Because there are seven shades of ink, the amount of overlapping inks makes for a set of curves that is more generous to sub-standard printers than would be a 3 ink or 4 ink curve. Because each density point in a K7 curve is comprised of so many inks, it allows for more printer anomalies to occur before the need for custom profiling.
But, I take it one level higher. Piezography K7 curves are unique in that I design them with back sloping curves. Most traditional QTR curves are bell shaped; the curve slopes up towards its peak amount of ink and then slopes back down in the shape of a bell. Piezography K7 curves slope up in a similar bell shape towards the peak, but they do not trail off quickly. Their trail off actually contributes to the density of the next darker ink.
My Piezography profiler can make bell shaped curves, but I have a feature that adds an amount of back slope that allows the ink that is trailing away (when the light dilution begins to stop printing in place of the next darker dilution) to extend further. I can control how much back slope I use. And I have a way to compensate for this back slope as it adds density when I am designing the curves. It is consciously an important part of my curves architecture.
Originally, I added back slope to eliminate any potential spaces between dithering dots of dark ink as it first began to print over a lighter dilution. I try to keep each dilution printing for as long as I can at maximum frequency dithering. But, when a dark ink begins to come into a lighter ink the darker ink’s dithering frequency is low and there is space between the dots of ink that might allow them to become visible to the human eye. Printing dark inks over back slopes of lighter ink prevents the dots from being visible. They all become part of what what appears solid continuous tone. K7 curves are not overlapping, progressively darker shades of ink. Rather they are complex overlaps of many ink shades at nearly every possible pixel value.
In comparing the K7 curves on multiple examples of printer models, we noted that those produced with more back slope would allow us to make curves that worked equally well across the printer models we had when not using enough back slope in curves produced variations in them. This is how back slope became a part of the standard K7 curve format.
We did not offer any custom profiling until two years after the introduction of Piezography K7. At the time I thought it would become necessary for aging printers, but it hasn’t. The overwhelming majority of custom K7 curves (profiles) that we produce for customers are for papers which we have not supported in the release.
As Epson print heads age they begin to lose the ability to form the micro-dots. That can change output density that we can correct with a custom. But, the amount of ink shades at any one point in our K7 curves is very forgiving to one or two print heads which may begin aging. Usually, a customer needs new ink dampers or print heads before they need a custom K7 curve. If you would like to check your linearization read this.
Before you contact us for a custom, make sure that you have been shaking your inks regularly and printing regularly. Usually when a printer is allowed to sit for a long time between printings, the pigment ink will settle and this will produce a non-linear state. Remove the carts, shake them gently. Run enough power cleans to move fresh inks from the cartridges to the print head. (3 on a large format printer, 1 on a desktop). If you’re using a CIS, drain the ink back into the bottles and shake gently. Then re-install to bring fresh ink back through the ink lines into the CIS dampers. Almost always this saves the need for a custom.
Back slope is just another part of the Piezography process that is oriented towards the highest possible standard.
Learn more about what a K7 curve set is by reading “Piezography Profiles”