# Evaluation of a Vein Biometric Recognition System on an Ordinary Smartphone
## 1 Introduction
Vascular patterns are formed by the structure of blood vessels within the body. The typical acquisition is based on sensing in an optical way the absorption of near-infrared (NIR) light by the hemoglobin in the bloodstream. Under illumination from NIR LEDs, vessels appear as dark structures and the surrounding tissues appear bright because vessels have much more light absorption in that spectrum. Although arteries or veins can be employed as biometric traits, veins are easier to detect, and their images are clearer. There are more veins than arteries in the human body and veins are closer to the skin surface. The body parts usually considered for biometric recognition based on veins are fingers, hands, and wrists [2]. There are several properties that should be considered for a biometric trait [1]: (1) universality (if all individuals possess the biometric trait); (2) distinctiveness or uniqueness (if the biometric trait is sufficiently different for individuals); (3) permanence (if the trait does not vary over time); (4) performance (if the recognition is accurate and fast); (5) circumvention (if the trait can be imitated using artifacts); (6) collectability or measurability (if the acquisition is easy to carry out); and (7) acceptability (if the individuals accept the acquisition of the trait and the use of the recognition system). Depending on the biometric trait, these properties vary. In the case of fingerprints and faces, universality, distinctiveness, performance, and acceptability are high. Permanence is medium because fingerprints (by the uses of the fingers) and faces (by the individual aging) change over time. Regarding collectability, contact is usually required in the case of fingerprints, which means that skin oiliness, dust, or creams may reduce the quality of the images acquired. In addition, specific sensors are required. For these reasons, the collectability of fingerprints is medium. Collectability for faces is also medium. In this case, non-specific cameras can be employed, but significant changes in the appearance due to the growth of a full beard or wearing masks affect facial recognition. Circumvention is medium in both cases since it is possible to spoof fingerprints and faces by using silicon fingers (for fingerprints) and photos, videos, or 3D face masks (for faces). In the case of veins, universality, distinctiveness, permanence, performance, and protection against circumvention are high because everyone has veins, vein patterns are different among individuals, the veins are usually invariant throughout the life, the error rate can be low and the recognition can be at real time, and finally, veins are inside the body and are difficult to spoof using artifacts. However, collectability and acceptability are medium. Although acquisition techniques are contactless, environmental factors, such as light, temperature, or humidity, and biological factors, e.g., the thickness of the layers of human tissues and diseases, such as anemia, hyperthermia, or hypotension, affect vein recognition. Furthermore, specific and large-sized acquisition devices, not suitable for ordinary smartphones, are typically employed to acquire veins [3; 4; 5; 6]. There are only a few works focused on the vein acquisition with portable devices and smartphones [7; 8; 9; 10; 11; 12]. However, a specific device is designed in [8; 9], additional illumination is employed together with a modified smartphone to remove the NIR blocking filter in [10], smartphones with an infrared camera are employed in [7], and very preliminary works are presented in [11; 12]. This work aims to evaluate acquisition methods and vein recognition algorithms using ordinary smartphone cameras, i.e., without modification or additional modules. The sophistication of ordinary smartphone cameras improves the quality of the captures. Besides, there are smartphone applications (apps) that acquire vein images in the visible domain and apply filtering techniques to improve the contrast of superficial veins, simulating the acquisition in the infrared spectrum.