Natural Plate Compensation Curves: Simplified flexo plate calibration with SCTV
Stefano d’Andrea, Flexographic Printing Process Trainer & Consultant
This method describes a simplified procedure to determine and use plate compensation curves within any flexographic workflow by making best use of tonal value measurement according to ISO 20654:2017, also known as SCTV.
During the phases of flexographic process optimization and fingerprint, we adjust the tonal values on plate to match a desired condition in printing.
In case of individual color adjustments, we normally target to a reference curve that was derived from legacy offset printing characteristics, onto paper substrate, and was adapted to flexography and published in earlier editions of ISO 12647-6.
Following this method, we are happy when a plate (mechanical) dot area of 50% results in a printed (optical) dot area of ~65÷70%.
The use of SCTV (ISO 20654:2017) measurements during plate calibration allows a common metric for mechanical tonal value on plate and optical tonal value on printed substrate: 50% on plate would read 50% on print, when measured in SCTV. For this convenient reason, SCTV has already been implemented at many users worldwide.
But the use of SCTV in plate calibration reveals another very interesting aspect about the shape of the curves.
When we look at TVI compensation curves, measured and calculated in SCTV, we can notice that they all have a similar shape: they are symmetric, thus they are arches of circumferences.
The shape of these curves is easy to be calculated and be described: we just need 1 value for compensation at 50%.
We start the calibration from a linear and proportional condition, with equidistant values on a linear plate. The resulting compensation to match a flat target becomes a sequence of equidistant and proportional values: a “Smooth Plate Compensation Curve” that maintains uniform mechanical and visual spacing of the tones between 0% and 100% area coverage.
In fact, the main purpose of SPC Curve is to define a pre-compensated starting condition that preserves the proportionality and equidistance between the tonal values.
The ideal usage of the SPC Curve is of course as “plate curve” values, leaving the possibility, for any imaging RIP, to use the “print curve” to further match other conditions, like grey balance (ISO TS 10128 / G7™).
A further benefit is the possibility to build pre- defined compensation curves for most common printing conditions, easy to share and to reproduce across different plate systems.
Any xy values of the compensation curve can be calculated to fulfil the needs of any rasterization platform, in whatever necessary format including ISO 18620:2016, with a formula that could be implemented on any RIP.
The SPC Curve can also incorporate any existing highlight bump value in case such correction is required when working with plate systems that do not have 1:1 file:plate reproduction capabilities: it is enough to move the x0y0 point to the necessary value that is defined with the bump.