Vertical resolution of a scanned image subjectively evaluated is consistently shown to be less than the geometrically-predicted resolution.

Observations are usually stated in terms of the ratio of perceived television lines to active lines present in the display. From the time that R. Kelt published his studies (conducted on a progressive scanned image), there have been numerous numerical values and substantiating theories proposed for this effect. The range of results suggests that many details of the experiments influence the result and make defining a single “Kell Factor” impossible. Reported experimental results range at least between 0.5 and 0.9. In an otherwise comparable display, the “ratio” is lower for interlaced scanning than for progressive scanning.