Method to determine refractive index by measurement of flat plates

Minimal refractive index data in the near-infrared spectrum are available for optical plastics. Typically, refractive index measurements are made by fabricating a prism of candidate optical material and using appropriate metrology equipment. Few plastics are available in thicknesses adequate to fabricate appropriate size prisms; however, almost all optical plastics can be acquired in a flat plate form. The investigation considered two fundamental approaches to measure the refractive index by (i) rotating a flat plate and measure the beam displacement and (ii) measuring the optical focal shift. The rotation method was determined not accurate enough. An optical focal shift method optical mechanical (n toptical / tmechanical ) = was developed that utilized existing laboratory equipment. The shift of focus when the plastic plate sample was inserted was located by determining the position of maximum contrast of the projection of a Ronchi ruling target when illuminated by flux from a Cary Eclipse covering the spectral range of 550–975 nm. The instrumentation, data processing, and measurement performance are presented.