At present the formulae proposed by G Boensch and E Potulski in 1998 (Boensch and Potulski 1998 Metrologia 35 133–9) are mostly used to calculate the air refractive index. However, the humidity correction equation in the formulae is derived by using the light source of a Cd lamp whose light frequency stability is poor and at a narrow temperature range, around 20 °C. So it is no longer suitable in present optical precision measurements. To solve this problem, we propose a refractive index measurement system based on phase step interferometer with three frequency stabilized lasers (532 nm, 633 nm, 780 nm), corrected coefficients of the humidity are measured and a corresponding revised humidity correction equation is acquired. Meanwhile, the application temperature range is extended from 14.6 °C to 25.0 °C. The experiment comparison results at the temperature of 22.2–23.2 °C show the accuracy by the presented equation is better than that of Boensch and Potulski.

A solution refractive index (SRI) is one of the key parameters to indicate important information of materials such as optical properties, solute composition and so on. Samples are usually made of low concentration solutions, so that some properties and parameters can be determined by detecting SRI variations in chemical or physical reactions. In this connection, a kind of SRI measurement method based on surface plasmon resonance (SPR) and dual-frequency laser interferometric phase detection is presented. The theoretical model based on the Kretschmann excitation structure shows the variation of phase difference between p and s polarization components of the reflected light is approximately linear with SRI at the range of 1.333–1.336 RIU. The measurement formula is derived and corresponding experimental system is built based on the heterodyne interference optical path by using a dual-frequency laser. Error analysis shows the maximum value of the measurement uncertainty is less than 3.0  ×  10−5. The experiment results of measuring glycerine solution refractive index agree with the theoretical analysis. Comparison results show the measurement differentia between the presented method and the formula by Abbe refractometer is less than 2.0  ×  10−5.