Radioactive specialty glass lenses from Kodak, Pentax, and Zeiss with high refractive index for exceptional sharpness. Yellowing from gamma radiation produces a characteristic warm aesthetic.
Technical Details
Thorium glass (Schott LaK-9, Ohara LAH-10) achieves an Abbe number of 47-52 with an exceptionally high refractive index. Kodak primarily used thorium glass in wide-angle and telephoto lenses of its Professional series, including the legendary Ektar 32mm f/1.9. The radioactivity measures 0.01-0.1 mR/h directly at the lens. Pentax employed the material in Super-Multi-Coated Takumar lenses, particularly in the 50mm f/1.4 and 55mm f/1.8. Discoloration occurs due to color center formation when gamma radiation alters the crystal structure of the glass.
History & Development
Schott developed the first thorium glasses for military optics in 1939. Kodak introduced commercial thorium lenses in 1946, followed by Pentax (1964) and Mamiya (1967). The peak period was between 1950-1975, when manufacturers like Schneider-Kreuznach and Rodenstock used thorium elements in large format lenses. Pentax ceased production in 1982, with other manufacturers following suit by 1990. Modern lanthanum glasses (La₂O₃) now replace thorium without radioactive side effects.
Practical Use in Film
Thorium lenses shaped the aesthetic of the 1960s-70s with their exceptional sharpness and characteristic contrast. Stanley Kubrick used the Zeiss Planar 50mm f/0.7 (containing thorium) for the candlelight scenes in "Barry Lyndon" (1975). Discolored thorium lenses produce a warm, golden look that some cinematographers utilize deliberately. UV treatment with 6-8 hours of direct sunlight can temporarily reduce discoloration. Professional cleaning requires radiation protection certification.
Comparison & Alternatives
Modern ED (Extra-low Dispersion) glasses and fluorite lenses achieve similar optical properties without radioactivity. Lanthanum crown glasses offer comparable refractive indices with neutral color rendition. Thorium lenses remain sought after for their unique rendering characteristics – their minimal vignetting and characteristic bokeh are difficult to replicate with modern designs. Collector's items like the Canon 50mm f/0.95 or Leica Noctilux today use aspherical elements instead of thorium.