Ulbricht’s sphere

Reflection spectroscopy. Spectroscopy of the reflected radiation of substances having a surface of diffuse scatter. Reflection spectroscopy is performed by means of spectrophotometers or Ulbricht s sphere and is particularly suited for examining the light absorption of substances that are impervious to light or relatively insoluble, since reflection and light absorption are linked in a way which is described by the theory of Kubelka and Munk. 

The reflectometer (an instrument for measuring reflection) is either a special spectrophotometer or a measuring instrument of special construction with a built-in Ulbricht s sphere. 

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. 

Fig. 46. Seralyzer reflection spectrometer (cross-section) 1 microprocessor, 2 xenon flashlight, 3 detector, 4 test module, 5 keyboard, 6 reagent slide, 7 test strip, 8 Ulbricht s sphere.

The rays reach the sample detector via an interference filter. A reference detector receives the rays reflected by the wall of the Ulbricht s sphere. The concentration of the analyte can be inferred from the difference between the two reflectance values. In addition, by this formation of difference values the optical system equalizes fluctuations of the xenon flashlight. A sapphire window placed before the sample and reference detector minimises excessive evaporation of surface moisture from the test area of the test strip. The interference filter, which is also situated before the detectors, is method-dependent and is inserted into the instrument with the plug-in module (Fig. 47). 

Figure 4-51. Ulbricht sphere for integral (total) reflectance. I inlet port S sample (positoned at the sample window) O outlet port Sc screen (blocks the direct light path from the inlet to the outlet) Pw white plate a angle of about 8° (schematic drawing).

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