An optical straightness measurement sensor for the KRISS watt balance

•We developed an optical sensor to measure straightness error of a stage.•The sensor is vacuum compatible, non-magnetic, thus suitable for watt balance.•Resolution is 10 nm, nonlinearity is less than 0.5%, sensitivity is 0.003624 μm−1.•Sensitivity is obtained as a function of the position of cube-corner prism.•Measurement results agree with that of laser interferometer within ±60 nm over 40 mm.

We present an optical straightness measurement sensor applicable to the watt balance experiment. Key parts of the sensor are a cube-corner prism and a right angle reflector having a sharp edge. Beam reflected from the cube-corner prism is split into two opposite directions by the right angle reflector, and collected by photo detectors connected to a lock-in amplifier. Straightness error is measured by monitoring the normalized intensity difference of the two lock-in signals. We describe in detail about the implemented sensor and evaluated performance on its resolution, sensitivity, and nonlinearity comparing with a laser interferometer. Straightness measurement result of a translation stage agreed with that obtained by a laser interferometer within ±60 nm over the measurement range of 40 mm.