What does REF IND stand for?

Refractive Index

(ASTM D 1218) number indicating the angle through which a ray of light is deflected as it passes through a solid or fluid medium; the number that expresses the ratio of the sine of the angle of incidence to the sine of the angle of refraction. Typically used for monitoring water-based fluids or solutions. The ratios of the velocities of light in a medium and in air under the same conditions. Measured by the ratio of the sines of the angles of incidence and refraction.