1 Introduction

One of the reasons face recognition has attracted so much research attention and sustained development over the past 30 years is its great potential in numerous government and commercial applications. In 1995, Chellappa et al. [5] listed a small number of applications of face recognition technology and described their advantages and disadvantages. However, they did not analyze any system deployed in real applications. Even the more recent review [35], where the set of potential applications has been grouped into five categories, did not conduct such an analysis. In 1997, at least 25 face recognition systems from 13 companies were available [3]. Since then, the numbers of face recognition systems and commercial enterprises have greatly increased owing to the emergence of many new application areas, further improvement of the face recognition technologies, and increased affordability of the systems. We have listed 10 of the representative commercial face recognition companies, their techniques for face detection, the face features they extract, and face similarity comparison methods in Table 1. These 10 companies are also the participants of the latest face recognition vendor test (FRVT 2002) [27] carried out independently by the U.S. government to evaluate state-of-the-art face recognition technology. Although some of these techniques are not publicly available for proprietary reasons, one can conclude that many others have been incorporated into commercial systems.