Difference between revisions of "Radioactive lenses"

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Many lenses produced from the 1940s through the 1970s are measurably radioactive. The main source of radioactivity is the use of [http://en.wikipedia.org/wiki/Thorium_oxide thorium oxide] (up to 30% by weight) as a component of the glass used in the lens elements. Thorium oxide has a crystalline structural similar to calcium fluoride ([http://en.wikipedia.org/wiki/Fluorite fluorite]). Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and utilize lenses of lower curvature, which are less expensive to produce. Contrary to often seen statements to the otherwise, lenses containing lanthanum are not appreciably radioactive - lanthanum is only 1/10,000th as radioactive as thorium. Radioactivity in lanthanum containing lenses is due to the intentional inclusion of thorium in the optical glass mix.  The presence of thorium can sometimes, depending on the mixture of other elements in the lens, cause moderate to severe browning of the lens element(s), which can be reversed by the action of photons (bright light) and UV light.<ref name="OSTIRad"/>
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Many lenses produced from the 1940s through the 1970s are measurably radioactive. The main source of radioactivity is the use of [http://en.wikipedia.org/wiki/Thorium_oxide thorium oxide] (up to 30% by weight) as a component of the glass used in the lens elements. Thorium oxide has a crystalline structural similar to calcium fluoride ([http://en.wikipedia.org/wiki/Fluorite fluorite]). Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and utilize lenses of lower curvature, which are less expensive to produce. Contrary to often seen statements to the otherwise, lenses containing lanthanum are not appreciably radioactive - lanthanum is only 1/10,000th as radioactive as thorium. Radioactivity in lanthanum containing lenses is due to the intentional inclusion of thorium in the optical glass mix.  The presence of thorium can sometimes, depending on the mixture of other elements in the lens, cause moderate to severe browning of the lens element(s), which can be reversed by the action of photons (bright light) and UV light.<ref name="OSTIRad">[https://www.osti.gov/biblio/10178461/  Wirtenson, G R; White, R H, 1992 Effects of ionizing radiation on selected optical materials: An overview. Technical Report Lawrence Livermore National Lab., CA, USA.]  doi:10.2172/10178461 </ref>
  
 
==Radiation levels==
 
==Radiation levels==
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* [http://hps.org/hpspublications/radiationfactsheets.html Office of Civilian Radioactive Waste Management - Fact Sheet]
 
* [http://hps.org/hpspublications/radiationfactsheets.html Office of Civilian Radioactive Waste Management - Fact Sheet]
 
* Rudolf Kingslake, ''A History of the Photographic Lens'', Academic Press, 1989, Chapter 5, section 4
 
* Rudolf Kingslake, ''A History of the Photographic Lens'', Academic Press, 1989, Chapter 5, section 4
<ref name="OSTIRad">[https://www.osti.gov/biblio/10178461/  Wirtenson, G R; White, R H, 1992 Effects of ionizing radiation on selected optical materials: An overview. Technical Report Lawrence Livermore National Lab., CA, USA.]  doi:10.2172/10178461 </ref>
 
 
[[Category:Lenses]]
 
[[Category:Lenses]]

Revision as of 17:57, 30 July 2021

Many lenses produced from the 1940s through the 1970s are measurably radioactive. The main source of radioactivity is the use of thorium oxide (up to 30% by weight) as a component of the glass used in the lens elements. Thorium oxide has a crystalline structural similar to calcium fluoride (fluorite). Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and utilize lenses of lower curvature, which are less expensive to produce. Contrary to often seen statements to the otherwise, lenses containing lanthanum are not appreciably radioactive - lanthanum is only 1/10,000th as radioactive as thorium. Radioactivity in lanthanum containing lenses is due to the intentional inclusion of thorium in the optical glass mix. The presence of thorium can sometimes, depending on the mixture of other elements in the lens, cause moderate to severe browning of the lens element(s), which can be reversed by the action of photons (bright light) and UV light.[1]

Radiation levels

Typical radiation levels can approach 1 mR/hr as measured at the lens element's surface, decreasing substantially with distance; at a distance of 3 ft. (0.9 m) the radiation level is difficult to detect over typical background levels. For reference, a typical chest X-ray consists of about about 10 mR, a round-trip cross country airline flight exposes a passenger to 5 mR, and a full set of dental X-rays exposes the patient to 10 mR to 40mR.

Kodak lenses

By far the most prolific producer of radioactive lenses was Eastman Kodak. From the 1940s through the 1960s, substantial numbers of amateur cameras were produced and sold with "thoriated" lenses (containing thorium oxide), including some of the Pony, Signet, and high end Instamatic cameras. In addition, many professional level Ektar lenses from this era contain thorium. Perhaps the most famous radioactive lenses of all were the Kodak Aero-Ektars.

Curiously, in his book, A History of the Photographic Lens, Rudolf Kingslake (head of the Eastman Kodak lens design department 1937-1968), makes only a single passing comment on the possible use of thorium in Kodak lenses.

Kodak lenses tested radioactive (by John Hufnagel)

Kodak lenses reported elsewhere as radioactive

  • Kodak Aero-Ektars (various models)
  • Kodak Ektanon 50mm f/3.9 (Kodak Bantam RF camera)

Non-Kodak lenses reported as radioactive

Lenses with elements made of contaminated glass

Some lenses of the 1960s have elements made of glass sorts which include small traces of radioactive rare-earth elements. Sometimes this accidental radioactivity causes a significant yellowing of these lens elements. Some users of such lenses reported in camera blogs that they healed the yellowing by exposing these lenses to the ultraviolet light of the sun. The procedure needs several days of sunny weather to have a positive effect. Lens elements with such yellowing radioactive impurity are in the following lenses:

  • Minolta MC W. Rokkor-SI 1:2.5 28mm (early variant, before radioactive glass impurity could be banned)
  • Minolta MC Rokkor-PG 1:1.2 58mm (early variant, before radioactive glass impurity could be banned)

The healing of yellowing by sunlight is also reported for some lenses with thorium glass elements, for example for the Nikkor 35mm f/1.4 lens and the Super Takumar 50mm f/1.4 lens.

Links/Sources

  • Wirtenson, G R; White, R H, 1992 Effects of ionizing radiation on selected optical materials: An overview. Technical Report Lawrence Livermore National Lab., CA, USA. doi:10.2172/10178461
  • 2.0 2.1 Gerjan van Oosten, 2021, "The Definitive ASAHI PENTAX Collector's Guide 1952-1977". 2nd Edition ISBN978-90-9034415-7
  • Takumar 6x7 Field Guide blog and site by David Rounsevell and Gordon McLellan