Editorial Feature

Considerations for Choosing Between Arc or Halogen Lamps

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Jul 30 2007

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There are several important factors that would help in deciding the best type and size of lamp for any application. These factors include spectral distribution, radiance, total output, source size and shape, and source stability.

Spectral Distribution

Spectral distribution represents energy radiation is distributed in different wavelengths at a visible spectrum. In considering this factor, specific wavelength standards (e.g. wavelengths causing stray light or unwanted sample heating) shall also be considered.

It is ideal to choose a lamp with high output in the preferred spectral region and low output at wavelengths that can cause stray light or other problems. Arc lamps are primarily ultraviolet to visible light sources. Mercury arc lamps have very strong peaks in the ultraviolet region. Meanwhile, tungsten halogen lamps are a good choice for longwave visible to near-infrared applications.

Radiance

This factor deals with the brightness requirements of a particular area where the lamp would be installed. There are lenses available (e.g. monochromator slit, fiber optic, detector, target) between the source and the area to be illuminated. With lenses or any kind of imaging, irradiance, but not radiance may be altered; an image that is brighter than the source cannot be achieved in this situation. When radiation is required to pass through optical components, then radiance is important. Smaller sources are easier to collimate and therefore better to focus. If, for example, there is a need to irradiate a slit, fiber, or pinhole (i.e. if the irradiated area is of the same size or smaller than the source), then the radiance of the wavelength is important.

Total Output

The area to be irradiated should also be considered in choosing appropriate lighting. If the area to be irradiated is large, then total output is more relevant than radiance. The radiance of a 75 W Xenon arc lamp is similar to that of a 1000 W Xenon arc, but if the target area is smaller, a 1000-watt source will produce about 30 times the irradiance of the smaller lamp.

In cases where the raw lamp output is used and collimation is not important, the irradiance curves are a reliable basis on choosing the correct lamp.

Source Size and Shape

The size and shape of the source determine how much light can get on target. An elongated source can be a better match to a slit target. With non-imaging reflectors such as elliptical reflectors, the size, shape, and angle of the secondary image are more important than the primary arc or filament.

Source Stability

It is also important to consider the stability of the source in choosing current lighting. Source stability involves both spatial and temporal stability. Spatial stability refers to the ability of an object or source to remain stable in the space where it is positioned. Meanwhile, temporal stability refers to the ability of the source to remain in such a position over time.

Because spatial and temporal stability of the source radiation is an important factor in measurements, companies would usually apply a double beam design as their standard. In general, tungsten halogen lamps are more stable than high radiance arc lamps.

Photofeedback can improve long-term source stability; however, it is also essential to utilize good designs in application. For instance, convection currents inside an arc lamp lead to fluctuations on the low radiance outer regions of the arc, but a well-designed system will not use these unstable zones.

Arc Lamps

Features

High radiance in the ultraviolet and visible spectrum (mercury lamps have spectral lines of very high radiance in the ultraviolet region)
High ultraviolet output
Small concentrated arc
In some cases, they have a spectral distribution resembling the sun (e.g. xenon lamps)