The sensor 110 may capture a vein image. The sensor 110 may include a light source and a camera. The light source may emit an infrared ray, etc., and the camera may capture the vein image. While the infrared ray is emitted from the light source, the camera may capture an image of the back of user's hand, or another part of the body, to acquire the vein image. Since a portion in which a vein is disposed has high infrared reflectivity compared to other portions of the back of hand, the camera may capture a reflected infrared ray to capture the vein image.

According to an exemplary embodiment, the sensor 110 may capture an image of biological information capable of identifying the user. The identifying the user may include determining who is the user currently wearing the wearable device 100, or who is the user from among a plurality of registered users, which is wearing the wearable device 100. For example, the sensor 110 may capture an image of biological information on a user's vein, iris, or finger, etc. As another example, the sensor 110 may capture vein images of both the user's hands. As yet another example, the sensor 110 may capture vein images of the back of the user's hand and the palm of the user's hand. According to an exemplary embodiment, the sensor 110 may capture iris images of both of a user's eyes.

In response to a function or an application, which requires authentication, being selected, the processor 140 may control the sensor 110 to capture the vein image of the user wearing the wearable device 100. The selecting the function or the application may include a user selecting the function or the application by using an input unit (e.g., inputter) in order to execute the function or the application.