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What are the price advantages of light color regulation via feedback control?

When looking at color correction or control solutions in LED lighting systems - these are often regarded as an expensive add-on. But actually it is possible to reduce the overall costs and future maintenance costs of a lighting system by using direct feedback solutions. This article demonstrates how to resolve various issues LED engineers have to cope with.

The current state-of-the-art LEDs demonstrate a high luminosity intensity but are in an early development-stage when it comes to brightness relations between chip temperature as well as aging-effects of the light-emitting layer of LEDs.

TEMPERATURE DEPENDENCE

The dependence of the brightness and the wavelength in correlation to given temperatures are known negative issues. Image 1 demonstrates the behavior of red, green, blue combinations of a LED light source.

Image 1: Dependence of wavelength and brightness regarding to the temperature of an RGB-LED

Image 1 shows that the dependence of a red LED is greater than compared to a blue LED. The brightness decreases by almost 40% during temperature drifts of 5°C (41°F) to 70°C (158°F).

Shifts 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, consisting 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.

Image 2: Tiling Wall illumination simulation at tolerances of ∆E=3

Example: A specific characteristic of the human eye is to react to color differences - whereas the evaluation of theabsolute color value is problematic without comparison values. These differences in color perception can even be increased by varying the brightness levels. Therefore slight color variances in LED panels or backlighting solutions can result into a major technological issue.

AGING EFFECTS

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.

Far before the 70% loss-level, differences and heterogeneous lighting effects can be experienced, which are proven by scientific tests in lighting laboratories.

Image 3: Brightness drift of RGB-LEDs related to time in hours (Source: OSRAM)

Image 3 demonstrates the brightness characteristics of a RGB LED within the first 10000 hours of operation. An interesting fact is that not only brightness loss could be experienced, 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 even within the first 5000 operating hours can vary between 5% to 25%. In other words - drift effects that can be seen by the average human eye. Achieving long-term color point stability is also a requirement to meet EnergyStar regulations.

Exchanging single LEDs within a lighting system, 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.

The question is: “What kind of cost-efficient solution can resolve these issues?”

THE CONCEPT OF OPTICAL FEEDBACK SYSTEMS

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 4.

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 4 demonstrates an RGB system. But this concept works for any LED light source, such as RGB white or RGB amber-white.

What is not demonstrated is how the color sensor detects the light. Therefore it is useful to generate simple solutions whereas reflecting light can be send to the color sensor. In other words existing reflectors of manufacturers can be used to achieve this goal at no or hardly any cost increase.

Image 4: Concept of optical feedback via light color of LEDs

BENEFITS OF OPTICAL FEEDBACK SOLUTIONS

The LED driver used in 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 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.

If one wants to use the binning segments highlighted in green, the only requirement is to adjust regulation range. The regulation range is usually set to a value of 30% based on aging- and temperature drift effects, therefore the segment needed to eliminate binning effects is of little importance. But it can save money during the purchasing operation, since tolerance ranges of LED Binnings can be strongly reduced or even eliminated. Often it is hard to buy the same classes/ranges of LEDs at one manufacturer without waiting periods. When taking the availability of specific LED classes into consideration, this can ease the entire purchasing process and limit waiting times until the next class of the same manufacturer is ready.

The visual light color range is shown in Image 6. The continuous line demonstrates the presentable color gamut based on all tolerances including the regulation range.

 

Image 5: Binning of LEDs by LED manufacturer Luminus

THE OVERALL SAVINGS POTENTIAL

In the end of the calculation one needs to recognize the costs benefits and compare. Eliminating negative issues and effects of LEDs such as binning or color inconstancies as well as creating a long-term stable solutions are just some elements, which can reduce the overall development costs and future running maintenance costs. Therefore it is essential to find the right solution for the given problem and weight the benefits and costs.

 

Image 6: Visual range within the color gamut with RGB-LEDs based on color tolerances (Binning)

The given solution sets itself apart from traditional lighting systems by utilizing control options of LED sources, eliminating negative aging, temperature and light color shifts, generating long-term stability and reducing potential future maintenance costs. It is important to choose a sensor that does not show aging-effects itself and is based on the perception of the human eye. The JENCOLOR sensors by MAZeT offer exact these characteristics.

BIBLIOGRAPHY:

  1. MAZeT-Homepage: http://www.mazet.de
  2. Color sensor product information: http://www.mazet.de/en/products/jencolor
  3. A. Wego, Korrekte Erkennung von Farben und Oberflächen mit Farbsensoren, Photonik, 5/2010, pp. 38-42, 2010
  4. OSRAM (LED aging-characteristics) http://www.osram.de

 

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