Figure 3 Presents an Example of the [Olis and PE] 983's Exponential Detector Output

Figure 3

The waveform in gray corresponds to the actual light intensity. The black curves represent the thermocouple detector system's output, which approaches a level corresponding to the light intensity. To obtain a %T reading (equation 1), accurate estimates of the light during the sample/reference and dark periods are necessary. Since the detector system's output varies exponentially, one can fit the signal profile to an exponential function and thereby accurately predict where the signal will go if one waited long enough. This is shown in Figure 4.

The chopper system sequences through six segments during each cycle (figures 1, 2 and 3). During each successive segment, the Olis 983 system collects ten values, each derived from 60 A/D converter operations and transfers these ten values to another microcomputer which fits the values to an exponential and returns the value 'b' (Equation 2¹). After all six segments have been analyzed, the six values (figure 2) are sent to the host computer for processing via Equation 1.

Figure 4

Equation 2

Since these 'b' numbers accurately reflect the true values of the light intensity, equation 1 must produce the correct value for %T without any adjustment or calibration.

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