Retinal vessel tree as biometric pattern

In current society, reliable authentication and authorization of individuals are becoming more and more necessary tasks for everyday activities or applications. Just for instance, common situations such as accessing to a building restricted to authorized people (members, workers,...), taking a flight or performing a money transfer require the verification of the identity of the individual trying to perform these tasks. When considering automation of the identity verification, the most challenging aspect is the need of high accuracy, in terms of avoiding incorrect authorizations or rejections. While the user should not be denied to perform a task if authorized, he/she should be also ideally inconvenienced to a minimum which further complicates the whole verification process Siguenza Pizarro & Tapiador Mateos (2005). With this scope in mind, the term biometrics refers to identifying an individual based on his/her distinguished intrinsic characteristics. Particularly, this characteristics usually consist of physiological or behavioral features. Physiological features, such as fingerprints, are physical characteristics usually measured at a particular point of time. Behavioral characteristics, such as speech or handwriting, make reference to the way some action is performed by every individual. As they characterize a particular activity, behavioral biometrics are usually measured over time and are more dependant on the individual’s state of mind or deliberated alteration. To reinforce the active versus passive idea of both paradigms, physiological biometrics are also usually referred to as static biometrics while behavioral ones are referred to as dynamic biometrics. The traditional authentication systems based on possessions or knowledge are widely spread in the society but they have many drawbacks that biometrics try to overcome. For instance, in the scope of the knowledge-based authentication, it is well known that password systems are vulnerable mainly due to the wrong use of users and administrators. It is not rare to find some administrators sharing the same password, or users giving away their own to other people. One of the most common problems is the use of easily discovered passwords (child names, birth dates, car plate,...). On the other hand, the use of sophisticated passwords consisting of numbers, upper and lower case letters and even punctuation marks makes it harder to remember them for an user. Nevertheless, the password systems are easily broken by the use of brute forcewhere powerful computers generate all the possible combinations and test it against the authentication system. In the scope of the possession-based authentication, it is obvious that the main concerns are 6