Before there were CCD and CMOS-sensors, there were Vidicon tubes.
Why to mention? These light receiving tubes influence till today the names for the sizes of our imaging sensors.
The dark grey round area of the tube is the light sensitive part. Obviously the dark gray area can not reach the full diameter of the tube.
Lenses have a so called image circle, the round area on the image side of the lens that receives light. A lens has an image circle thats large enough to expose the dark gray part to light. if the dark area was 6mm in diameter, we talk of a 1/3″ lens, because the outer diameter of the Visicon tube is 1/3″ = 25.4/3mm = 8.467mm.
But has a 1″ lens an image circle which is 3x as large as a 1/3″ lens ?
A third inch lens has 6mm Image circle, so a one inch lens should have 3 times as much, say 18mm. It is 16mm only, however, because a vidicon tube with an 16mm diameter dark area had an outer diameter of one inch (25.4mm).
That’s why 1/3″ has 6mm and 1″ has 16mm image circle 🙂