Graphic Arts Manufacturing Company

Warren Childers

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.

Optical densitometer

In a preferred embodiment of the present invention, an optical densitometer is provided having an improved output scale resolution. An output scale reading from 0.000 to 4.000, having a resolution to 0.001, is obtained by improved method and apparatus for obtaining the reference operating characteristics for 'zero-mode' parameters. An analog reference anode signal is formed by making adjustments in a generally antilogrithmic relationship to the difference between the reference anode signal being generated and the anode signal produced under the zero-mode conditions. The antilogrithmic relationship improves scale resolution near the zero point and increases densitometer stability during a subsequent measurement cycle. The combination analog reference anode signal generating circuit and an improved exponential characteristic selection circuit enables the improved output resolution and accuracy to be obtained. The exponential characteristic selection circuit enables the measurement circuitry to be adjusted in accordance with the actual photomultiplier tube in the circuit to produce and maintain a measurement having an improved accuracy.

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Houston, TX, United States of America

Warren Childers