|
Tungsten Quartz Halogen Lamps are the most efficient incandescent lamps, offering precise beam control, better lumen maintenance, longer life and compact design.
The halogen cycle ensures that the bulb wall is kept clean to ensure better lumen maintenance, the linear construction of tungsten filament provides precise beam control and slim & strong quartz body enables the lamp to operate at higher pressures to give compact design and longer life.
- Construction of Halogen Lamp :
- Quartz (fused silica) body :The lamp is typically made from of quartz tube (10 mm OD, 8 mm ID). Softening point of quartz is 1800ºC as compared to 700ºC for ordinary soda glass. Processing of quartz requires very high temperature and therefore, oxy-hydro or oxy-LPG flame is used in lamp making. Quartz has very low coefficient of thermal expansion (5x107/ºC) and therefore, very high thermal shock resistance. If water is sprayed on a GLS bulb, the bulb cracks but halogen lamp can easily withstand such a thermal shock.
- Filament : Like any other incandescent lamp (GLS bulb), filament of halogen lamp is also made of tungsten wire in single or coiled-coil configuration. The filament is placed along the axis of the tube with the help of a series of support rings (separated by 10 to 20 mm) also made out of tungsten wire. The ring ensures that filament does not touch the quartz tube and a perfect alignment is maintained.
- Pinch Seal :For glass-to-metal seal in a halogen lamp, a molybdenum foil (with feather edge) is used. A typical moly foil is 3 to 4 mm wide and 0.028 mm thick in the middle and has feather edges. For higher current rating, greater than say 12 amps, thicker and/or wider foil is use.
- Contact Button :In a typical halogen lamp for lighting and in some halogen IR lamp, R7s contact buttons are welded or crimped to the molybdenum lead wires. The spherical concave curvature of the button facilitates electrical contact with a spring loaded holder.
- Ceramic Cap :A cylindrical cap made of a ceramic called steatite is commonly used in halogen lamps. A cement is used to bond the cap and the quartz body of the lamp.
- The Halogen Cycle : In any incandescent lamp, tungsten vaporizes from the filament and condenses in the bulb wall causing blackening leading to lower light output. In halogen lamp, a trace amount of halogen (most commonly bromine) is added along with an inert gas (most commonly argon). The halogen reacts with the tungsten deposition on the wall forming volatile halides and oxy-halides of tungsten (such as WBr6, WO2Br2, etc). These volatile tungsten bearing molecules decompose when they hit the hot filament, depositing tungsten on the filament and releasing halogen to again react with tungsten deposited on the wall. Thus, a trace amount of halogen is able to return continuously all the evaporated tungsten back to the filament. This is the halogen cycle. For tungsten to react with halogen on the wall a certain minimum temperature (Tmin) is required. To achieve a temperature higher than Tmin, halogen lamps have to be very compact; typical volume being 1% of the conventional bulb of the same wattage.
- Filament Temperature and Colour Temperature : For lighting applications, typical filament temperature is between 2800°K to 3400°K. On the other hand, for infrared heating applications, tungsten temperature could be anywhere from 2000°K to 3200°K, most commonly used temperature being 2500°K. As the filament temperature is raised, the radiation spectrum changes. For a given filament temperature (T°K), the wavelength at which the radiant energy is maximum can be estimated by the Wien’s Law :λ max = 2898/T°K.
The filament temperature can be estimated from the ratio of the hot resistance (RH ) and cold resistance (RC @ 25°C) of the filament. Since the data of resistance of tungsten as a function of the temperature is accurately known, the ratio RH / RC gives a fairly accurate estimate of filament temperature. Emissivity of tungsten at blue end is higher than at red end and therefore, the apparent colour temperature of a tungsten filament is higher than the actual filament temperature by 50 to 100°K around 3000°K.
- Lamp Life and Voltage Relationship : Halogen lamp like any other incandescent lamp is rated for a certain light output (lumens) at a rated watt (W) when operated at the rated voltage. Efficacy of the lamp is calculated as lumen/watt and life of a lamp changes with the applied voltage. For small changes in voltage (<10%) life is inversely proportional to be applied voltage, the exponent being 13. Thus a 5.5% higher voltage will reduce the lamp life by a factor of 2. Efficacy (lumen/watt) increases with voltage, the exponent being 1.9.
Rated lamp lives are therefore a strong function of colour temperature and efficacy. A few rated values are given below in a table for a 1000W lamp.
|
Colour temp |
Lumen/watt |
Rated lamp life |
| i. |
3400º K |
33 |
50 hr |
| ii. |
3200º K |
26 |
200 hr |
| iii. |
3000º K |
21 |
2000 hr |
| iv. |
2500º K |
9 |
> 5000 hr |
- Lamp Life, Lumen Maintenance and Fill Pressure : Halogen cycle returns all the evaporated tungsten to the filament and therefore the lamp remains clean right upto the very end of the lamp life. Lumen maintenance of halogen lamp is practically 100% as against ~ 80% for non halogen incandescent lamp.
Although all the tungsten evaporating from filament returns to the filament, the halogen cycle by itself does not significantly enhance the lamp life. Longer life of halogen lamps stems from higher fill pressure of the inert gas in the lamp which significantly retards evaporation rate of tungsten, thereby improving the lamp life by a factor of 2 and higher.
- Reflector Lamp :
Infrared halogen lamps are commonly coated with alumina reflector coating. This enhances the IR heating systems efficacy significantly by reducing heat losses. In some applications, a gold coating is used as a reflector coating on the lamp.
|
