Perceiving the dynamical environment both visually and tactilely is crucial for the survival of animals, and therefore, is considered of importance in robotics research. Recently, there has been an increasing interest in vision-based tactile sensors due to their high sensing resolution and robustness to environmental changes. However, almost all vision-based tactile sensors make only partial use of the camera, specifically, only when contact occurs, and stay idle at other times, which results in a waste of the camera information bandwidth. In this paper, we propose a new visual-tactile dual-modality sensor called SpecTac, which can visually inspect the environment and make tactile observations. The main novelty of the sensor is the use of ultraviolet (UV) LEDs and randomly distributed UV fluorescent markers. When the LEDs are on, those markers will be bright and can easily be distinguished and tracked from the background. Besides, by controlling the on and off of the UV LEDs, due to the switchable visibility of those markers, the sensor will switch between visual and tactile sensing mode. The qualities of tactile and visual perception are evaluated quantitatively by force estimation, visual triangulation and visual feature matching. By combining both modalities into one compact sensor, the information from the camera is better utilized, and it is hoped that the sensor will achieve more flexibility in the motion of the robot arm, especially in tasks where the workspace is narrow.