In the world of night vision optics, these differences are most commonly classified as "generations."

Generation I optics use a photocathode tube and provide good performance at a reasonable cost.
Generation II units add a micro-channel plate (MCP) to decrease image distortion and increase light amplification.
Generation III adds Gallium Arsenide to the intensifier tube, producing an even brighter and sharper image.
Generation IV optics employ the latest technological advance, Gated Filmless tubes, to provide excellent low light level performance and triple the high light level resolution.

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