Precise Measurement of Dynamic Ink Spreadability Using High-Speed Camera and Nonlinear Regression

Printability on thin heterogeneous media using ink is related to the spreadability of ink. Thus, this study focused on quantifying the spread of ink droplets by performing amorphous morphology measurements using a measurement system that generated uniform ink droplets while simultaneously acquiring high-speed images of the ink spread on a thin medium. The directions of the droplet and camera inspection were unified to ensure measurement accuracy. Back-light illumination was adopted to increase visibility and distinguish the ink spread during image acquisition. The spreadability was assessed using digital filters, a boundary detector, nonlinear regression, and statistical analysis. Digital filters removed atypical patterns caused by the heterogeneity and porosity of the medium from the images. Subsequently, boundary points were extracted from the binarized images using a Canny edge detector. The ink spread was represented by a radial model to identify the overall movement, whereby a shaded region was considered for the kinematic structure of the measurement system. The radial model was estimated from the boundary points using nonlinear regression and a simplex search. Various spreadability indices were derived from the geometric and statistical relationships between the radial model and boundary points. The proposed measurement method was verified using predefined test images. The measurement error was at the sub-pixel level. The spreadability of ink droplets with 10–100% density was further measured, and the physical properties were investigated.