A subjective number of lines of resolution that can be visually perceived in a video display system, expressed as a percentage of the total number of lines of resolution. In 1933, RCA engineers Kell, Bedford and Trainer determined that the effective resolution of their electronic video system was 64% (0.64) of the total scan lines. As the electronics became more capable and generated more scan lines, Kell and Bedford increased the factor to 85% in 1940.