The instrument consists of a vacuum photoelectric cell feeding into a vacuum tube voltmeter with logarithmic input characteristic. The voltmeter employs a type 6D6 tube which is provided with a properly chosen biasing resistor in the cathode lead. This gives a plate current proportional to the logarithm of the input voltage. Since this relationship never extends to zero plate current, a portion of the plate current is balanced out, giving in effect, a suppressed zero instrument. Suitable shunts and a choice of input resistors afford several ranges of sensitivity. The output meter can be calibrated directly in concentration units. Other methods of achieving the same result are discussed.
RALPH H. MÜLLER and GILBERT F. KINNEY, "A Photoelectric Colorimeter with Logarithmic Response," J. Opt. Soc. Am. 25, 342-342 (1935)