Methods and apparatus, including computer program products, for determining a presence of an object located close to a lens (301) of a camera monitoring a scene. A first infrared illumination source (302) arranged to illuminate the scene from a first angle is activated. A first image (304) is acquired by the image capturing device (300). The first infrared illumination source (302) is deactivated, and a second infrared illumination source (306) arranged to illuminate the scene from a second angle is activated. A second image (308) is acquired by the image capturing device (300). Intensity information of the first image (304) and the second image (308) is compared to determine the presence of an object located close to the lens (301).