Selenium meters convert solar radiation into direct current electricity (photovoltaic). These cells produce a current proportional to the light that’s shining on them, so the meters don’t need batteries to function.
Although these cells put out very little power, they became the basis of the first electronic light meters (as seen on the original Canonet).

Cadmium-Sulphide, commonly abbreviated to CdS, unlike simple Selenium meters, which are powered by light, are more complex and require a battery. They became common in the late 1960s, and replaced the less sensitive Selenium meters.

Both systems are designed to measure light reflected from the subject and are calibrated to show the appropriate exposure for “average” scenes. The meters on these rangefinders are mounted on the lens barrel which means they automatically compensate for filters and
other attachments.

NB Light meters are not essential to the operation of our cameras and are only needed for Automatic modes. Shutter speed and aperture stops can be set for specific lighting conditions and judged mentally with knowledge of EV numbers, EV tables and simple arithmetic