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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 

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 colour 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 colour 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 colour. Especially in larger projects, that consist of multiple light sources it is problematic to maintain stable light colour conditions. Even inexperienced viewers can see colour differences and heterogeneous light colour conditions at colour point tolerance levels of ∆E= 2.5 to 3.

Ageing 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 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 colour mixing effects. Achieving long-term colour 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.

Figure 2: Video wall illumination simulation at tolerances of greater ∆E=3.

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 colour rendition and unity of light colour 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 colour detection sensor for the specific application (JENCOLOR True Colour 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 Colour Rendering Index one must say that the colour 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 optimised with adequate side conditions (like CRI) it is also possible to improve the output quality/ colour 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 minimise the required number of LED light sources within a system to achieve an equal or better CRI value.
 • To optimise the light colour or temperature output of multiple light sources and eliminate problems such as manufacturer based replacement LEDs or binning.
 • To implement control options via optimised regulation algorithms to make it possible to reduce costs or adapt LED lighting solutions to green technology standards like Energy Star.

About the author
Kevin Jensen is from the International Sales & Marketing group of MAZeT GmbH.

To download the PDF version of this article, click here.


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