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RESOLUTION, ARBITRARY UNITS
EXTINCTION POINT
Fig.
1. Comparison of photographic contrast with television depth of modulation over total range of resolution.
Photography; resolution in terms of lines /mm.
Television; resolution in terms of detail frequency.
the detail fineness approaches the limiting resolution or extinction value, but a mere statement of the resolving power does not disclose the rate at which the fall-off takes place.
In an attempt to reconcile the television and optical points of view, the authors propose to use a term which has come to be used, namely "detail frequency," which is the product of the number of lines per millimeter into which the object is dissected and the scanning speed. Detail frequency in television is thus the electrical counterpart of detail fineness in photography and its use permits comparisons to be made. It must be recalled, however, that 1 line/mm in photographic practice conventionally represents one white and one black line, whereas in television the black and white lines are counted separately, i.e. one photographic line equals two television lines. It must be added, moreover, that the detail frequency is to be regarded as the fundamental frequency generated by scanning a repetitive pattern. No account is taken of harmonic development at this stage.
Figure 1 shows an arbitrary comparison between the detail-frequency re
sponse of a television system and the detail response of a lens and photographic emulsion in comparable terms. To illustrate the point, the lens and film combination have been shown as having something approaching a normal aperture/distortion curve, whereas the television-system response has been maintained at 100% almost up to a sharp cutoff. The limiting resolution is the same in both cases.
It is believed that of the two reproducing systems, television will present a picture giving a greater subjective impression of sharpness and boldness of detail than the other, even though the detail cut-off frequencies are the same in both cases. The theory is advanced that subjective impression of definition can in some way be related to the ratio of the respective areas below the curves. The determination of this effect is complicated — like all comparisons of definition between television and photography — by the fact that television pictures are discontinuous in the vertical plane, whereas photographs are continuous in both planes. However, this does not necessarily invalidate the truth of the conception.
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December 1952 Journal of the SMPTE Vol. 59