The relevance of aviation security has increased dramatically in recent years. Airport security technology has evolved remarkably over the last decade, which is especially evident for state-of-the-art X-ray screening systems. However, such systems will be only as effective as the people who operate them. Recognizing all kinds of prohibited items in X-ray images of passenger bags is a challenging object recognition task. In this article we present a method to measure screener detection performance based on signal detection theory. This method is applied to measure training effects resulting from individually adaptive computer based training (CBT). We have found large increases of detection performance and substantial reductions in response time suggesting that CBT is a very effective tool for increasing effectiveness and efficiency in aviation security screening.