1878: The differential arc lamp
With the differential arc lamp of 1878, Siemens brought electric light to streets and plazas, thus also helping expand the delivery of electricity. In continuous competition with gas lighting, electric light slowly proved its worth in urban centers, initially with the arc lamp and then with the incandescent-filament lamp.
But in carbon arc lamps, the inevitable burn-down of the electrodes, which increased the distance between them, ultimately caused the arc to burn out. So the electrodes had to be adjusted constantly by hand – a complicated procedure for routine operation.
True, Werner von Siemens had already discovered the principle of differential regulation in 1873. But Siemens chief designer Friedrich von Hefner-Alteneck was the first to embody it in a design, in 1878. He invented an automatic control that autonomously adjusted the carbon rods so that the burned-out electrodes had to be replaced only occasionally. And now several arc lamps could be run from a single generator, where previously each lamp needed a generator of its own – a major step in the direction of general electric lighting.
1882: The first permanent street lighting in Berlin
Siemens brings light to darkness: The Berlin Industrial Exposition of 1879 saw the first series-connected differential arc lamps, powered only by a single dynamo. They were installed in the city's Kaisergalerie, a shopping galleria based on models from Paris and Brussels, now in the Mitte district of town. After this successful test, in 1882 Siemens got the order to install Berlin's first permanent electric street lighting on Potsdamer Platz and Leipziger Strasse.
This was followed by railroad stations, office buildings, factories and port facilities. It was the beginning of Germany's large-scale use of arc lighting. In 1888, finally, Berlin's grand Unter den Linden avenue was illuminated by 108 lamps.
Even though the investment was three times what it would have cost to improve the existing gas lighting, many municipalities decided on electric light. Yet the conflict between gas and electricity continued for several decades more, until electric street lighting finally won out.
Because it was so large, and its light was too brilliant for home use, the differential arc lamp was gradually replaced by the incandescent-filament lamp.
1905: Tantalum lamp
What would be the right filament for light? Werner and his son Wilhelm were already interested in the problem back in 1882. As they experimented looking for a suitable incandescent filament lamp, they first tried metal wires, but soon moved on to carbon filaments – which they did in grand style. Because that same year, Siemens built the first incandescent-lamp factory in Germany, producing the first Siemens carbon filament lamps.
But the investigators weren't satisfied. Wilhelm asked Werner Bolton to search for a better filament. It was Bolton who discovered that tantalum was a metal with all the desirable characteristics for conducting electric light. It has a high melting point (around 3,000 degrees Celsius), a low vapor pressure, and easy deformability. Ideal conditions for replacing fragile carbon filaments with a stable metal filament. In 1905 – some 20 years after the first experiments – Siemens presented its customers with the first commercial metal-filament lamp, the tantalum lamp.
Siemens had bet on the right horse. In the coming years, the tantalum lamp would become one of the company's biggest sales drivers. By 1914 Siemens had sold more than 50 million lamps worldwide. These lamps continued to be made in the USA, England and France even after World War I began, with filaments always provided by Siemens.
1980: The Circolux fluorescent lamp
In 1936, Siemens subsidiary Osram caused a genuine sensation at the World Exposition in Paris, with the first marketable fluorescent lamp. These low-pressure mercury-vapor lamps, incorrectly called "neon" tubes, had a phosphor-coated glass tube that emitted a relatively more natural light than sodium-vapor lamps. Within a few decades they had overshadowed all competitors, thanks to their advantages of high light output, uncomplicated handling and very good color fidelity.
A further advance was the Lumilux energy-saving fluorescent lamp of 1978. It worked with the 3-band principle, meaning it contained three phosphors (and rather expensive ones at that): one for blue light, one for green, and one for red. The color of the light could be varied by changing the phosphor composition.
In 1980, the Circolux using an incandescent-bulb socket achieved another milestone, emitting as much light as a 75-watt incandescent bulb while consuming only 25 watts (W). The Dulux EL compact fluorescent lamp lowered consumption from a 75 W incandescent bulb still further, to only 15 W – while offering a service life six times longer.
2005: The brightest LED
At the turn of the century, LEDs (light-emitting diodes) more and more began replacing conventional light sources like incandescent or fluorescent lamps. They were smaller, cooler and most importantly, consumed much less electricity.
In 2005, Siemens subsidiary OSRAM introduced the brightest white LED to date. The Ostar Lighting had an output of 200 lumens, literally putting conventional incandescent and fluorescent lamps into the shade. An Ostar Lighting LED had an average service life of 50,000 hours; burning eight hours a day, that came to nearly 18 years.
Just two years later, light output had been boosted again by a factor of five. For the first time, an LED now emitted more than 1,000 lumens. That outperformed the brightness of a 50-watt halogen lamp, making the little LED suitable for broad uses in general lighting. The Ostar Lighting LED could provide enough output to light a desk adequately from a height of two meters. And since it was so small, it offered the possibility of entirely new lamp shapes.
2009: First OLED for general lighting
In 2009, OSRAM presented its first light source made of plastic. The Orbeos brought the market an OLED area light for professional applications. OLED stands for Organic Light Emitting Diode – diodes made of thin organic layers that emit light when an electric current passes through them.
The extremely thin OLED panel was only eight centimeters in diameter, and weighed just 24 grams. Unlike conventional LEDs, OLEDs were not point sources of light; they were large-area emitters that provided low-glare, soft lighting combined with high energy efficiency. The Orbeos delivered a warm white light with a color value similar to that of an incandescent bulb. It could be switched on and off with no delays and could be dimmed smoothly. Unlike an LED, it offered easy heat management. The panel contained no mercury and emitted no UV or infrared radiation. In optimum use, it had a service life of about 5,000 hours.
These extremely thin, super-lightweight large-area emitters opened up hitherto unknown opportunities for lighting, such as luminous room dividers or entirely new luminaire designs.