How to convert optical inches to image circle diameter?

Please do not mix ‘real world” inches with optical inches.
1″ (real world) = 25.4mm, but
1″ (optics) = 16mm
1/3″ (real world) = 8.47mm
1/3″ (optics) = 6mm
1/3″ in optics is not (1″ (optics) / 3 !!
A sensor is called Third inch sensor for example, when the vidicon tube needed for this image circle would have “real world” 1/3″ in diameter

Here’s a calculator to do the math for you: