A method of receiving an input by determining information regarding a point of contact (48; 62; 102) of an object on a surface of a solid body, the method comprising: a) placing an input device (4; 4'; 4") in contact with the surface of the solid body, wherein the input device (4; 4'; 4") comprises a plurality of optical vibration sensors (2; 2'; 2"; 402; 500) mounted in a common housing (16; 16"), each optical vibration sensor (2; 2'; 2"; 402; 500) comprising: a diffractive optical element; a light source arranged to illuminate said diffractive optical element such that a first portion of light passes through the diffractive optical element and a second portion of light is reflected from the diffractive optical element; and a photo detector arranged to detect an interference pattern generated by said first and second portions of light (30, 32); wherein the optical vibration sensor (2; 2'; 2"; 402; 500) is configured so that in use, after the first portion of light passes through the diffractive optical element, the first portion of light is reflected from a reflective surface onto the photo detector; b) bringing the object into physical contact with the surface of the solid body, thereby causing vibrations in the solid body; c) detecting the vibrations using two or more of the optical vibration sensors (2; 2'; 2"; 402; 500); and d) using the relative phase(s) and/or the relative amplitude(s) of the vibrations to determine information regarding the point of contact (48; 62; 102) of the object on the surface of the solid body. a) placing an input device (4; 4'; 4") in contact with the surface of the solid body, wherein the input device (4; 4'; 4") comprises a plurality of optical vibration sensors (2; 2'; 2"; 402; 500) mounted in a common housing (16; 16"), each optical vibration sensor (2; 2'; 2"; 402; 500) comprising: a diffractive optical element; a light source arranged to illuminate said diffractive optical element such that a first portion of light passes through the diffractive optical element and a second portion of light is reflected from the diffractive optical element; and a photo detector arranged to detect an interference pattern generated by said first and second portions of light (30, 32); wherein the optical vibration sensor (2; 2'; 2"; 402; 500) is configured so that in use, after the first portion of light passes through the diffractive optical element, the first portion of light is reflected from a reflective surface onto the photo detector; a diffractive optical element; a light source arranged to illuminate said diffractive optical element such that a first portion of light passes through the diffractive optical element and a second portion of light is reflected from the diffractive optical element; and a photo detector arranged to detect an interference pattern generated by said first and second portions of light (30, 32); b) bringing the object into physical contact with the surface of the solid body, thereby causing vibrations in the solid body; c) detecting the vibrations using two or more of the optical vibration sensors (2; 2'; 2"; 402; 500); and d) using the relative phase(s) and/or the relative amplitude(s) of the vibrations to determine information regarding the point of contact (48; 62; 102) of the object on the surface of the solid body.