Thoriated glass

[2] 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.

[3][4] Over extended time periods, thoriated glass may develop significant discoloration.

This is due to induced F-centers forming in the glass as the radioactive decay of the thorium progresses.

[5] The formation of F-centers is due to the ionizing effect of the high energy thorium decay products.