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Optical format is a hypothetical measurement approximately 50% larger than the true diagonal size of a solid-state photo sensor. The use of the optical format means that a lens used with a particular size sensor will have approximately the same angle of view as if it were to be used with an equivalent-sized video camera tube where the actual sensitive target is smaller than the overall size.
The optical format is approximately the diagonal length of the sensor multiplied by approximately 3/2. The result is expressed in inches and is usually (but not always) rounded to a convenient fraction. For instance, a 6.4x4.8 mm sensor has a diagonal of 8.0 mm and therefore an optical format of 8.0*3/2 = 12 mm, which is expressed as the convenient 1/2 inch in imperial units. The reason why it is expressed in inches is historical, dating back to the early days of television. The TV tube produced a circular image, but only part of that image was usable. The optical format is the diameter of the circular image for which the stated rectangle in millimeters can be used, and the circular image is wider than the diagonal by a factor of about 1.5. Wikipedia
For larger systems the size is usually given as the true rectangular dimensions of the imaging sensor in millimeters, such as 36 x 24 mm in the case of 35 mm film sized sensors.
More info http://www.dpreview.com/articles/8095816568/sensorsizes
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There is nothing worse than a sharp image of a fuzzy concept ;~) Mike Grace