Optical Properties

Refractive Index - The ratio of the velocity of propagation of an electromagnetic wave in a vacuum to its velocity in the glass. This determines how much a light wave is "bent" when entering and leaving the surface of the glass. This property is important in producing certain optical devices or effects, such as lenses. The refractive index differs depending on the specific wavelength of light. Therefore, the refractive index is always specified at stated wavelengths, generally corresponding to characteristic material spectral emission lines, such as the sodium "D" line - the most commonly used wavelength value.

Dispersion - A measure of the separation of light into its component colors, such as a prism dispersing white light into a color band or a rainbow effect.

Abbe Value - The ratio of refractivity to dispersion in an optical medium. Specifically: (nd-1)/(nF-nC), where n is the index of refraction for the spectral lines Fraunhofer d. F and C, respectively.

Transmission - The passage of light through a material; light not reflected back to its source or absorbed is transmitted through the glass. Values are given as a percentage of the original source light energy versus the transmitted light energy. These values vary with wavelength and for each material. For example, soda-lime glass has very low transmission at a wavelength of 265nm, but fused silica transmits well at this wavelength.

Reflectivity - The return of light from a surface with no change in wavelength. This is generally given as a percentage of the source light energy versus the reflected energy and is dependent upon the angel of incidence (the angle between the light source and the glass surface, usually specified from "normal" - a line perpendicular to the glass surface) and wavelength.

Birefringence Constant - Birefringence is the separation of a beam of light into two beams as it passes through a doubly refracting glass. Differing stress layers in a glass will cause birefringence and therefore birefringence may be used to measure the amount of stress in a material. Since the amount of birefringence differs by material, the birefringence constant must be considered when determining stress levels.

Absorption - The amount of light energy converted to heat within a material that is not transmitted nor reflected. The amount of absorption varies by material type and wavelength. Tinted materials will absorb more light than clear materials.