Technologies LED – light emitting diodes History of these light sources dates back to 1907 when H. J. Round, PA to G. Marconi, observed electroluminescence on crystals of silicon carbide. O. V. Losev described this phenomenon thoroughly in 1927. K. Lehovec provided deep theoretical explanation in 1951.

Several infrared-emitting semiconductors were created in 50s. In 1962, N. Holonyak presented the first red light emitting diode. Yellow LED followed soon and the efficiency was growing. LED price dropped dramatically in the beginning of 70s so they were used as indicators and displays of portable calculators. Green LED followed after a while, but the blue one was invented only in 1989. First white LED appeared soon after this discovery. Inventions made in 90s improve the efficiency of LEDs dramatically, thanks to multi-stage, multi-layer or resonant structures. The power of LEDs grown from miliwatts to units of watts and their efficiency has already beaten fluorescent tubes and HIDs.

LEDs can be found as indicators in most of the electronic appliances, they backlit the displays of cellular phones, hand torches and are even used as a source of light in some TVs and data projectors. LEDs are new to lighting industry and they can be used in exterior, interior, architectural and transport lighting.

LED is a semiconductor source of light. Layers of silicon-based materials emit light as electric current passes through them. The spectrum of the emitted light is a bell-shaped in a relatively narrow band around a dominant wavelength. So LEDs are primarily sources of colour light.

White light can be achieved by combining several colours, blue LED with yellow phosphor being the most commonly used. The blue light partly passes the phosphor layer and partly gets converted to the yellow light. The resulting light looks white. LEDs of warm to daylight colour can be obtained, however, the R_ais only about 75 to 80. Special phosphors can be used to push R_ato about 90 for sake of efficiency. Another approach is to use a (ultra)violet LED and a mixture of colour phosphors. R_aof 97 can be achieved and high lm/W are expected. Yet another approach is mixing the white from several colour LEDs. Using the most common RGB, R_aof about 70 can be achieved. Using newer components with four colours, RGBA (A for Amber), R_aaround 90 can be reached. Ra of 97 has been reached with RGBAW (W for White). The colour components can be driven independently which offers a wide range of applications. LED/Light/Engin units with an RGBW matrix, cooler and driver introduce a complete solution for creating a comfortable light with variable parameters, dimming, chromatic temperature changes, mood, decorative or biodynamic light.

The advantage of LEDs over other light sources is their great efficiency and a potential to increase it even more. The fluorescent and HIDs have reached their maximum around 100 lm/W while LEDs are already at 150 lm/W and it’s expected that more than 220 lm/W can be reached. Another advantage is longevity: LEDs live 3 to 5 times longer than fluorescent tubes. LED’s start-up time is much shorter than that of incandescent bulb, which makes LEDs great for car brake lights. Fast switching (PWM) can be used to control the average luminous flux of LEDs from almost zero to 100% with tiny losses.

The basic drawback of LED is high initial cost, which is not yet balanced by their longevity and efficiency. The LED way mostly equals new luminaries too. There are some retrofit lamps with LEDs on the market, but it’s often just a halfway solution. A paradoxical disadvantage is their potential efficiency in combination with a long lifetime. Some investors rather choose a cheaper traditional solution and wait. The longevity of LEDs is often overrated and the expected 30,000 to 100,000 hours can be achieved with expensive cooling system. A quaint con can be their high efficiency too – LED traffic lights freeze and get covered with ice while the old incandescent still work.

LEDs are often criticized for low colour rendering. Tests show that people judged some LED lights better than a plain R_acomparison would suggest. It is possible that wider use of LEDs will require a new metric for quality of colours that would reflect the subjective feeling from a given light better than Ra, which does not work very well for LEDs.

The year 2015 is estimated to be a breakthrough in LEDs use.