ELSEVIER 0963-8695(95)00015-l NDT&E International, Vol. 29, No. 1, 3-I 1, 1996 pp. Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0963~8695/96 $15.00 + 0.00 Contrast transfer function measurement of X-ray solid state linear detectors using bar/space pattern methods V. Kaftaindjian”, Y. M. Zhut, G. Peix” and D. Babot” “Laboratoire CND par Rayonnements lonisants, INSA Lyon, Bat 303,20 avenue A. Einstein, 69 621 Villeurbanne Cedex, France t Creatis, UIRA CNRS 1216, INSA Lyon, Bat 502,20 avenue A. Einstein, 69 621 Villeurbanne Cedex, France Received 70 October 7994; revised 73 March 7995 This paper presents a detailed analysis of bar/space pattern methods used in the contrast transfer function (CTF) calculation of linear solid-state X-ray sensitive detectors (LD). The analysis presented centres on the effect of cell aperture on LD response as well as on the measurement of the LD’s CTF along its main axis. Various theoretical simulations are designed to obtain a better understanding of the behaviour of the LD. The simulation results are also compared with the experimental CTF measurements. Keywords: contrast transfer function, linear detector, X-ray Linear solid-state X-ray sensitive detectors (LD) are a new generation of X-ray imaging devices. This type of detector, also often called a discrete detector in the literatureI’], offers significant advantages over conven- tional detectors such as film and X-ray image intensifier (XRII) based radioscopic techniques for inspection of displacing objects, and has now become a cost-effective and practical tool for nondestructive testing (NDT) applications[2-41. The imaging performance of LDs can be described by using many possible quality parameters. The modulation transfer function (MTF), however, is a major parameter making it possible to quantify the contrast response of a system as a function of spatial frequency (lines pairs/ mm). Many MTF evaluation methods have, in the past, been proposed in the context of classical detectors (or continuous detectors with respect to the term ‘discrete detectors’). Among them, the bar/space pattern method seems to be the most commonly used. This direct and intuitive method indeed appears very practical and requires very simple apparatus. Note, however, that the bar/space pattern method enables the contrast transfer function (CTF) instead of the MTF of the system to be measured, and that the CTF values are slightly greater than the MTF ones. Nevertheless, in practice, it can often be considered that the CTF is approximately equal to the MTF except for a constant difference” ]. Therefore, to some extent, we can alternatively employ the term CTF and MTF. Conceptually, the bar/space pattern method can be simply employed for LD cases. However, anomalies can appear in the resultant periodic rectangular signals, altering the accurate measurement of CTF values. The purpose of this paper is to study in detail the use of the bar/space pattern method for measuring the CTF of LDs, to show the problems which can occur and to propose possible solutions to them. The paper is organized as follows. In the next section the features and operating principle of a LD based system are recalled. The following section focuses on the discrete characteristics of the detector, and how they affect the CTF measurements. After presenting a number of numerical simulations, experimental measurements of the LD’s CTF are given. Also, a comparison of simulated and experimental results is presented. Description of a LD based imaging system The general architecture of a LD based radioscopic system is shown in Figure 1. An X-ray beam passing 3