Various quartz tungsten halogen Lamps.
Quartz Tungsten Halogen (QTH) lamps are popular visible and near infrared sources because of their smooth spectral curve and stable output. They do not have the sharp spectral peaks that arc lamps exhibit, and they emit little UV radiation, see the spectral irradiance curves of our QTH lamps.Our QTH lamps use a doped tungsten filament inside a quartz envelope. They are filled with rare gas and a small amount of halogen. Current flowing through the filament heats the tungsten to >3000 K. The filaments of our 10 to 250 W lamps are dense planar structures for highest image brightness. The white light produced radiates through the clear quartz envelope.
Fig. 1 shows the spectral distribution of the irradiance from the 6315 Lamp at its rated voltage. The shape of the curves for all our lamps are similar, but the location and height of the peak depend on the model of lamp and operating conditions. The filament temperature, emissivity and transmission of the envelope determine the radiated energy and its spectral distribution.The luminous output is particularly sensitive to the filament temperature, and the shape of the filament is important in the directional distribution of the radiation. The total irradiance at 50 cm, from the 1 kW 6315 Lamp, is much less than ten times that of the 6333 100 W lamp. This is because of differences in filament temperature and shape.
Fig. 2 shows the spectral irradiance from 250 to 500 nm for the 6333 100 W lamp at different voltages (the filament plane was parallel to the slit of the radiometer for maximum irradiance). As voltage is reduced, total output is reduced and the peak wavelength shifts only slightly to the red. The output at a wavelength in the blue end of the spectrum can change significantly with a slight change in voltage.
Color temperature and total radiated lumens are useful for comparison of lamp performance in the visible. The efficiency in the visible, the number of lumens per watt, increases with color temperature because the spectral distribution curve moves to shorter wavelengths, i.e. towards the visible, with increasing temperature (Fig. 3). The emissivity of tungsten is about 0.4, in the visible, so the output is less than that of a full radiator of equivalent temperature. Because the emissivity varies with wavelength, there is not an exact match of spectral distribution with a full radiator (see Fig. 4).The color temperature listed for our lamps, in the Ordering Information Table, is the correlated color temperature. This is the temperature in K (Kelvin) of a full radiator (black body), which emits light of similar color balance (chromaticity) as the tungsten halogen lamp. It is a recognized mathematical best match. For a filament temperature of 3300 K, the color temperature is approximately 3390 K.
All our QTH lamps use a high temperature quartz envelope with molybdenum foil hermetic seals. This allows the filament to be run at higher temperatures, which shifts the spectral output curve to shorter wavelengths. The result is increased output in the short wave blue end of the visible spectrum and a whiter light.
The 10 to 250 W lamps have closely packed filaments; we call these rectangular dense filaments. These lamps may be operated vertically or horizontally. Orient the emitting plane with consideration to the target. If you are imaging the filament onto a vertical monochromator slit, for example, operate the lamp horizontally so the filament image matches the slit shape.The larger lamps have coiled filaments. Give special consideration to light uniformity when imaging these filaments. Refer to the outline drawings for the filament shapes and and the Ordering InformationTable, which lists the dimensions for all our QTH lamps.
In all tungsten filament lamps, the tungsten evaporates from the filament and is deposited on the inside of the envelope. This blackens the bulb wall and thins the tungsten filament, gradually reducing the light output. With tungsten halogen lamps, the halogen gas effectively removes the deposited tungsten, and returns it to the hot filament, leaving the inside of the envelope clean and providing greater long term stability (see Fig. 5). This thermo-chemical process is called the Halogen Cycle, and it greatly increases lamp life.
The filament parameters are chosen for a specific operating voltage. Lamp life is the average life of a number of lamps run in open air at the rated voltage. Operating these lamps at a higher voltage greatly reduces the life. A 6% increase in voltage reduces the operating life by 50% (Fig. 6). A small reduction in voltage gives a longer lamp life; a reduction of more than 10% may drastically shorten lamp life as the halogen cycle cannot operate effectively under this condition.
Light is emitted by a heated filament. The temperature of the filament does not immediately follow changes in voltage, so the light output does not follow rapid (kHz) voltage changes. Smaller filaments have lower thermal mass and therefore exhibit more variation.