which is located on detector board 52 detects light passing through the sample and sends a signal along signal line 54 through connector board 56 and to a microprocessor not shown located on microprocessor board 58 in addition to the photodetector signal the microprocessor receives signals indicating the temperature of the detector board and the sample these signals are transmitted to the connector board along signal lines 60 and 62 respectively and then to the microprocessor components of the detector board are shown in greater detail in fig3 the photodetector 70 is a silicon photodiode e g a hamamatsu s1337 photodiode the output current of which is proportional to the intensity of the light impinging on it the photodetector is connected across pins 2 and 3 negative and positive respectively of op amp 72 which is for example a harris 3160t op amp the output voltage of the op amp at pin 6 is measured by the microprocessor as the detector signal as noted above the temperature of the photodetector as well as the temperature of the sample is measured and fed to the microprocessor this is accomplished using thermistor 74 which is for example a betatherm 10k3d409 to this extent the detector board components are as known in the art with respect to the op amp feedback path however it is modified to provide dual gain on the photodetector signal op amp output at pin 6 specifically the feedback path has a pair of resistors r 1 and r 2 arranged in parallel with r 1 having a significantly greater resistance than r 2 for example r 1 is preferably 1000 megohms and r 2 is preferably 25 megohms switch 76 is.