LEDS: AN EFFICIENT ALL-ROUNDER

From domestic households and the catering sector to industrial applications – light emitting diodes (LED) are becoming ever more popular, because high quality LEDs in particular from renowned manufacturers such as the lighting expert Osram feature significant benefits compared to standard light sources. They feature good colour rendering, high energy efficiency and durability, consequently reducing running costs such as power overheads or replacement costs. The structure and principal of LEDs Light emitting diodes consist of several layers of semiconductor material that generate light upon the application of DC voltage. In contrast to incandescent lamps an LED is a monochromatic light source, and according to its composition emits light with a specific colour. White-emitting LEDs also initially generate monochromatic light that must then be converted, and this is usually achieved with a combination of blue LEDs with a yellowish phosphor layer or by mixing the light of red, green and blue LEDs (RGB). Furthermore, specific RGB colours can be mixed to create an infinite number of colour tones by varying the mixing ratios. PET PCBs for greater flexibility Typical LED modules consist of several combined LEDs, a fixed PCB and various other components. A practical alternative to fixed fibreglass PCBs are flexible materials based on polymers, as these enable the production of LED modules with a high diversity of forms, in turn opening the way to new designs and lighting solutions. Flexible light engines can also be manufactured on rolls, providing simpler handling as well as automatic manufacturing and the reduction of stock keeping overheads. A robust, non-conductive material with a high optical reflection factor is ideal for producing such flexible PCBs, and the material should also be environmentally compatible and chemically stable. Such requirements are best fulfilled with polyethylene terephthalate (PET) versions. With regard to these advantages, the lighting manufacturer Osram developed its PrevaLED Flex Linear light engine that fully exploits the benefits of flexible PET PCBs. The light engine consists of a thin and pliable copper layer in which the requisite circuit has been etched and sealed between two PET layers. An efficient direct current solution Due to the electrical properties of LEDs, their drivers should directly supply a constant current to the module. The reason for this is that in contrast to LED systems requiring constant voltage, other current control components on the module are not needed with systems supplied by constant current directly from the LED driver. This provides efficiency increases of 5 to 15 per cent. A sophisticated system for grouping and mixing the various LEDs (various bins) is used to achieve homogeneous voltage distribution in the LED lines various (also switched in parallel) with simultaneous constant current operation, thereby also avoiding brightness and colour deviations within the partly long and flexible systems. This in turn enables a uniform distribution of light colour and brightness, across each section and the complete module. Light on a strip In addition to flexibility, a special benefit of the strip solution is cutting the strip to suit the desired length, enabled by switching the LEDs in parallel. With the Osram PrevaLED for example, the strip can be cut after each segment without the module forfeiting any quality of light. One segment has a length of either 93.3 mm equipped with 10 LEDs or 140 mm with 12 LEDs, and the length itself is restricted merely by the supply of current, which must be as homogeneous as possible for all LEDs to create light with high uniformity. In this regard the distance between the current source and the LED is an important factor – the maximal length of a flexible LED module with single-sided power input is currently 1.5 metres, and module lengths of three metres can be achieved with central power input. As a consequence, the length of the light strip can be individually modified between approximately 10 cm and three metres, and installations that previously required the back-to-back switching of several modules can now be produced as one unit. PrevaLED also cuts installation costs by up to 30% compared to standard lighting solutions. The LED strip can be bonded simply into luminaires or luminous elements, enabling the saving of valuable work time. The broad flexibility of PrevaLED comes to the fore during simple installation and the implementation of sophisticated design projects, and the PET strip can for example be bent to form a circle with a radius of just 15 cm. With a colour temperature of 4000 Kelvin, a good colour rendering index of over 80 and up to 170 lumens per watt, PrevaLED Flex is not only a genuine alternative to fluorescent tubes but also opens up new possibilities for designing light.