Dot Gain in Colour Halftones
Abstract: The incomparably most common way of reproducing images in print is by halftoning, whereby varying levels of grey or colour are simulated by small dots with maximum colour density but with a varying local fractional area coverage, printed on a white substrate. Whenever such a reproduction is used, a ubiquitous effect called dot gain comes into play and makes the actual image appear darker than what would have been expected from a perfect reproduction.Dot gain is actually two effects that occur for two very different reasons. Physical dot gain occurs because the dots gain physically in size due to imperfections in the image transfer from the original to the print. A typical reason for physical dot gain is ink smearing and spreading in the printing process.Optical dot gain is an effect that occurs because the halftone dots are printed on a scattering substrate. The lateral spreading of light in the substrate yields a shadow around the rim of the halftone dots, whereby the dots appear larger and the image appears darker.Dot gain has been widely known and studied for half a century, but analysing and modelling it has presented a problem. Most models presented have been narrowly focused on providing a simple parameterized curve for the image signal transfer to facilitate dot gain compensation. The parameters of such models have been measured experimentally, but no real connections to physical material properties have been made.With the rather recent introduction of computerised image processing, tools have now become available to develop a better model that closely mimics the physical reasons for dot gain, and which could not only be used to pre-compensate for the effects of a known system, but also for prediction and simulation of expected dot gain effects. In the work summarized by this thesis, we have developed a model for dot gain in image processing and signal transfer terms.We concentrate on optical dot gain, and describe it in image processing terms with a fairly simple equation that centres on a point spread function for diffuse bulk reflection in turbid media. Calculating this point spread function presents a spatial problem of multiple scattering of light. A newly developed spatially resolved simulation method for isotropic light scattering in planar, layered turbid media is presented, which under certain restrictions is capable of accurately predicting the bulk reflection point spread function of turbid media like paper.Using these simulation results, we present simulations of optical dot gain for various halftones, and we show that the dot gain characteristics depend heavily not only on the halftone frequency, but also on the halftone geometry. The models traditionally used are incapable of accurately predicting these detailed properties of dot gain.Colour imaging is quickly becoming an everyday commodity, and halftone colour reproduction is a very active field of research right now. Good models are needed to accurately predict the colour rendering properties of various imaging systems, and we demonstrate how our model can be easily extended to handle colour. By applying the extended model to a few typical halftone colour imaging systems, we show that the colour rendering properties of halftone printing is greatly influenced by dot gain effects. A somewhat surprising fact is that a large dot gain can actually increase the range of reproducible colours, the colour gamut. We present experimental evidence which clearly supports this theoretical finding, and which shows that our model is capable of predicting the outcome of colour halftone imaging on scattering media.Finally, we present a few ideas on the design of experimental equipment to measure the parameters of our model and to exploit the model for making new kinds of measurements on printing substrates that more closely predict their halftone imaging properties.
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