Image sensors (CCD / CMOS) are commonly specified in inches ("), such as 1/1.8" or 1/3". Many people are informed that this figure stands for the diagonal length of the sensor, yet actual measurements prove these nominal dimensions do not equal the physical diagonal of the sensor chip.For instance, the nominal value of 1/3" converts to roughly 8.4582 mm, while the true diagonal length of a 1/3" image sensor is only 6 mm.

This sensor sizing convention dates back to the cathode ray tube (CRT) television era in the 1950s. Back then, the inch designation referred not to the diagonal of the active imaging area, but to the outer diameter of the entire glass envelope of the CRT tube. Engineers soon discovered that the usable active imaging area inside the tube amounted to approximately two-thirds of the nominal outer diameter, for multiple structural reasons.