Measuring Register Shift and Investigating Its Effect on Color Appearance for Different Halftoning


Document ID: 
Sasan Gooran, Daniel Nyström, Mahziar Namedanian, and Shahram Hauck


Digital, Non-Member: 
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In commercial prints the halftone dots are seldom placed exactly at their corresponding positions in the digital bitmap, mostly due to the imprecise transportation of the printing substrate. In this study, we firstly present an image processing model to measure the displacement of the dots in different color separations by using a high resolution camera. By using a filter wheel equipped with a set of interference filters and sending light in different wavelength bands, it is possible to separate the different color inks. For example, for the combination of cyan and magenta inks, only cyan will be visible in the captured image if the wavelength band of the incoming light is  concentrated around 700 nm. On the other hand, for the wavelength band concentrated around 500 nm only magenta will be visible. By comparing the positions of the dots in the captured images with those in the original bitmap we can measure register shift. Secondly in this study, we investigate how misregistration affects the color appearance of the final print for different halftoning techniques. We use AM, FM first generation and FM 2nd  generation halftoning methods and investigate and compare their sensitivity to register shift.

In the present work we measure the register shift for color patches printed in offset by the proposed image  processing model. In order to study the effect of misregistration on the resulting color appearance we first  simulate the misregistration in the digital bitmap. Then we print the simulated bitmap using an office laser printer. Since the misregistration is usually negligible for digital prints, especially in lower resolution, we can  examine the accuracy of our model for measuring dot displacement by comparing the simulated displacement with the measured one. Finally, by using a spectrophotometer to measure color coordinates, we can study the effect of misregistration on the resulting color appearance for different halftoning methods by calculating the DElab color difference.

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