The Aero-Ektars

Introduction

The Aero-Ektars are lenses for aerial reconnaissance photography made by Kodak. Most were made for War World II. For their time, the Aero-Ektars were very advanced technology with a new optical design and use of the latest optical cement, coatings and high-tech optical glasses. Today their most notable feature is that the special optical glasses used are radioactive.

I have been researching the Aero-Ektars. Unfortunately much of the information on the internet is wrong; regrettably too many people are guessing and their guesses are being accepted as fact. I have based my conclusions on patents and research articles from the 1940's and on laboratory gamma-ray spectral measurements. I am by profession a gamma-ray astrophysicist: further information on my qualifications may be found on my professional web page.

I have tried to base my conclusions either on "primary sources", documents nearly contemporaneous with the production of the Aero-Ektars and the special glasses, or on examination and laboratory measurements of the lenses. Documents that I have used include three U.S. Patents on the Aero-Ektar and the special glasses used in them, and three scientific journal articles that appeared shortly after the end of War World II. I have tried to state the evidence for my conclusions and to distinguish between conclusions directly reached from solid evidence and conclusions that incorporate judgments.

I own a 7" f2.5, a 12" f2.5 and a 24" f6.0 Aero-Ektar. The 7" (178 mm) f2.5 model is by far the most common. The 24" f6.0 has a different optical design from the others and the documents I have provide little information about it. The remainder of this web page focuses on the faster optics; however, measurements show the 24" f6.0 to be radioactive in a similar manner to the faster optics.

This web page will be revised and augmented as I have time and as I obtain additional information.

The cause of the radioactivity

By design, the Aero-Ektars incorporate glass lens elements that contain significant amounts of Thorium. That the cause of the radioactivity is Thorium is indisputable, both on the basis of the documents from the the World War II era and from gamma-ray spectral measurements that I have made. The Thorium-containing glasses were used because these glasses have a high refractive index with a low dispersion (variation of index with wavelength), a highly desirable combination.

Some of the decays of Thorium and its daughters result in daughter isotopes that are in excited states. These isotopes quickly transform to their ground or unexcited state by emitting one or more gamma-rays. Because of quantum mechanics, the energies of the excited states, and therefore of the gamma-rays emitted to transition from an excited state to the ground state, can only have certain values. The discrete photon energy values are specific to particular isotopes and serve as "fingerprints" to identify the isotopes. The fingerprint analogy isn't perfect: the gamma-ray energies are more certain proof of the identity of an isotope than a fingerprint is certain evidence of the identity of the person who left the fingerprint. The gamma-ray spectral measurements that I have made are inconvertible evidence that isotopes of Thorium and its daughters are present in the Aero-Ektars that I have measured. The most distinctive energy seen is that of the 2.6 million electron volt photons emitted by the Thorium-daughter Thallium-208, which decays into the stable isotope Lead-208.

All three Aero-Ektars that I have easily unscrew into two optical assemblies. This is probably to permit mounting the lenses onto the aerial reconnaissance cameras that they were designed for. For all three models that I own, the radioactive glass elements are in the rear group. For the f2.5 models, the rear optical assembly consists of four lens elements cemented into two groups: the radioactive lens elements are the two inner elements.

The location of the radioactive elements causes the rear of the lens to emit much more ionizing radiation than the front--the front elements block most of the radiation that is emitted in the forward direction. Even in the rear portion significant self-shielding is occurring: the two radioactive elements are surrounded by the metal barrel and the two outer non-radioactive glass elements. Disassembly of the rear optical assembly exposes a significantly higher radiation flux.

The rear half of every Aero-Ektar that I have seen is brownish, with a color somewhat like tea. I believe that the brown color is from radiation damage to the glass accumulated over the years. Ionizing radiation can damage glass, creating "color centers" which absorb some wavelengths of light, creating a color cast. A similar brown color seen in any lens would be highly suggestive of radioactivity content. The brown color can be missed if one doesn't know to look for it. One way to see the color is to view a uniformly illuminated white sheet of paper or a lightbox through the lens. The human visual system is good at adjusting for color shifts, so the best approach is to view the white paper or lightbox and look through the lens simultaneously, comparing the two views. The color is very clearly seen if you unscrew the lens into its front and rear assemblies and compare the view of the white paper seen through the front and rear optical assemblies.

Optical Properties

Even by today's standards, these lenses are exceptionally fast for the size negatives they were designed to cover. For example, the 7" f2.5 model was designed for a 5" X 5" negative. In comparison, current lenses intended for large-format 4" X 5" negatives typically have apertures of f5.6 or slower and only have excellent optical performance at f11 or even slower.

However, if you are thinking of using an Aero-Ektar for photography, you may be disappointed. Because of the radiation-induced browning, they no longer deliver the quantity of light that an f2.5 lens should. I have not yet tested their optical performance, but it probably doesn't reach the levels of current commercial large-format lenses. The resolution of film has significantly improved since the 1940's and so the expectations for lens resolution and sharpness have changed.

Thorium

Thorium is one of the three significantly-radioactive elements naturally present in non-trace amounts (the others are one isotope of Potassium and two of Uranium). The fact that it has survived from an origin before the creation of the solar system indicates that it must have a long half-life; the half-life has been measured to be 14 billion (thousand million) years. This very long half-life means that only a very minute fraction of Thorium atoms decay in any particular second, year or human lifetime, and that naturally occurring Thorium is comparatively weakly radioactive compared to many artificially produced radioactive isotopes. Thorium is naturally present around us, in rocks, the ground and in various building materials.

Thorium decays by many steps into an isotope of lead; the isotopes produced in the intermediate steps are referred to as "daughters" of Thorium. Thorium and its daughters emit almost every form of ionizing radiation emitted by radioactive isotopes. The isotopes in the decay chain transform into the next isotope in the chain by the emission of either alpha particles (Helium nuclei) or beta particles (electrons). Some of the daughter isotopes are created in excited states which decay to the ground state of the same isotope by the emission of one or more gamma-rays (high-energy photons). Finally, the atomic electrons typically rearrange themselves after the nucleus changes its atomic number, or position in the periodic table, and in the process frequently emit X-rays (medium high-energy photons).

Radiation Level

Shielding to block alpha and beta radiation takes relatively little material; most of the alpha and beta particles are absorbed by materials of the lenses. Shielding X-rays, and especially gamma-rays, requires thick shields. Blocking the gamma-rays emitted by the Aero-Ektars requires a layer of lead MUCH thicker than the foils used in the film shields sold to protect film from airport X-ray machines. Without evaluation of shielding by a trained person, the most reliable way to reduce exposure to radiation is to use distance and thus reduce the radiation level by the inverse square law. Of course, shielding material of any sort (the more the better!) in conjunction with distance will help. I store my Aero-Ektars in the far corner of a rarely used room. When you store an Aero-Ektar, realize that some of the gamma-rays will go through some walls and floors, depending on the mass of the wall or floor.

My conclusions on the intensity of radiation from the Aero-Ektars are PRELIMINARY. Because our laboratory was recently moved, my calculations have not been tested by measurements of the intensity. I do NOT guarantee the correctness of these calculations. My very-approximate calculations show that a few hours exposure close to an Aero-Ektar causes a smaller additional radiation dose than a trans-Atlantic plane flight. However, a one-year long close exposure would cause a significant radiation dose, one that would be several times higher than the naturally occurring dose. Clearly, you should not store Aero-Ektars under your bed!

While caution is in order, one should not exaggerate the dangers of the radiation from a few Aero-Ektars. Many people will take trans-Atlantic plane flights for pleasure, while others will experiment with or use Aero-Ektars for their pleasure.

A further indication that Aero-Ektars are reasonably safe (nothing is perfectly safe) is that their possession by ordinary persons, i.e., persons without a radiation license, is legal in the United States. Title 10, Chapter I, Part 40, Section 13 of the Code of Federal Regulations says in part:

Any person is exempt from the regulations in this part and from the requirements for a license set forth in section 62 of the Act to the extent that such person receives, possesses, uses, transfers or delivers source material in ......

(7) Thorium contained in finished optical lenses, provided that each lens does not contain more than 30 percent by weight of thorium; and that the exemption contained in this subparagraph shall not be deemed to authorize either:

(i) The shaping, grinding or polishing of such lens or manufacturing processes other than the assembly of such lens into optical systems and devices without any alteration of the lens; or

(ii) The receipt, possession, use, transfer, or [sic: the original is missing a word] of thorium contained in contact lenses, or in spectacles, or in eyepieces in binoculars or other optical instruments.

Two elements of the f2.5 Aero-Ektars are made of glass with 11% and 13% Thorium by weight, so this regulation clearly allows possession of Aero-Ektars.

The most basic principle of radiation safety is that radiation exposure should be lowered to the minimum practical level. Regardless of safety calculations, the level of radiation exposure should be further reduced if there is a practical means to do so. This means keeping people far away (many yards/meters) from Aero-Ektars except when one is being used or examined.

Of course, in their original war time use, the danger of the radiation from Aero-Ektars was utterly trivial compared to the more obvious risks that America's soldiers, sailors and aviators took in order to win the war. The substitution of night-time reconnaissance flights for daytime flights made possible by the fast aperture of the f2.5 Aero-Ektars undoubtedly saved American lives.

Myths

The following italicized statements that appear in several places on the internet are incorrect or misleading:

false: The Aero-Ektars are radioactive because of the Lanthanum content of some of the glasses. Gamma-ray spectral energies are more distinctive than a human fingerprint and the gamma-ray spectral measurements that I have made prove that the radiation originates with Thorium and its daughters. The documents from the World War II era very convincingly show that the inclusion of Thorium was intentional. Also, Lanthanum is barely radioactive: only 0.1% of naturally occurring Lanthanum is radioactive, and the radioactivity of even this portion is very low because of the extremely long half-life of 100 billion years.

false: Thorium is present in Aero-Ektars as a contaminant in the rare earth elements such as Lanthanum or Cerium which were intentionally included in the optical glass. The documents are clear that Thorium was intentionally used in the Aero-Ektars. Also, the measured amount of radioactivity is much too high to be caused by levels of Thorium that could be termed a contaminant.

false: Thorium decays to Lead by alpha and beta emission and these radiations are easily shielded and so the radiation is not dangerous. The statement that Thorium and its daughters decay via alpha and beta emissions is correct, but neglects the fact that many of the daughter isotopes are created in excited states. These excited isotopes quickly transition to their ground states by the emission of gamma-rays. The atomic electrons also readjust their states by the emission of X-rays. Blocking the X-rays and especially the gamma-rays require much more shielding than blocking alpha and beta particles.

false: Since the radioactivity is due to the presence of Thorium as a contaminant in other rare earth elements, various lens specimens have different levels of radioactivity, depending on the purity of the rare earths in the glass batch used in their construction. As explained above, the Thorium is not present as a contaminant. As an intentional and significant component of the glass, the quantity of Thorium probably varies little from batch to batch of the glass. It is possible that, without changing the lens name or informing the public, some radioactive lenses were redesigned to use non-radioactive glasses. This is quite possible with commercial lenses produced after War World II for amateur and professional photographers. However, every Aero-Ektar that I have seen has brown glass, so it seems likely that the Aero-Ektars were never redesigned to avoid the use of Thorium.

probably false: The brown color of the glass results from the chemically unstable nature of the glass and not because of the radioactivity. I cannot disprove this idea but inherent chemical instability seems less plausible than radiation-induced damage. The browning of glass from radiation damage is a well known effect, so it would not be surprising for the decades of self-induced radiation exposure to brown the radioactive glass of the Aero-Ektars. I don't know of any evidence for the idea that the Thorium-containing glass of the Aero-Ektars is chemically unstable.

Request for Information

If you own an Aero-Ektar, I would like to receive via email the data written around the front of the lens. This will help me determine how many types of Aero-Ektars were made, the years of production and the approximate numbers produced. For example, "f:2.5 7 in 178mm 5x5 EE11492". Please note whether the information is marked on the front rim of the lens or around the side of the lens, whether there is a colored dot and any other information written on the lens; there is no need to include the manufacturer or the place of manufacture unless these differ from "Kodak" and "Rochester, NY". Thanks to all who contribute!

BlueRule

This web page will be revised, so please visit again.

Any questions or comments? Then send email.

Copyright 2002 by Michael S. Briggs.

Version 0.4: revised 2002 January 16.


Home Home Page