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Television Lines (TVL) are derived from the EIA 1956 testing standard, where they are defined as the number of light and dark vertical lines that are visible (i.e., distinguishable) over the height of the screen. TVL defines “lines” as distinct dark and light lines. This is equivalent to Line Widths (LW), and should not to be confused with Line Pairs (LP), which represent a complete cycle, i.e., one TV line is equivalent to half a line pair. Although TVL is not the best measurement of digital imaging system quality, it can be useful for comparing digital systems with older analog systems.
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The definition of TVL is problematic because “resolved” or “visible” are not clearly defined. There is no description of the viewing conditions, no objective definition of the contrast representing “light” and “dark”, and no description of how much contrast is lost before detail is considered “no longer resolved”. Is a 90% loss of the signal (MTF10) enough to be considered “no longer resolved?” This is a relatively common definition of resolution (derived from the Rayleigh diffraction limit), and it has been standardized for Camera Monitor Systems in the ISO 16505:2015 standard. We have proposed an improved definition of resolution, but it has only gained limited traction. Using wedge patterns, some observers state that lines can no longer be resolved at 70% (MTF30) or 80% (MTF20) loss of contrast. Relying on subjective assessment of observers makes it difficult to get a consistent objective definition of TV lines.
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