Sony DSC-P1 Specifications
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What that means is that if it were a vidicon tube it would be 0.556 inches in diameter.
This measuring system is a leftover from the days of video TUBES and has nothing to do with the surface area of the chip in a direct sense. The Edmund Scientific Optics catalog has a sidebar that completely backs up the idea.
For instance, a "one inch" chip has a 16mm diagonal sensor array. A "one half inch" chip has an array of only 8mm in the diagonal. This means that the 1/1.8 = 0.555 inch measurement means NOTHING about the sensor array size. Unless they have changed the entire measurement system just for this chip, the actual sensor patch is 11% larger than the 8mm of a 1/2 chip or 8.88mm in diagonal.
(Mark Schubin of Videography Magazine)