Exposure spreading (also called trapping) is imposed in the printing copy to even out inaccuracies when superimposing and adjoining the individual colour surfaces during multicolour printing (mis-registration). In colour separation the lighter picture sections are slightly magnified, while the darker sections remain unchanged. The trapping occurs later in the printing together through the exposure spreading of the light and darker surfaces. The ideal width varies depending on the printing process, printing substrate and printing press.