Direct modulated infrared source introduced by Heraeus

Citation

(2002), "Direct modulated infrared source introduced by Heraeus", Sensor Review, Vol. 22 No. 4. https://doi.org/10.1108/sr.2002.08722daf.001

Direct modulated infrared source introduced by Heraeus

Direct modulated infrared source introduced by Heraeus

Keyword: Infrared

A new direct modulated infrared source for infrared (IR) gas analysers is now available from Heraeus Noblelight, of Bromborough, UK. The new source utilises carbon fibre element technology to eliminate the need for mechanical modulation and employs gold reflectors to direct the IR beam (see Plate 1).

IR gas analysers often use a thermal source of IR, which is modulated by means of a rotating mechanical chopper. For most applications in process gas analysis, this modulation is in the frequency range 2-6Hz and is important in ensuring a clean signal with the minimum of noise. However, to achieve the degree of modulation necessary, mechanical choppers must be very precisely manufactured and are consequently very costly.

The new IR source features carbon fibre heated ribbons, which offer relatively high emission coefficients (around 0.85) together with a low thermal capacity. As a result it is now possible to modulate the source electronically in the output range 5-10Watts, so eliminating the need for a mechanical chopper. The source can be used in the standard wavelength range (3-5µm) of automotive exhaust gas and process gas analysers, with modulation frequencies in the range 2-6Hz. A modulation depth can be achieved similar to that available with mechanical choppers.

Plate 1

The new source can also be used in out-of-phase modulation in double beam analysers. Unlike conventional sources using beam choppers, where compensation to adjust the sample and reference beam synxnetry is achieved by means of attenuators, the direct modulated source employs electronic compensation, which may be analogue or digital. If a digital technique is used, setting of the automatic null may be automated. In addition, the element heating periods can be phase shifted to allow control by the analyser instrumentation.

The compact new IR source operates as a vacuum tight inert gas system behind a 20mm calcium fluoride window, which is suitable for all sample cuvette sizes. It features a central divider for adaptation to split cuvettes used in out-of-phase modulation.

Heraeus Noblelight, part of the multi-national Heraeus Group, specialises in the development, production and application of high quality energy sources covering the electromagnetic spectrum from IR to ultraviolet.

Inquiries to: Gary Thomas, Heraeus Noblelight Ltd, 8 Caldbeck Road, Croft Business Park, Bromborough, Merseyside L62 3PI, UK. Tel: 0151343 0545; Fax: 0151343 9883.