The volume of mass unsolicited electronic mail, often known as spam, has recently increased enormously and has become a serious threat to not only the Internet but also to society. This paper proposes a new spam detection method which uses document space density information. Although it requires extensive e-mail traffic to acquire the necessary information, an unsupervised learning engine with a short white list can achieve a 98% recall rate and 100% precision. A direct-mapped cache method contributes handling of over 13,000 e-mails per second. Experimental results, which were conducted using over 50 million actual e-mails of traffic, are also reported in this paper.

The double-probe piezodriving specimen holder that was recently developed by some of the present authors is modified to introduce a laser irradiation port in one of its two arms. As a result, the new specimen holder consists of a piezodriving probe and a laser irradiation port, both of which can be three-dimensionally controlled by using piezoelectric elements and micrometers. While the piezodriving probe interacts with the specimen set in the holder in several ways, the laser beam causes photo-induced phenomena to occur. By performing electron holography using the new specimen holder, we demonstrate that it is possible to evaluate the change in the electric field resulting from the discharging effect of laser irradiation on organic photoconductors.

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