Reference signal, optical

Figure 7.12. Chronology of message capacity showing exponential increase with time. The number of voice channels transmitted per fibre increases rapidly with frequency of the signalling medium. The three right-hand side points refer to optical-fibre transmission (after MacChesney and DiGiovamti 1991, with added point).

A laser-based detection system for NO2 gas (which is an industrial hazard and common environmental pollutant) was developed by Koybayashi et al.4. This was achieved by splitting light, from an Ar-ion multi-line laser, into two paths, one passing through a measurement gas, and the other being transmitted directly to the measurement unit as a reference signal. The detection unit contained two filters to separate the two chosen laser lines, and these were then detected on separate optical receivers. One of these chosen laser lines coincided with a strong absorption line in the NO2... 

Most common lock-in amplifiers can be operated at frequencies ranging from a few Hz up to 100 kHz. This fact is important in analyzing the temporal evolution of optical signals for example, fluorescence decay time measurements. Although this particular application of lock-in amplifiers is beyond the scope of this section, it is instructive to mention that this can be done by tuning the relative phase (the time delay) between the signal intensity and the reference signal provided by the chopper. 

With diffuse-reflection optics a reference signal cannot be measured without a standard. With reference to diluted samples, the matrix particulate is best suited. In this way, contaminations of the matrix which are virtually inevitable due to the large specific surface, are compensated for. For undiluted, specially compact samples, gold covered erasive paper has proved to be an excellent ready-to-use standard (Otto, 1987). 

Absorbance may be derived from the photoelectric signal either directly on a precalibrated meter scale or by a null method where a reference signal is provided optically or electrically. In null methods, readings... 

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