In an example embodiment, rather than a single calibration object, a plurality of calibration objects are placed within the field of view of an inspection camera at the same time. Each of the calibration objects is or includes a semi-spherical reflective object having known sizes. During a calibration process, the distance between a light source and camera may be determined by activating the light source and capturing an image of how the light source bounces off each of the calibration objects, and more particularly the location on each of the calibration objects that the light source appears to reflect from. This allows the angle that the light source strikes each calibration object to be calculated, and these angles can then be used to determine the distance between the light source and the camera.

A refractometer and a method for measuring refractive index is disclosed. The refractometer comprises a light source, a reflecting module, a lens module, an array sensor, and a processor, wherein the reflecting module is configured to receive optical beams from the light source, the reflecting module comprises a detection surface configured to totally reflect at least part of the optical beams; the lens module is configured to converge optical beams from the detection surface onto a focal plane of the lens module; and the array sensor is located on the focal plane of the lens module.