The linearity of incident radiation and photo current measured at the photocurrent measuring circuit of photovoltaic cell is limited by interrelation between ampere-voltage characteristic of photovoltaic cell and performance of photocurrent measuring circuit.
In this paper, it is described how to evaluate constants of equivalent circuit of Si photovoltaic cell by least squares, and the measuring range is made clear of the photocurrent measuring circuit of a cellamperemeter coupled circuit, Campbell-Freeth circuit, and Galvanometer amplifier by investigating the linearity of each circuito by means of equivalent circuit of the Si photovoltaic cell. Especially the Campbell-Freeth circuit to measure short circuit current of photovoltaic cell has been considered optimum precision method of radiation measurement but the upper limit of the measuring range for precision measurement is restricted by a series resistance of equivalent circuit of photovoltaic cell and the lower limit by null detector's voltage resolution. It is suggested that the Campbell-Freeth circuit for measurement of very weak radiation must be constructed by null detector with nv resolution and galvanometer amplifier must be useful for the measurement.