Whether it be in businesses or in public buildings, in shops, restaurants, hotels, or private homes, thanks to their environmental and economic advantages, LEDs are gradually taking over as illuminants from traditional light sources. Unfortunately, however, the world of LED lighting is not entirely straightforward. Good and efficient LED lamps require an intelligent activation system. This activation system must solve a number of technical challenges, among them, the color and intensity of individual LEDs which depend on age, current and temperature.
Thanks to their better efficiency than other light sources, the heat developed by LEDs is relatively low. The operating temperature range for the application must therefore be more carefully considered. In mobile situations (such as in vehicles), LED systems must function reliably within a wide temperature range.
Since the power dissipated in an LED is, due to the low temperature difference to the surroundings, carried away primarily by conduction or by forced convection (fans), a considerable additional outlay may be incurred here. Temperature changes cause changes in the color and brightness of LED light. For example, temperature fluctuations from 25 to 70°C (77°F - 158°F) during operation lead to a color drift of up to ?u’v ‘= 0.03. This is equivalent to a factor of 6 above the perception threshold of the human eye for color differences. Also, high working temperatures cause LEDs to age faster. Additionally, the brightness of LEDs from the same batch can vary by up to 20 percent and the dominant wavelength of LEDs of the same type may be spread over ±8nm. Even after being classified (i.e. sorted into one of several ‘bins’ in a process known as ‘binning’) by wavelength group, the wavelength can still vary by ±3nm.
Figure 1: Color control systems using Jencolor color sensors ensure a high constancy of brightness and color of light. They are also used in modern lighting systems that can automatically modify their brightness to suit ambient conditions.
Overall, these factors result in a change in or distortion of the required color that is perceived by our eyes. Up to now, it was generally accepted that the technical limit for the measurement of possible color deviation over the device lifetime was ?u’v’ < 0.007 (e.g. Energy Star). More recent research, taking the comparability of adjacent light sources into account, assumes future limiting values of the order of ?u’v’ < 0.001, which are to be realized as the minimum target/actual color deviation in lights in operation and variation of ambient conditions.
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This article briefs about the color changes in the LED light output over a period of time and how it can be controlled to have minimum perceivable to our eyes.The active color control techniques said here can solve this.The cost of the lighting will increase accordingly.The color sensors mentioned here also will age and this to be matched with the life of the LEDs. As such the wave length of light output from the LED varies as the operating current and the temperature changes.So it is a multi computational effect and the circuit design need to address this effect with good dynamic range.
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