A number describing the loss of vertical resolution from that expected for the number of active scanning lines. Named for Ray Kell, a researcher at RCA Laboratories. Many researchers have come up with different Kell factors for progressively scanned television systems. These differences are based on such factors as aperture shape, image content, and measurement technique. A generally accepted figure for the Kell factor is around 0.68, which, multiplied by the 484 active NTSC scanning lines, yields a vertical resolution of 330 lines, matched by NTSC’s 330 lines of horizontal resolution per picture height (see Square Pixels). It is important to note that most studies of the Kell factor measure resolution reduction in a progressive scanning system. Interlaces scanning systems suffer from both a Kell factor and an interlace coefficient.