Radioactive lenses

From Camera-wiki.org
Revision as of 16:06, 27 January 2023 by Titrisol (talk | contribs) (→‎Links: Oakland Ridge reference)
Jump to: navigation, search

Many lenses produced from the 1940s through the 1980s 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.

5098188400_0aa7824ac7_m.jpg
Kodak Bantam RF 1955
image by vincentnip (Image rights)

In 1945, Eastman Kodak Company, filed a patent for Glass formulation[1], which introduced thorium and lanthanum oxides in boric optical glass for optical properties. This application relates to glass having optical values in a range that is useful for the designing of optical instruments. Specifically, this application relates to such glass having an index of refraction for the D line (n) in the range between 1.65 and 1.68 and an Abbe value (v) between 52.5 and 57.0.

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.[2]

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.[3] 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. Studies carried out by the US Army show that glass attenuates alpha radiation and any residual will be absorbed in a surface layer of less than 100 micrometers. [4]

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)

49024200977_ca33c0219e_m.jpg
Kodak Aero Ektar f2.5 / 7"
image by SBA73 (Image rights)

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

50000265646_45e1d5fa83_m.jpg
Voigtländer Zoomar 36-82 f/2.8
bayonet and rear element showing yellowing
image by Michael Khan (Image rights)
2591235726_59f47b8020_m.jpg
SUPER-TAKUMAR 50mm f/1.4
image by Jeffk42 (Image rights)

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. 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)

Discoloration of lenses

Sometimes the radioactive glass elements show a significant yellowing, which not only changes the color of the picture but can cost 1/2 to 1 stop of light. Early research by the US military showed that glass exposed to radiation can be discolored in this way, and that exposure to bright light (fotons) will bring the glass back to its natural clear state. [2] Over the years, the users of discolored lenses have reported that exposure to sunlight, UV lamps, or bright LED lamps healed the yellowing. The sunlight procedure is carried out by placing the lens on a window sill and needs several days of sunny weather to have a positive effect. UV and LED lights might need more or less time depending on the intensity of the light.

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, Voigtlander Zoomar, and the Super Takumar 50mm f/1.4 lens.

Notes

  1. ↑ United States Patent 2466392A for Optical Glass by Paul F. De Paolis
  2. ↑ 2.0 2.1 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
  3. ↑ Wang J, Henningson V. 2013. An Analysis of Residual Radiation in Thoriated Camera Lenses. Department of PhysicsSchool of Engineering SciencesRoyal Institute of Technology (KTH)Stockholm, Sweden, 2013
  4. ↑ Robert C. McMillan & Steven A. Horne: Eye Exposure from Thoriated Optical Glass U.S. Army Mobility Equipment R&D Center. Date unknown
  5. ↑ 5.0 5.1 Gerjan van Oosten, 2021, "The Definitive ASAHI PENTAX Collector's Guide 1952-1977". 2nd Edition ISBN978-90-9034415-7
  6. ↑ Takumar 6x7 Field Guide blog and site by David Rounsevell and Gordon McLellan

Links

Retrieved from "http://camera-wiki.org/index.php?title=Radioactive_lenses&oldid=220821"