Jeroen van Roy

A high contrast 400-2500 nm hyperspectral checkerboard consisting of Acktar material cut with a femto second laser

Visible-near infrared (Vis-NIR) and short wave infrared (SWIR) hyperspectral imaging (HSI) are gaining interest in the food sorting industry. As for traditional machine vision (MV), spectral image registration is an important step which affects the quality of the sorting system. Unfortunately, it currently still remains challenging to accurately register the images acquired with the different imagers as this requires a reference with good contrast over the full spectral range. Therefore, the objective of this work was to develop an accurate high contrast checkerboard over the full spectral range. From the investigated white and dark materials, Teflon and Acktar were found to present very good contrast over the full spectral range from 400 to 2500 nm, with a minimal contrast ratio of 60% in the Vis-NIR and 98 % in the SWIR. The Metal Velvet self-adhesive coating from Acktar was selected as it also provides low specular reflectance. This was taped onto a near-Lambertian polished Teflon plate and one out of two squares were removed after laser cutting the dark coating with an accuracy below 0.1 mm. As standard technologies such as nano-second pulsed lasers generated unwanted damages on both materials, a pulsed femto-second laser setup from Lasea with 60µm accuracy was used to manufacture the checkerboard. This pattern was monitored with an Imec Vis-NIR and a Headwall SWIR HSI pushbroom hyperspectral camera. Good contrast was obtained over the full range of both HSI systems and the estimated effective focal length for the Vis-NIR HSI was determined with computer vision to be 0.5 mm, close to the lens model at high contrast.