| 摘要 |
For indication of the amount of a constituent in a substance, for example the proportion of moisture in hay, a pair of light-emitting diodes 1 and 2 have their emissions reflected from the substance being analyzed. The diodes emit radiation in narrow bands selected so that there is differential reflection depending on the proportion of the constituent in the substance. The supply currents to the diodes are switched in alternate sequence and their relative magnitudes adjusted to give minimum output at the switching frequency. For determining the proportion of moisture in hay suitable wavelengths are 1450 nm and 1300 nm. The diodes may be constructed as InGaAsP/InP diodes with the proportions of the constituents adjusted to give emission at the required wavelengths.
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