US Patent 7975393 Spirit level

A spirit level (1) comprises a planar base (14) for resting, in use, on a surface whose level is to be determined, and levelling means (27) comprising a vial (41) filled with sufficient liquid that an air bubble (47) is formed within the vial (41). The spirit level (1) comprises indicia (43, 45) relative to which the bubble (47) moves as the orientation of the spirit level (1) is altered so as to indicate the orientation of the spirit level. Optical means (29) is provided on the spirit level (1) to provide an image indicative of the position of the bubble (47) relative to the indicia (43, 45).