<title>MTF as a figure of merit in visual surface inspection</title>

Abstract

The quality of the image is crucial for the performance of a visual surface inspection system. The final image quality is affected by the properties of the illumination and imaging units. In order to evaluate the quality of these units a designer should be able to 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 is considered. An uncomplicated test set-up for measuring the system MTF of an imaging unit is presented and MTFs of typical web inspection imaging arrangements are introduced as a function of the f-number of the imaging lens. An appropriate set of defective samples from a paper manufacturing process is collected and imaged using the same arrangements as in the MiT measurements. Defect contrasts and signal-to-noise ratios are determined from the images and the results are compared to the corresponding MiT values.The objective of the study is to demonstrate the relationship between the MiT and the capability of inspection system to detect certain 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 . Defects whose detectivity is critically dependent on the MiT and 2. defects whose detectivity is practically independent of the MiT. As a conclusion, examples are introduced on how the MiT measurement can be used as an imaging system evaluation and design tool.

Dynamic range in automated visual web inspection

“…Image noise. 4 For the determination of the image noise the original data is processed by subtracting the average of an nϫn neighborhood from the value of each pixel. The standard deviation or peak-to-peak value of the filtered image is used as a measure of noise in the image.…”

“…The quality of the imaging components together with the properties of the illumination determines the actual image quality. We have previously proposed methods for the evaluation of imaging in web inspection 2,3 and defined some image quality criteria with respect to an inspection application 4,5 in order to systemize the design of the imaging and to develop tools for the comparative evaluation of different imaging solutions. The proposed methods concentrate on the determination of the spatial resolution and factors affecting the dynamic range are only briefly discussed in one paper.…”

Method for evaluation of imaging in automated visual web inspection

“…In the papers of Laitinen and Moring 18,19 an evaluation procedure for imaging was proposed. It is the aim of this paper to examine more thoroughly the special requirements for the imaging in web inspection and the theoretical bases of the modulation transfer function ͑MTF͒ measurement for a sampling imaging system.…”