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When added to glass, thorium dioxide helps increase its refractive index and decrease its dispersion. Such glass finds applications in high-quality lenses for cameras and scientific instruments. The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal. Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides such as lanthanum oxide in almost all modern high-index glasses, as they provide similar effects and are not radioactive.
This picture shows a yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre) and a lens without yellowing (right).Photograph credit: El Grafo