is the ratio of the velocity of light at a specified wave
length in air to its velocity in a substance under examination. The
refractive index of light-colored petroleum
liquids can be determined by test method
ASTM-D-1218, using a refractometer and a monochromatic light source.
Refractive index is an excellent test for uniform composition of
solvents, rubber
process oils, and other petroleum products. It may also be used in
combination with other simple tests to estimate the distribution of
naphthenic, paraffinic,
and aromatic carbon atoms in a process oil.