A useful way to categorize built-in camera meters is by both sensor type and metering method.

Common built-in meter technologies in film cameras include:

• selenium cells
• CdS (cadmium sulphide) cells
• silicon-based sensors, including later TTL and multi-segment designs

Beyond those, some earlier or less common approaches also existed:

• chemical actinometers
• optical extinction meters
• in theory or special cases, other electronic sensors such as germanium or gallium-arsenide devices

It’s also worth separating the sensor from the metering layout. For example, CdS meters were not limited to simple external windows: some cameras used multi-segment and even TTL CdS systems. Likewise, “TTL,” “center-weighted,” “spot,” “partial,” “matrix,” and “evaluative” describe how the camera measures the scene, not necessarily what photosensor material it uses.

So there isn’t one single neat exhaustive list unless you define your categories first. A practical classification is:

1. non-electric: chemical and optical
2. electric by sensor material: selenium, CdS, silicon, and rarer alternatives
3. metering method: external vs TTL, and averaging vs spot/segment-based patterns