This photometer is based on a Wheatstone bridge that must be balanced by the linear potentiometer P1. The value of this potentiometer can be in the range of 100k ohms to 2.2 M ohms depending on the range of light intensities to be measured. The P1 itself can also be equipped with a light intensity scale, just by lifting the characteristic curve of the sensor, in this case a common LDR.
