Kell factor
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Kell factor is a parameter used to determine the effective resolution of a discrete display device. The number was first measured in 1934 by RCA engineer Raymond D. Kell and his associates. It has no fixed value, but is usually taken to be about 0.7, for electron-gun scanning. Kell originally defined this as 0.64, and then later revised it to about 0.85. The number can be higher than 0.9, when CCD scanning and displays are used.
For example, a 625-line analogue (e.g., 50Hz PAL) television picture is divided into 576 lines from top to bottom. Suppose a card featuring horizontal black and white stripes is placed in front of the camera. The effective vertical resolution of the TV system is equal to the largest number of stripes that can be within the picture height and appear as individual stripes. Since it is unlikely the stripes will line up perfectly with the lines on the camera's sensor, the number is slightly less than 576. Using a Kell factor of 0.7, the number can be determined to be 0.7×576 = 403.2 lines of resolution.
In addition, the actual sampled resolution will depend on the spot size and intensity distribution. For electron-gun scanning systems, the spot usually has a Gaussian intensity distribution. For CCDs, the distribution is somewhat rectangular, and is also affected by the sampling grid and inter-pixel spacing.
Kell factor can be used to determine the horizontal resolution that is required to match the vertical resolution attained by a given number of scan lines. For 576i50, given its [[4:3]] aspect ratio, the required horizontal resolution must be 4/3 times the effective vertical resolution, or (4/3)×0.7×576 = 537.6 lines of resolution. Taken further, since 537.6 lines is equal to 268.8 cycles, and given 576i50 has an active line period of 52µs, its luminance signal requires a bandwidth of 268.8/52 = 5.17 MHz.
Kell factor also applies to digital display devices. Using a Kell factor of 0.9, a 1080i HDTV using a CCD display will only have 1728×972 lines of resolution.
Note that the presence or absence of interlacing is irrelevant to Kell factor.
References
- M. Robin, "Revisiting Kell", Broadcast Engineering, May 2003.
- S. Mullen, "Just What is 1080?", HDV@Work, Feb. 2006.
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