How to compute resolution in TV Lines (TVL)
- Henry Koren
- Norman Koren
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.
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.
TVL measurements do not consider changes in sharpness across the image that are typical of conventional lenses. Measuring the center-only may produce optimistic results. The EIA 1956 and ISO-12233:2000 charts had wedges in the corners that were easy to ignore. You should consider points all across the image to get a full understanding of your system’s resolution. You can use a mean of these values if you only want a single number. Remember that the tilt of your lens can degrade resolution in some corners, but boost it in opposite corners, without having much effect on the mean MTF.
A convenient, stable method of measuring TV lines in displays (i.e.,monitors) is given in Display (Monitor) Sharpness. For most other images we recommend using MTF20 (or better, MTF20P) in units of LW/PH (Line Widths per Picture Height), measured with (near) vertical slanted edges. (MTF20P is less sensitive than MTF20 to excessive software sharpening.)
note: for cropped images enter the original picture height into the more settings dimensions input.
If you can control the processing of your imaging system so you can disable sharpening or analyze unsharpened RAW images, you can make an approximate TVL measurement using “MTF10 LW/PH” or “MTF20 LW/PH” from vertical edges across the image. You can use the Multi-region summary plot from SFR, SFRplus, eSFR ISO, Checkerboard, or SFRreg modules to obtain a weighted mean MTF10 or MTF20 in LW/PH units, which are good objective approximations to the TVL metric. This is superior to the wedge measurement described below since it is less affected by aliasing, but it may not be the best measurement if you cannot disable your cameras sharpening.
If you have a “black box camera” system where you cannot disable sharpening, we recommend the hyperbolic wedge pattern to measure high-frequency MTF or vanishing resolution. Otherwise, the slanted edge MTF10 or MTF20 numbers may be exaggerated by the sharpening. Our best target for measuring multiple wedges across the field is our ISO 12233:2017 target with “extra wedges”. The best Output is our Multi-Wedge plot. The Weighted Mean Wedge MTF10 or MTF20 in units of LW/PH is very close to TV Lines.