| 摘要 |
PROBLEM TO BE SOLVED: To improve S/N ratio and calibrate the fluctuation of circuit characteristics even in the case with large light absorbance of measuring objects by changing the gain of a gain changing circuit based on a measured direct current voltage or alternating current voltage. SOLUTION: The output of an LPF circuit 11 is selected by controlling a switch circuit 13 with an operation control circuit 18 and based on the direct current voltage having passed the LPF circuit 11 among the output of a light reception means 9, the gain of a gain changing circuit 14 is changed so as not to saturate the input range of an A/D converter 16, for example to suppress the increase of noise component of the A/D converter 16. Also, by controlling the switch circuit 13 and selecting the output of an HPF circuit 10, signal component (alternating current component) having passed the HPF circuit 10 is converted into digital signal with the A/D converter 16 and taken in to gain an interferogram. Then, the output of a reference signal generator circuit 12 is selected with the switch circuit 13, the gain of the previously measured interferogram is compensated with the actual gain of the gain changing circuit 14 obtained by measuring the reference signal to calibrate the changed circuit characteristics.
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