Submitted by Al ….
The Imperial View-Finder CDS Exposure Meter is a precision instrument with an extremely high sensitivity using a jeweled movement. The Cadmium Sulfide Photocell is powered by a 1.3volt battery. Unlike a selenium cell which creates current, CDS cells are resistors that vary with the amount of light that hits them (resistance drops as light increases). Theoretically, a CDS cell would never wear out because you could simply replace the battery as required.
CDS cells are more sensitive than selenium cells, and better in very low light conditions. CDS cells are a lot smaller: a typical selenium cell might require a square inch of area, while a CDS cell is usually about 1/8" or so in diameter. CDS cells made it possible to put the cell behind the lens in cameras, and took camera automation to a whole new level.
A minor drawback to a CDS cell is that it sees light similarly to the human eye, which is not the same way most films see it. A major drawback is, like humans, it experiences temporary night-blindness—a slowness to change from a bright light level to a dimmer one. Although controversial, over time, CDS cells all but replaced selenium meters.
Reproduced below is the instruction book for the Imperial View-Finder CDS Exposure Meter.
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