Linearization of Halftones

Linearization of Halftones

The word ‘linearization’ seems confusing, it seems to have no association with screen printing and yet the process is often overlooked, but very crucial to halftone print quality. The truth is if you print halftones you need to understand how important linearization is in creating smooth tonal halftone prints. Linearization refers to adjusting a tonal ramp within a RIP program to produce accurate tonal values on film. Huh? Lets look at these terms in detail to understand what is going on.

Tonal Ramp: Tonal values run from 0% (no tonal values or halftones) up to 100% which is a solid black image on your film. From 1% to 99% become halftones in the tonal ramp. Think of a gradation from white to solid black and all the shades of grey in between as shown in the example above. The more accurate your film halftone values are to the original art tonal values in Photoshop, the better the screen print reproduction. So just remember this, your film must have accurate tonal values, a 50% tonal value in the original art needs to be 50% on the film. (There are additional tonal adjustments due to printed dot gain, right now we need to control the film first.)

RIP Program: A high quality RIP program, (Raster Image Processing) reads the tonal values in the art and outputs halftones on film. Some RIP programs can be linearized, some can't. We sell Filmgate RIP which contains a linearization assistant to help control halftone values.

The image setter of choice for film nowadays is an inkjet printer mainly because of cost. Epson, HP, and other manufacturers have ink jet printers capable of printing good quality film positives. However there are higher end image setters available using real film with very sharp halftone imaging that exceeds the quality of an ink jet imagesetter’s halftone dot.

We have an Epson 9880 which produces very good positives using Filmgate RIP. The linearization process within Filmgate uses a target image tonal ramp that is assigned a halftone value (55 lpi for example) then printed on the inkjet printer and then measured with an X-rite Transmission Densitometer Model 331. The halftone measurements are entered into Filmgate’s linearization assistant which uses the data to control future halftone output to be accurate, ie a 50% tonal value in an art program becomes a 50% halftone.

So here is what is needed to linearize the halftone output on an ink jet printer: 

1. A good RIP Program, we use and sell Filmgate from Colorgate for it's top quality controls and ease of use. 

2. A good ink jet printer like the Epson 4880 thru 11880 series with stock K

3 Ink. 

4. A Transmission Densitometer - we use X-rite's Eye One's Transmission Densitometer Model 331. Next a look at common issues with inkjet printers before the linearization process.