is problematic to maintain stable light color conditions. Even inexperienced viewers can see color differences and heterogeneous light color conditions at color point tolerance levels of ∆E= 2.5 to 3.

Aging drifts – Even though the lifecycle of LED data sheets document beneficial statistics compared to the traditional light bulb – LEDs also experience brightness drifts from the first day of usage. Usually the lifecycle of an LED is based on a brightness loss of more than 70% compared to the original values as well as the used electronics at hand. Far before the 70% loss-level, differences and heterogeneous lighting effects can be experienced, which are proven by scientific tests in lighting laboratories.

Brightness drift of RGB-LEDs related to time in hours (Source: Osram)

Brightness characteristics of RGB high brightness LEDs change within the first 5000 hours of operation. An interesting fact is that not only brightness loss has been measured, but also a temporary increase of the light output has been recorded. These characteristics differ, depending on the used materials within red, green and blue LEDs. It has been proven that the brightness loss within the first 5000 operating hours can vary between 5% to 15%. In other words – drift effects that can be seen by the average human eye are are even increased through color mixing effects. Achieving long-term color point stability is also a requirement to meet international standards like the EnergyStar regulations.

Exchanging single LEDs or lamps within a lighting system or network, as a result to environmental effects or damage can be a difficult and expensive maintenance task. These maintenance and running costs can be drastically reduced via feedback control options.

Image 3: Video wall illumination simulation at tolerances of greater ∆E=3 (Source: MAZeT)

What are the benefits?

The increasing demands for high quality products and market trends for light network solutions, demonstrate that it is essential to use a feedback control-loop solution if long-term stable color rendition and unity of light color or temperature output of multiple LED lighting solutions are the main goal of the application.

To achieve the best quality possible one needs to focus on the following four aspects:

• The right choice of the color detection sensor for the specific application (JENCOLOR True Color based on CIE 1931 for human eye perception)

• The right choice of LEDs regarding quality, quantity and cost-efficiency

• Calibration of the sensors to the specific application at hand

• Finding an idea control and regulation algorithm

Reviewing the initial question regarding intelligent feedback control algorithms to improve the overall perception based on the Color Rendering Index one must say that the color rendition quality is not solely enhanced by increasing the number of used LEDs. The quality also depends on the spectrum produced by the LED combinations. Via feedback control algorithms optimized with adequate side conditions (like CRI) it is also possible to improve the output quality/ color rendition.

When evaluating the further aspects, for example reducing maintenance costs, one needs to consider a feedback control solution during the development phase due to the following facts it is possible:

To minimize the required number of LED light sources within a system to achieve an equal or better CRI value.

To optimize the light color or temperature output of multiple light sources and eliminate problems such as manufacturer based replacement LEDs or binning.

To implement control options via optimized regulation algorithms to make it possible to reduce costs or adapt LED lighting solutions to green technology standards like Energy Star.

Author: Kevin Jensen, International Sales & Marketing, MAZeT GmbH


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