a bubble vial having a vial body defining a fluid chamber with a curved upper surface,

a quantity of fluid partially filling said chamber, said fluid defining a bubble in the upper portion of said chamber which moves along said upper surface in dependence upon the orientation of said vial,

a centrally positioned light source above said bubble vial directing light into said chamber,

four light sensors positioned above said bubble vial and arranged circumferentially around said centrally positioned light source, a first pair of said sensors being positioned on opposite sides of said light source along a first axis, and a second pair of said sensors being positioned on opposite sides of said light source along a second axis, said second axis being substantially perpendicular with respect to said first axis, and

a reflector beneath said fluid chamber, such that light emitted from said light source passes through said bubble and said fluid and is reflected by said reflector to said light sensors, whereby the relative levels of light detected by said first pair of sensors indicates the orientation of said first axis with respect to horizontal, and the relative levels of light detected by said second pair of sensors indicates the orientation of said second axis with respect to horizontal.