This International Standard specifies two test methods for determining the refractive index of plastics, namely: — Method A: a refractometric method for measuring the refractive index of moulded parts, cast or extruded sheet or film, by means of a refractometer. It is applicable not only to isotropic transparent, translucent, coloured or opaque materials but also to anisotropic materials. The method is recommended when great accuracy is required. It is not applicable to powdered or granulated material. — Method B: an immersion method (making use of the Becke line phenomenon) for determining the refractive index of powdered or granulated transparent materials by means of a microscope. Monochromatic light should, in general, be used to avoid dispersion effects. The accuracy of this method is about the same as that of method A. It is applicable to isotropic translucent, coloured materials but is not applicable to opaque materials nor to anisotropic materials. NOTE 1 The refractive index is a fundamental property which can be used for checking purity and composition, for the identification of materials and for the design of optical parts. The change in refractive index with temperature may give an indication of transition points of materials. NOTE 2 The accuracy of method B is approximately the same as that of method A when an experienced operator uses the method with extreme care (see clause 7).