a Figure of Merit in Visual Surface Inspection

ABSTRACT The quality of the image is crucial for the performance of a visual surface inspection system. The final image quality is affectedby the properties of the illumination and imaging units. In order to evaluate the quality of these units a designer should be ableto characterize their performance using quantitative figures of merit.In this paper the suitability of the modulation transfer function (MTF) for the figure of merit in visual web inspection isconsidered. An uncomplicated test set-up for measuring the system MTF of an imaging unit is presented and MTFs of typicalweb inspection imaging arrangements are introduced as a function of the f-number of the imaging lens. An appropriate set ofdefective samples from a paper manufacturing process is collected and imaged using the same arrangements as in the MiTmeasurements. Defect contrasts and signal-to-noise ratios are determined from the images and the results are compared to thecorresponding MiT values.The objective of the study is to demonstrate the relationship between the MiT and the capability of inspection system to detectcertain kinds of surface defects. The existence of this relationship is a necessity when MiT is considered as a figure of merit.Based on the experimental results surface defects are classified in two categories: 1 .