Spot Color Inclusion mechanism (SIM)

Posted: December 5, 2016 in Color

Spot Color Inclusion mechanism (SIM) for printing with 3 color process inks and spot color replacing black as fourth process color.

Wiki Defines Spot color as below

“In offset printing, a spot color is any color generated by an ink (pure or mixed) that is printed using a single run.” And it defines the CMYK color model (process color, four color) “as a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black). Though it varies by print house, press operator, press manufacturer, and press run, ink is typically applied in the order of the abbreviation”.

Generally the cost and potential for problems for a print job increase as one adds more spot colors, due to the increased cost and complexity of added process inks and plates, and requiring more runs per finished print. However, because of the complicated process, spot colors are effective at preventing forgeries of money, passports, packaged products, bonds and other important documents. Money printing for example, uses secret formulae of spot colors, some of which can be seen by the naked eye and some that can only be seen by using special lights or applying certain chemicals.

In today’s competitive market, all the packaged products are printing spot color as part of their job work. Let’s see a below image as an example

Figure 1: Image courtesy : www1.macys.com

In the above mentioned image, the image is with process CMYK colors and background is with pantone Reflex blue C.

For a typical printer in India who is having a 4 color printing press, these jobs will involve printing the job in two passes thus increasing the production cost by double. In the price competitive market, this erodes his margin as he has to fight to keep these kinds of packaging jobs in hold against a competitor who is having a 5 color printing press.

So there is an urgent need to support these printers with pre-press tools so that he can do the printing in one-pass. One option is to convert the spot colors in to process color combinations of C, M, Y, K percentages which will give the equivalent color perception.

For ex: a pantone blue can be printed with c=90% m=83% and black=10%.

But this method is prone to issues of color variations and color in-consistency.

So there is a need for some other tools to make the printing efficient and hence the tool of “Replacing a process color with Spot color without compromising print and color quality”. Hence the method of “SIM” is developed.

Procedure:

1. The process color and spot color combination in the printing job to be evaluated to understand which color can be replaced.

2. In the example shown above figure 1, the spot color used is Pantone Reflex blue. This leads to two possibilities of removing process colors and replacing it with process color. Either we can replace the Cyan with Pantone Blue or the black can be replaced with the pantone color.

3. A Hybrid test form (Chart 1) is printed to determine the best possible density for the spot color mentioned in the job. The density is determined on the basis of Hybrid Model Approach” (HMA)

4. The test chart is specially designed with a continuous wedge of solids along with dot gain patches. The sheet is printed with different densities from one end to other end. The variation is kept at 0.05 densities.

5. The wet densities were measured and sheets are allowed to dry. After 8 hours, the ‘Lab’ and ‘Print Contrast’ are measured on each density patches.

6. A curve depicting different lab and contrast values are generated. The lab values are compared with customer job specified standard lab values. It is observed that most of the cases, Delta E with medium deviation and maximum print contrast are matching.

7. Instead of choosing the lowest Delta E density, if we choose the point where both “print contrast” and “medium deviation Delta E”, the print results are better.

8. To evaluate this a ECI 2002 test chart is printed with the possibilities

a. Normal CMYK Printing

b. Pantone blue, M, Y, K process printing (replacing with Cyan)

c. C, M, Y and Pantone blue process printing (replacing with black)

9. Then these charts are measured with Heidelberg Color Tool box to understand the gamut of the above mentioned ECI 2002 test charts. It is observed that ECI test chart with option “C” closely suited the gamut required in the job.

10. A “Device-link” profile is created with the characterization data obtained from the ECI test chart.

11. A new dot gain curve is created to compensate the effect of Pantone blue over cyan and magenta regions. This dot gain curve is also obtained from the test chart.

12. Using image conversion software, the image is then converted to the destination profile with pantone blue elimination black color.

13. The job is then printed with C, M, Y, Pantone Reflex blue with compromising the use of pantone colors and print quality.

14. The printing quality is compared with 4 color + Process color printing and decided to make the analysis by human perception. The same sheets are given to “10 Standard Observers” and their visual assessments are compared.

15. Interesting the standard observers were not able to identify noticeable difference in print quality and they perceived it as usual CMYK + Spot color printing.

Further Scope:

The SIM system can be improved with an automatic mechanism to identify the image and recommend the right replaceable process color and corresponding “device link profile” and dot gain curve accordingly.

Chart 1

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