Refractive index measurment apparatus and refractive index measurment method

A refractive index measurement apparatus comprising:

• a light source;
• a measurement cell including a sample as an object of refractive index measurement, and diffracting light incident from the light source;
• a detector that detects the amount of diffracted light exiting from the measurement cell in at least one diffraction order other than zero order;
• and
• a control unit that determines refractive index of the sample corresponding to measured value of the amount of diffracted light in the at least one diffraction order detected by the detector using an equation expressing the relationship between diffraction efficiency in the at least one diffraction order and refractive index of the sample;
• wherein the measurement cell includes:
• a first transparent substrate and a second transparent substrate disposed so as to be opposed to each other;
• and
• a diffraction grating that is formed from transparent material having known refractive index and disposed between the first substrate and the second substrate, and diffracts light from the light source in accordance with difference between refractive index of the sample and refractive index of the transparent material;
• wherein the diffraction grating is a binary grating having a plurality of members formed from the transparent material whose width in a first direction parallel to a first surface of the first substrate opposed to the second substrate decreases stepwise as approaching from the first substrate to the second substrate, the plurality of members being periodically arranged at a prescribed pitch along the first direction;
• and
• wherein the sample is filled in the space between the first substrate and the second substrate not occupied by the diffraction grating.