Thoriated glass is a

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

material used in the manufacture of optical systems, specifically photographic lenses. It is useful to this process due to its high

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

. Thoriated glass is

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

due to the inclusion of

thorium dioxide Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is mainly a by-product of lanthanide and uranium production. Thorianite is the name of the mineralogical for ...

, oxide of radioactive element

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

. It has therefore been succeeded as a material of choice by glass including lanthanum oxide. Thoriated glass can contain up to 30% by weight of thorium.http://www.irpa.net/irpa3/cdrom/VOL.3B/W3B_13.PDF Robert C. McMillan & Steven A. Horne: ''Eye Exposure from Thoriated Optical Glass'' (U.S. Army memo) The thoriated glass elements in lenses over time develop a brown tint reducing transmission and interfering with neutral color reproduction. Many Kodak, Fuji and Asahi Takumar lenses that were produced prior to the 1970s are radioactive.

Radiation browning

Over extended time periods, thoriated glass may develop significant discoloration. This is due to induced

F-center An F-center or color center or Farbe center (from the original German ''Farbzentrum'', where ''Farbe'' means ''color'' and ''zentrum'' means ''center'') is a type of crystallographic defect in which an anionic Vacancy defect, vacancy in a crystal ...

s forming in the glass as the

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

of the thorium progresses. The formation of F-centers is due to the ionizing effect of the high energy thorium decay products. This process can potentially be reversed by annealing the glass or exposing it to

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

References

Thorium Glass engineering and science Glass compositions {{Glass-engineering-stub