The Index Color Separation is one of many kinds of color separations used mainly in screen printing. It’s used as a substitution for CMYK color separation – an inexpensive, but a bit outdated method. Depending on a purpose CMYK is being gradually replaced by index or simulated process color separation, mainly because these methods allow using more than just 4 colors. Using multiple colors allows transferring more complex color patterns on garments making the prints appear more opaque and sharp. However it has its good and bad sides, and it can’t be used for all kinds of graphics. But first things first…
What is Index Color Separation
The Index Color Separation is the process of breaking down complex designs into a few colors, not necessarily Cyan, Magenta, Yellow, and Black. Theoretically, the number of colors is limited just by the number of available printing heads on the machine. What distinguishes this method from the simulated process color separation is the use of bitmap instead of halftones. Index color separation converts an image into a diffusion dither which is a computer-generated stochastic raster displacing the dots closer or further apart from each other to create a pseudo-random square pattern, popularly referred to as pixels. It is easier to color match with index color final prints are bright and high contrast and the index separations also work great and print equally as well on both light and dark shirts! Also, the good news is that you can use Pantone! It’s one of the greatest assets of this separation technique. It makes the colors appear opaque and leaves the edge sharp on the outcome.
Without the dithering, the banding effect is visible – a problem of inaccurate color presentation in computer graphics. In some cases, there is a risk of producing abrupt changes between shades of the same color. For instance, displaying natural gradients, like sunsets, dawns, or clear blue skies, can show minor visible bands on the computer screen. When colors graduate from one to another, they never transient smoothly and therefore, due to lossy compression, transmit to a limited color palette with a visible error. It doesn’t even appear on some separations, as colors don’t overlap on all projects, and on some, it just requires using more colors. The key is to make the transition invisible to the human eye. The more colors are used the better an image looks. For reference, a 12-index color image looks twice as smooth as a 6-index color image. Diffusion dither is adding noise perceived as a delicate blur if seen from distance and fixes the problem.
Bitmap keeps you away from the common moiré pattern problem, a waveform pattern caused by interference between half-tone dot angle and screen mesh. Therefore it creates a pattern consisting of equal size adjoined square dots of different colors. As dots on a bitmap never overlap, it’s commonly used to represent sharp edges often appearing with AM screens too.
The dark side of index color separation
It’s hard to transcribe any gradient or shading to a bitmap, without setting a proper resolution. The resolution, in this case, is just the size of the “bits” from the so-called bitmap. Resolution is just how many rasters, which in our case are the adhering squares, one can fit in an inch-long line. 135 – 155 pixels/inch is the standard resolution for most index separation prints. Briefly, the higher the resolution the more detailed the picture can be. Then a lot relies on a screen mesh. Finer mesh size allows to obtain of more precise graphics, but the left-alone dots may still be visible. That’s why we disadvise using this method of color separation when we work on too many details. Especially when there are a lot of transcending colors within a project. Another disadvantage is that it takes more time. Photoshop can automatically generate a starting point of a sep, but it’s simply not as smart and requires an intervention of a human eye and potentially a manual edit. You cannot apply any “grayscale” adjustments like tone curves to the final separations. You pretty much get what you get and if you don’t like the outcome you have to go back, re-pick colors, and re-separate the job. But there is no need to be afraid that the image looks dotty. In cases like the above, we can always use a different method of color separation – like simulated process color separation to use halftones. You can read about it here.
The ink application is a little bit different when you are using index color separation. Using lower mesh size allows you to achieve higher dot gain making the print look like the halftones were used, but the colors will remain very sharp. It’s usually a kind of separation that requires wet on wet printing. The reason to print wet on wet is that the dots blend forming a continuous tone much like a photograph.
This kind of separation is recommended when the image has sharp edges, like with text, logos, geometrical shapes. Especially, when there are not so many color transitions, even complex patterns can be printed with a good effect. It makes the prints vibrant and sharp which is harder to achieve with other techniques.