Determining point spread function of pressure sensitive labels facestock

Abstract

Pressure sensitive labels (PSL) are a growing part of graphic industry. In the past, their design and application were rather limited to basic packaging and product information while currently labels are receiving more and more attention because they are customer's first contact with a product. PSL composition includes release liner, adhesive and facestock. Facestock can be considered as the most important part of the label since its properties determine the final label appearance. Nowadays, facestock material can be both paper and filmic. Therefore, it is necessary to perform a comprehensive characterization of pressure sensitive labels facestock in order to investigate the future impact of optical dot gain on the print. Point spread function is a measure of scattering and absorption of light in the given substrate. It carries information regarding paper composition and its future behaviour in term of optical dot gain. In the study, seven different facestock substrates were examined, two made from biogenic polyethylene, and five paper based PSL facestocks, three of which are made from 15% agro industrial waste by-products (barley, citrus and grape). Point spread function of the substrates were obtained by projecting collimated red laser light source on the facestock of pressure sensitive labels. Although the laser light used in this experiment was characterized by a narrow beam width of 0.5 mm and low divergence, <1 mrad, the beam was further converged and then passed through a space filter to ensure collimation of the laser beam which was then incident perpendicular to the set sample. Laser profile was firstly obtained by photodiode profiling. Based on the measured values of light intensity, the laser light profile was determined, as well as the average beam width that was needed for further image analysis. Image of the light scattering within the substrate was obtained perpendicular to the surface with Canon EOS 5DS camera. From the obtained image, laser profile was removed, and the resulting profile gives the point spread function of the substrate. The resulting images were processed with commercially available image analysis software (ImageJ). The mean background intensity subtracted from each individual pixel was determined to remove the background effect, and the distribution of the scattered light profile was achieved. The results showed that facestock composition has a high influence on PSF shape and width.