On estimating local defects of softwood kraft fibers stained with congo red and assessed with a novel fiber analyzer

•Stained softwood fibers were analyzed with a new algorithm for the overall fiber damage index (OFDI).•Analysis using a prototype fiber analyzer equipped with a CCD camera.•Expectably higher reproducibility and statistical significance compared to manual analysis.•Excellent correlation with the widely applied zero span tensile test.•Allows new insights in the action of high and low consistency pulp refining.