Arc lamps or halogen lamps? Selection of the optimum lamp type
Selection of the optimum lamp type and lamp power for your application depends on a variety of factors.
Which wavelength do you need?
Which wavelengths scatter the light or heat the sample, which is undesirable?
One should ideally choose a lamp with high power in the desired spectral range at and low power at wavelengths that may be light-diffusing or tend to cause other problems. Arc lamps are primarily utilized as radiation sources for light in the ultraviolet to visible range. Mercury arc lamps emit particularly strong lines in the UV range. Halogen lamps, on the other hand, are a good choice for applications in the visible to the near infrared range.
The catalog contains typical spectra of these individual lamp types.
How bright should the image produced by the radiation source be?
How large is the surface to be exposed?
Usually there are one or two lenses between the radiation source and the surface to be exposed (monochromator slit, optical cable, detector, sample). These lenses resp. any kind of imaging can only be used to change the intensity of irradiation on the receiving surface, but it cannot affect the available radiance – which, after all, is a characteristic parameter of the respective radiation source. Images of the radiation source can never exceed the radiance emitted by the source itself.
The radial intensity per unit area is an important factor wherever light is to pass through certain optical components. Small radiation sources are easily collimated and therefore easy to focus. Whenever a fiber-optical system, a monochromator slit, or a pin diaphragm, for instance, is to be illuminated - in which case the surface area to be illuminated is of the same size or smaller than the radiation source itself – then the spectral radiance at the required wavelength is of importance. At a first approximation, the value of the irradiance or the luminous flux, divided by the surface area of the light source, affords a good reference value. The radiance of a 75 WXe lamp, for instance, is approx. 2.7 times as high as that of a 150 W Xe lamp, since the surface area of the arc in this case is approx. 8.8 times smaller.
Total output power
How large is the surface area to be illuminated?
If larger surface areas (several cm²) are to be illuminated, then lamps with a higher output power (luminous flux) will yield better results. The radiance of a 75 W Xe arc lamp approximately equals that of a 1000 W Xe arc lamp. Due to the larger-sized arc, however, a 1000 W light source will illuminate a surface that is 30 times as large as that covered by the smaller lamp. Irradiance curves provide a good basis for lamp selection in applications where collimation is irrelevant and only the output power of the lamp is crucial.
- high radiance in the UV and the visible range. Mercury lamps exhitibt spectral lines with very high radiance in the UV range.
- high UV output power
- small-sized electric arc
- Xenon lamps exhibit a spectral distribution that is very similar to sunlight.
- highest irradiance on small illumination surfaces
- simulation of daylight
- intense collimated beams due to small arcs with high radiance
- excellently suitable light sources for UV photochemistry
- Emission between 350 nm and 2700 nm
- good stability
- high output power in the visible range
- useful as photometric or radiometric sources
- continuous emitter; i.e., relative minor spectral intensity changes
- simpler detection during scanning
- less costly than arc lamps
Shape and size of the radiation source
What about shape and size of the object to be exposed?
The existing optical system as well as the shape and size of the radiation source determine how much light ultimately reaches the object to be illuminated. An elongated arc, for instance, is a better instrument to illuminate a monochromator slit. In the case of reflecting optical systems, such as housings with an elliptical reflector, the shape and size of the image are largely determined by the optics. The chapter on ‘condensor optics or elliptical reflector‘ discusses this topic.
In how far does your application require stability with regard to space and time?
In many cases, radiation stability in space and time is so important that a double-beam approach must be employed. Halogen lamps emit a more constant intensity than arc lamps. A light regulator may further contribute to a more constant light intensity, but a good system design is equally important. Thus, free-convection flows inside arc lamps cause fluctuation around the marginal areas of the arc. A carefully designed system will eliminate these unstable zones.