| 摘要 |
In a preferred embodiment of the present invention, method and apparatus are provided for converting an input light intensity representing an optical density to a standard industry scale reading. A photomultiplier tube is operated with a constant anode current corresponding to a reference, or "white" sample, and the dynode voltages varied to obtain the constant anode current. The resulting dynode voltage forms a signal linearly related to the input light intensity. A reference exponential characteristic is selected from an envelope of such characteristics, the selected characteristic cooperating with the photomultiplier tube characteristics. The exponential characteristic envelope may be conveniently formed by resistor-capacitor decay circuits with charging and discharging characteristics compatible with the range of photomultiplier tube parameters to be accommodated, where the envelope curve boundaries cross at a selected reference point. A counter is actuated when the dynode signal voltage equals the decaying exponential characteristic and the counter is disenabled when the exponential characteristic has decayed to a voltage approximating the crossing point of the envelope. The envelope crossing point may be directly detected and used to disenable the counter. The contents of the counter are displayed to present the optical density measurement to a preselected industry scale reading from 0.000 to 4.000.
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