Reflectometric measurement system

Light emitted by a tungsten halogen lamp is passed through one of five interference filters mounted on a filter rotor. The monochromatic light beam thus obtained is focused by a lens and directed into the optical fiber block. It is then distributed to a seven-path light guide and transmitted to the optical unit. 

The five wavelenghts available through the five interference filters of the filter rotor are respectively 405, 550, 575, 610 and 820 nm. 

The wavelength selection is performed by a photointerruptor, which outputs the signal synchronized with 5 kinds of optical fiber in the optical fiber block. 

The optical unit comprises six sensor blocks (one for each measuring channel) and an intensity monitor. A sensor block consists of two photodiodes, each placed at a 45° angle from the reagent table. 

During the measurement or calibration, these photodiodes measure the light reflected from the reagent strips or the standard reflectance plates previously moved to the measuring position. The output of each photodiode is taken out as short-circuited current, which is then converted to voltage by an A/D convertor. 

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The reflectometric evaluation procedure of the Ektachem system differs considerably from that of other dry chemistry systems. Ulbricht s sphere is not required in this reflectometric measurement. Measurement is effected from the underside of the slides. The arrangement of the measurement makes it possible to eliminate interfering substances that can influence the reflectometric measurement signal by their colour only. 

Advanced analytical techniques as well as special accessories evolve from custom-made laboratory units into commercially available devices. Toward a more comprehensive characterisation of swelling and switching layer systems, different analytical techniques are combined in a single set-up. Examples closely related to the topic of this chapter are a unit for simultaneous measurements by QCM-D and ellipsometry (see application notes at www.q-sense.com) as well as a combined set-up for microslit electrokinetic measurements and reflectometric interference spectroscopy or Fourier transform infrared spectroscopy, respectively (see www.zetascience.info). 

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