A projector and a dust detection device for an air filter thereof. More particularly, the system allows users to automatically enter and maintain contact-related information in a digital address book or similar application or device with minimal or no manual entry of the contact-related information by the user. The transducer senses acoustical energy, e. Therefore, light leakage may be minimized. The core andor the inner clad areis doped with a heavy metal. A mobile application tag may be identified in the webpage, where the mobile application tag is independent of a device type of the mobile device.