Introduction to Quartz Tungsten Halogen Light Sources

Quartz Tungsten Halogen (QTH) lamps are proven visible and near infrared sources, thanks to their smooth spectral curve and stable output. They generate little ultraviolet radiation and do not exhibit sharp spectral peaks. The QTH lamps offered by Oriel Instruments employ a doped tungsten filament within a quartz envelope. They are packed with a rare gas and trace amounts of halogen.

The tungsten filament is heated to >3000K by the application of current. The filaments of Oriel Instruments’ 10 to 250 W lamps are dense planar structures to achieve the highest image brightness. The white light generated radiates via the clear quartz envelope. Different QTH lamps are shown in Figure 1.

Figure 1. Various QTH Lamps

Spectral Output of QTH Lamps

The spectral distribution of the irradiance from the 6315 Lamp at its rated voltage is illustrated in Figure 2. All Oriel lamps have similar shaped spectral irradiance curve, but differ in the location and height of the peaks that rely on the model of lamp and operating environment. The radiated energy and its spectral distribution can be determined from the filament temperature, emissivity and transmission of the envelope.

Figure 2. Spectral irradiance at 0.5m from the 6315 1000 W QTH Lamp

The luminous output is especially sensitive to the filament temperature and the filament shape is crucial in the directional distribution of the radiation. The irradiance from 250 to 500nm for the 6333 100 W lamp at different voltages is depicted in Figure 3. There is a reduction in the output with reduced voltage, but the peak wavelength value changes only slightly to the red. Conversely, there is a considerable change in the output at a wavelength in the blue end of the spectrum with a slight change in voltage.

Figure 3. Spectral irradiance at 0.5m from the 6333 100 W QTH Lamp at different voltages. The lamp is rated for 100W at 12V.

Color Temperature

Color temperature and total radiated lumens are useful data to compare lamp performance in the visible. The efficiency in the visible (number of lumens per watt) improves with color temperature due to the shifting of the spectral distribution curve towards shorter wavelengths with increasing temperature (Figure 4). In the visible region, the emissivity of tungsten is roughly 0.4. Hence, the output is below that of a full radiator of equivalent temperature. An exact match of spectral distribution with a full radiator is not available owing to the fact that the emissivity changes with wavelength (Figure 5).

Figure 4. Correlated color temperature and voltage relationship

Figure 5. Spectral exitance of (1) full radiator, (2) a tungsten surface and (3) a gray body with emissivity of 0.425, all at 3100K

Lamp Construction

The 6336 600 W QTH lamp is illustrated in Figure 6. All Oriel QTH lamps employ a high temperature quartz envelope with molybdenum foil hermetic seals, thereby enabling the filament to be operated at higher temperatures. This causes the spectral output curve to move towards shorter wavelengths, thus leading to increased output in the short wave blue end of the visible spectrum and a 'whiter' light.

Figure 6. 6336 600 W QTH Lamp. The filament is shown on the right.

Geometrical Considerations

The closely packed filaments in the 10 to 250 W lamps are called rectangular dense filaments. It is possible to operate these lamps either vertically or horizontally. The orientation of the emitting plane is based on the target. For instance, horizontal operation of the lamp is recommended for imaging the he filament onto a vertical monochromator slit in order to match the filament image to the slit shape. The larger lamps feature coiled filaments, which require special consideration to light uniformity for imaging.

Lamp Life

All tungsten filament lamps involve evaporation and deposition of the tungsten from the filament on the inside of the envelope, causing thinning of the tungsten filament and blackening of the bulb wall. This, in turn, gradually decreases the light output. In tungsten halogen lamps, the deposited tungsten is effective removed and returned to the hot filament by the halogen gas. This leaves the inside of the envelope clean and yields greater long term stability (Figure 7). This thermo-chemical process is known as the ‘Halogen Cycle,’ which significantly improves the lamp life.

Figure 7. Typical Light Output vs. Time, of the 6333 QTH lamp and a conventional filament lamp.

The filament parameters are selected for a specific operating voltage. Lamp life is the average life of a number of lamps operated at the rated voltage in open air. The life of the lamp reduces significantly when it runs at a higher voltage. A slight decrease in voltage increases lamp life greatly. However, the reduction should not be more than 10% because the halogen cycle will not occur effectively under this condition. This again leads to reduced lamp life.

Short Term Light Output Stability

A heated filament emits light. Voltage changes do not have an immediate effect on the temperature of the filament as well as on the light output. The lower thermal mass of smaller filaments makes them to show more variation.

Complete and Calibrated QTH Systems

Oriel Instruments offers new CE marked QTH sources along with the housing, lamp, power source, and all necessary cables and hardware. The company also supplies the components individually to enable customers to build their own systems. The complete QTH systems offered by Oriel Instruments include low-cost 10 - 20 W QTH Source, Series Q Sources operate QTH lamps to 100 W, CE Marked 10 - 250 W Research Sources, and CE Marked 600 - 1000 W Research Sources.

Oriel Instruments also sells calibrated lamps, which are sources with measured, NIST traceable, spectral irradiance. These sources can be used for calibrating detectors from 250 - 2500nm or irradiating samples of known power density and spectral distribution.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.