The spirit level produces a signal when the bubble(29) is displaced w.r.t. the centre of the bulb(27). An illumination system for the bulb cooperates with a signal generator and a photodetector (22, 24) connected so as to detect the quantity of light transmitted or reflected by the bulb for each side of the bubble. The generator functions when the light level transmitted or reflected from one side is different from that transmitted or reflected from the other side of the bubble. The generator may be a laser(16) and the detector may be a bank of photocells.
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