A data entry system , including: a processor ; a sensor apparatus (201; 300; 400) configured to capture an image including the fingertips of a hand and an arbitrary physical surface; and a memory configured to store the captured image, the sensor apparatus further comprising: at least one mechanical sensor configured to detect contact by at least one fingertip of the hand with the arbitrary physical surface, wherein the data entry system further comprises a wearable structure constructed to support the sensor apparatus on any one of i) a dorsal surface of the hand, ii) a palmar surface of the hand, iii) a wrist of the hand, or iv) phalanges of the hand, or any combination of i), ii), iii), and iv), and wherein upon detection by the at least one mechanical sensor of contact by the at least one fingertip the processor is configured to: cause the sensor apparatus to capture the image, analyze the image to determine the respective positions of the fingertips, determine which fingertips are in contact with the arbitrary surface, and generate a signal representative of contact of one or more fingertips singly or in combination against the arbitrary physical surface in an arbitrary location. a processor ; a sensor apparatus (201; 300; 400) configured to capture an image including the fingertips of a hand and an arbitrary physical surface; and a memory configured to store the captured image, the sensor apparatus further comprising: at least one mechanical sensor configured to detect contact by at least one fingertip of the hand with the arbitrary physical surface, wherein the data entry system further comprises a wearable structure constructed to support the sensor apparatus on any one of i) a dorsal surface of the hand, ii) a palmar surface of the hand, iii) a wrist of the hand, or iv) phalanges of the hand, or any combination of i), ii), iii), and iv), and wherein upon detection by the at least one mechanical sensor of contact by the at least one fingertip the processor is configured to: cause the sensor apparatus to capture the image, analyze the image to determine the respective positions of the fingertips, determine which fingertips are in contact with the arbitrary surface, and generate a signal representative of contact of one or more fingertips singly or in combination against the arbitrary physical surface in an arbitrary location. at least one mechanical sensor configured to detect contact by at least one fingertip of the hand with the arbitrary physical surface, wherein the data entry system further comprises a wearable structure constructed to support the sensor apparatus on any one of i) a dorsal surface of the hand, ii) a palmar surface of the hand, iii) a wrist of the hand, or iv) phalanges of the hand, or any combination of i), ii), iii), and iv), and wherein upon detection by the at least one mechanical sensor of contact by the at least one fingertip the processor is configured to: cause the sensor apparatus to capture the image, analyze the image to determine the respective positions of the fingertips, determine which fingertips are in contact with the arbitrary surface, and generate a signal representative of contact of one or more fingertips singly or in combination against the arbitrary physical surface in an arbitrary location. cause the sensor apparatus to capture the image, analyze the image to determine the respective positions of the fingertips, determine which fingertips are in contact with the arbitrary surface, and generate a signal representative of contact of one or more fingertips singly or in combination against the arbitrary physical surface in an arbitrary location. <EOS>