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Autor: Dennis Jackstien


Lighting technology: Basics in LED technology (5 / 10)

After we have already briefly touched on some aspects of LED technology in the overview of lighting technologies, this article will take a closer look at what is undoubtedly the most important lighting technology at present.

In the following we will deal with the basics and characteristics of LED technology. A separate article then looks at the current problems and disadvantages of LED technology. 

Functional principle of LED technology

While traditional lighting technologies are based on heat radiation or gas discharge, LEDs make small semiconductors, often only 1mm² in size, glow. We will not examine the exact physical processes in detail, as these are not relevant for the application. But in the following we will show the structure of a classical SMD-LED:




By combining different materials in the semiconductors - among others Indium & Gallium - LEDs can be produced in different light colours. From UV over the visible range from deep blue to turquoise, green, yellow, orange and red to the no longer visible infrared, all types are available today.




Coloured LEDs have a very narrow-band spectrum and thus produce highly saturated, intense colours. By mixing different LED colours - usually red, green and blue (RGB) are used as a basis - pastel colours and of course white tones can also be achieved. Due to the narrow-band spectra, however, natural colour reproduction is not usually to be expected here.




This means that pure RGB LED spotlights, as they are often used, for example, for events or in theatres, can only be used for effect applications and never for classic, natural film or TV lighting.




White LEDs

By applying a phosphor layer on blue LED chips, a part of the narrow-band blue light can be converted into broadband, higher wavelengths, e.g. yellowish light (right-hand region in the spectra). Blue and yellow mixed together produces white light. The more phosphorus is used, the warmer (orange) the resulting white becomes.







With this method, significantly more filled spectra with better color rendering properties can be achieved. Modern processes today combine several phosphor layers and thus achieve excellent colour rendering values that are already close to halogen incandescence.




The entire manufacturing process of LEDs has been continuously improved so that even LEDs with excellent color quality today achieve efficiency values of 100lm/W and more in real operation. LED technology is therefore currently the most efficient lighting technology for high-quality white light.


Control

LEDs as semiconductors are operated with direct voltage. Since they also react very strongly to voltage changes, constant current is usually used for operation. Usual values are 20mA to 60mA, in the high-power range also 350 or 700mA.
Dimming of the LEDs is possible by lowering the constant current. However, as this leads to slight colour changes of the LEDs and is technically more complex, dimming is realised by high-frequency pulsing (fast on/off switching) in 99% of the available LED luminaires.
The frequency usually remains stable, but the duty cycle is changed. This means that the time in which the LED is switched on is shortened or extended accordingly. This is called pulse width modulation (PWM for short). A diagram is shown below:





The shorter the ON phase of the LED, the darker it appears (lower picture). Of course, this only works if the switching operations are carried out very quickly and therefore invisibly. With film and TV LED lights, at least 20,000 times per second (20 kHz), so that even cameras no longer perceive the pulsing.

Cheap LED lamps and lights for general use, however, are pulsed at much lower frequencies - sometimes only 200Hz. In this case a clear flickering in the camera image is immediately noticeable.


LED & Heat

LEDs do not generate any heat radiation (with the exception of special infrared LEDs). A glance at an LED spectrum shows that only visible light and no infrared radiation (= heat) is emitted.




However, since even the most efficient LEDs available today cannot convert all the energy introduced into light, power loss in the form of heat naturally occurs. This means that LEDs do not radiate heat, but they themselves become warm. If this heat loss rises above a certain level, the LED is irreparably damaged and loses its luminosity.

Therefore, thermal management must be considered for all LED luminaires with higher power, usually in the form of heat sinks and/or fans. LED luminaires with high luminosity and without good thermal management will not retain their full brightness for very long (possibly only a few hundred hours). These are also called "disposable LED lights".




Lifetime

LEDs can reach lifetimes of many thousands of hours (L80, B20), provided that they are well managed. The figures in brackets, i.e. L80 and B20, are decisive, because they indicate how many LEDs (here B20 = 20%) have a brightness drop of less than 80% (L80) after the specified hours. A total failure is very rare with LEDs, so the brightness drop is relevant.
Often other, worse L and B values are used. A specification like "Lifetime: 50.000h (L70, B50)" means e.g. that half of the LEDs do not have 70% luminosity after 50.000h. If nothing else is specified, a L70, B50 specification can generally be assumed.
With good thermal management, however, other components such as controllers or power supply units will usually fail much sooner in an LED headlight, so that the service life of the LEDs generally plays a subordinate role.

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