Refractive-index concentration sensor

A refractive-index concentration sensor that measures a concentration of a coolant including a water-soluble cutting oil comprising:

• a cable that includes a power line to supply power to the refractive-index concentration;
• and
• a housing that has a cable connector unit connected to the cable, the housing having a detection window located on a surface other than a part on which the cable connector unit is provided,
• wherein the housing accommodates:
• a light source;
• a light projecting lens that converts light emitted from the light source into substantially parallel light;
• a diffusion plate that forms a surface light source, the diffusion plate diffuses the substantially parallel light converted by the light projecting lens and converts the substantially parallel light into diffused light having a plurality of angular components at points of the diffusion plate;
• a prism that has a first surface to receive the diffused light transmitted through the diffusion plate, a second surface to reflect the diffused light depending on a refractive index of the coolant including the water-soluble cutting oil in contact with the coolant as a measurement target liquid, and a third surface to extract the reflected light generated by the diffused light being reflected at the second surface;
• a light receiving lens that receives the reflected light received by the third surface of the prism and that collects the reflected light;
• an imaging element that receives the collected light through the light receiving lens and that acquires a light amount distribution corresponding to the collected light;
• a main board that is supplied with power via the power line included in the cable;
• and
• a holder that is fixed to the housing such that the holder presses the prism from an inner side to an outer side and the second surface of the prism is exposed from the detection window of the housing, the light source, the light projecting lens, the diffusion plate, the light receiving lens, and the imaging element being fixed to the holder,
• wherein
• the refractive-index concentration sensor is configured to calculate the concentration of the coolant, based on a position of a bright and dark line generated depending on a critical angle determined by the light amount distribution of the collected light and a correspondence relationship between the refractive index and the concentration.