Vintage cameras use one of two types to determine the proper exposure of an image
A light meter is an instrument that measures the intensity of light that is falling upon a subject (incident light) or that is bouncing off the subject (reflected light). They are used to calculate exposure settings, taking into account the speed of the film being used. There are two main types found on older cameras. Both are designed to measure light reflected from the subject, and are calibrated to show the appropriate exposure for “average” scenes.
Selenium meters generate their own electric current which is 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). Multiple “insect eye” lenses covering the photocell are characteristic of a selenium meter (they are often mistaken today for LEDs). The cells inevitably deteriorate over time and eventually expire (especially if they are not stored properly or are exposed to light for long periods) so may no longer function on an old camera.
The second type use cadmium-sulfide (CdS) cells which require a battery to operate. They are smaller, so take up less room, and are more sensitive, working better than selenium cells in low light conditions. They became common in the late 1960s, and rendered selenium light meters obsolete.
N.B. Light meters are not essential to the operation of the mechanical cameras featured on this website 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.