ASTM C1648-12

4.1 Measurement—The refractive index at any wavelength of a piece of homogeneous glass is a function, primarily, of its composition, and secondarily, of its state of annealing. The index of a glass can be altered over a range of up to 1×10^-4 (that is, 1 in the fourth decimal place) by the changing of an annealing schedule. This is a critical consideration for optical glasses, that is, glasses intended for use in high performance optical instruments where the required value of an index can be as exact as 1×10^-6. Compensation for minor variations of composition are made by controlled rates of annealing for such optical glasses; therefore, the ability to measure index to six decimal places can be a necessity; however, for most commercial and experimental glasses, standard annealing schedules appropriate to each are used to limit internal stress and less rigorous methods of test for refractive index are usually adequate. The refractive indices of glass ophthalmic lens pressings are held to 5×10^-4 because the tools used for generating the figures of ophthalmic lenses are made to produce curvatures that are related to specific indices of refraction of the lens materials.

4.2 Dispersion—Dispersion-values aid optical designers in their selection of glasses (Note 1). Each relative partial dispersion-number is calculated for a particular set of three wavelengths, and several such numbers, representing different parts of the spectrum might be used when designing more complex optical systems. For most glasses, dispersion increases with increasing refractive index. For the purposes of this standard, it is sufficient to describe only two reciprocal relative partial dispersions that are commonly used for characterizing glasses. The longest established practice has been to cite the Abbe-number (or Abbe ν-value), calculated by:


where v_D is defined in 4.2.1 Some modern usage specifies the use of the mercury e-line, and the cadmium C′and F′ lines. These three lines are obtained with a single spectral lamp.


where v_e is defined in 4.2.2 A consequence of the defining equations (Eq 1 and 2) is that smaller ν-values correspond to larger dispersions. For ν-values accurate to 1 to 4 %, index measurements must be accurate to 1×10^-4; therefore, citing ν-values from less accurate test methods might not be useful.

1.1.2 Test methods presented by name only (see 1.1.2.1 Immersion refractometers,

1.1.2.2 Interferometry,

1.1.2.3 Ellipsometry, and

1.1.2.4 Method of oblique illumination.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

1.3 Warning—Refractive index liquids are used in several of the following test methods. Cleaning with organic liquid solvents also is specified. Degrees of hazard associated with the use of these materials vary with the chemical nature, volatility, and quantity used. See manufacturer's literature and general information on hazardous chemicals.