Light Emitting Diodes (LED)
LEDs generate the light inside a solid state material with an electrical current (most common is a gallium arsenic semiconductor crystal). The light generation takes place at room temperature (The LED remains cool). The conversion itself from electrical current into light is very efficient. The problem is to guide the light outside the crystal with little loss. The life time and reliability of LEDs is very high as there are very little chemical or physical reactions due to the low operating temperature. LEDs are well suited for applications requiring little power but high reliability.
Due to the semiconductor physic, the LED emit monochrome light only (e.g. red, green, blue). White light can be obtained by two different methods:
- mixing different colors from different monochrome LEDs
- partial conversion of the light of blue LEDs to yellow light. The result is white light made of yellow and blue.
Halogenlamps
As for regular incandescent lamps, the light is emitted by a tungsten filament. But for halogen lamps, the glass bulb remains always clear. This is achieved be the filling gas. This gas avoids that evaporated tungsten is deposited at the inside of the glass bulb. Instead, the evaporated tungsten reacts with the filling gas to a gas which then flows back to the filament. At the hot surface of the filament, the tungsten is deposit on the filament and so, the tungsten is recycled. The most advantage of halogen lamps is the low price and the simple operation.
HID-Lamps
(HID=High Intensity Discharge) Other common names are: metal halide lamps, short arc lamps and others. The light generation of HID lamps is much different from halogen lamps. HID lamps do not have a filament, the light is emitted by an electrical arc. This arc is generated inside a glass bulb between two tungsten electrodes. Inside the arc, the temperatures can be as high as 6000 °C. With additives to the gas filling, the efficiency and the color temperature can be controlled.