Set Ink Restrictions Like A Pro

One of the first and most important steps in creating profiles on a ‘halftone’ printer is setting the correct Ink Restrictions. In this blog post, I want to tell you how - and why! -  to set your Ink Restrictions like a pro.

Before we dive into this, it’s important to know that most RIP softwares already have an ‘automated’ Ink Restriction tool available based on measurement. In order to use this you need a spectrophotometer to read the chart generated from your RIP. Sometimes after reading the Ink Restriction chart, there’s still a lot to gain, for example, when the curve is based on the maximum density of a color. As you might know already, the density of a color doesn’t tell you anything about the pure color - it only tells you how dark or light your color is.

If your printer features light color inks there is even more to gain because this is normally based on the default presets and the curve generated from the measurements. I will also try to explain how to set your light/dark ink transition in order to create the perfect Ink Restriction settings for your profile!

Chroma & Density

Let’s kickoff with an intro into chroma values and ink densities. When profiling, it’s important that we set the correct chroma values per ink channel in order to avoid issues in a later stage of profiling. If you use too much ink per channel you will definitely get issues with reproducing the colors you want, and in a later stage you’ll have challenges with your total ink limit settings and your all colors will get mixed. Most RIP software supports different color tables ( e.g LAB, LCH, Density) to describe the measured color. The one you want to use for optimal ink restrictions is the LCH table. Here, the “C column” tells you the chroma value of your measured color. You can also convert your LAB values to LCH values if you don’t have the tools in your software to do this.

So what is a chroma value and why is it important to use this value for your color?

Munsell Color System - by Wikipedia

Chroma is the quality of color overall; it describes the strength of your color. The higher the chroma value, the more pure/better your color is. The LCH model is based on the Munsell Color System which was invented by Albert H. Munsell in the first decade of the 20th century. The model tells you the Lightness, Chroma and Hue in the color model.

In this model you can find the perfect spot for the purest ink color you can get. Some RIP software automatically selects the highest chroma value for your color, but if yours doesn’t, there’s always a way to manually find the optimal chroma value. In a lot of cases you will see that a chroma value of ink goes up, peaks at a certain value, and then goes down again. So for example; you have a chroma value 60.5 on 75% of ink, but you have almost the same or a slightly higher value on 100%. It's better to limit your ink at 75% because you save a huge amount of ink and there’s no difference visible for the human eye. Look at the table below, this is an example how the color (chroma) value looks compared to density. It also tells you that when you limit this color on the density method you will print too much ink without getting better color - this results in printing issues, drying issues etc.