Simple and flexible method for measuring the liquid refractive index with an electrically tunable lens

In the parallel-plate-based method for measuring the liquid refractive index (RI), the parallel light passes sequentially through a convex lens and a parallel plate containing the tested liquid, resulting in axial displacement of the emitted light. By moving the camera, the axial displacement caused by the introduction of the tested liquid can be obtained and used to calculate the liquid RI. To avoid the mechanical adjustment of the system, a simple and flexible method for measuring the liquid RI via an electrically tunable lens (ETL) is proposed. The convex lens is replaced with an ETL. By precisely controlling the driving current of the ETL, the focal position of the camera can remain unchanged after placing the tested liquid in the parallel plate. The focal length change, induced by variations in the driving current of the ETL, is equivalent to the axial displacement of the camera. Therefore, the RI of the liquid can be measured from the change in focal length, avoiding mechanical movement of the camera. The equivalence of the ETL is theoretically derived. The imaging characteristics of the ETL were thoroughly discussed based on Zemax simulations, validating the effectiveness of the proposed method. The experimental results of the RI for different liquids further demonstrate the validity of our proposal.