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Photoelectric Effect at War

The 4.5-inch M-8 rocket of World War II traveled at about 1,000 ft/s near its terminus. A traditional fuze on board the rocket would be set to go off at a predetermined time during the trajectory. Given the rate of engagement, fixed-time fuzes would not perform well. A similar problem existed with artillery shells that used contact fuzes. If the shells could be exploded in air above their targets (or rockets exploded when they got close to targets), they would be much more effective than simply ramming the shell into the ground.

The fuze problem was obvious and engineers around the world were busy trying to solve it.(1) British and American radio engineers had designed an RF proximity fuze, which was based on CW Doppler radar. The fuze included a miniaturized vacuum-tube radio transmitter, receiver, antenna, trigger, and power source. As an armed rocket or artillery shell approached a target, the reflected radio signal from the target increased and interfered with the transmitted signal to produce a low-frequency beat signal, which was then filtered and thresholded. If the amplitude of this beat reached a predetermined value, then a target is said to be within range and a thyratron trigger was fired to initiate the explosive charge. The RF proximity fuze was a closely held Allied secret. The fuzes included a self-destruct sequence to help keep unexploded ordnance from falling into enemy hands. It has been said that the atomic bomb ended the war, but radar and the proximity fuze won the war.

RF CW Doppler VT (“Variable Time”) Fuze

While it was decided to put most all of the effort behind the RF proximity fuze in October 1943, other fuze technologies were developed (e.g. electrostatic, acoustic). There were many more radio engineers around than optical engineers, however, there was a significant effort directed towards the development of a photoelectric optical proximity fuze.

“Photoelectric fuzes were designed to be used against airborne targets. The optical system of the T-4 fuze was designed to see a ring of sky about 5 degrees wide and 20 to 25 degrees forward of the equatorial plane of the projectile. This width was about equal to the minimum angular width of the target at the lethal range of the shell, and the direction was determined by the expected fragmentation cone of the M-8 rocket. The optical system consisted of a toroidal lens set in the outer case of the fuze and a ring slit surrounding a photocell (936 phototube) at the axis of the fuze.

The lenses were made of Lucite or Plexiglas and formed either by machining a plate of Lucite to the required shape or by molding the plastic and machining only the optical surfaces.

Since the smaller radius of curvature of the toroid was small compared with the larger radius, the focal properties of the toroid were approximately those of a cylindrical plano-convex lens. The image of a distant point of light was a line in the case of a cylindrical lens, and for the toroid it was roughly the arc of a circle of radius equal to the larger radius of the toroid.

To find the radius of curvature, it was convenient first to compute the focal length of a lens of unit radius and of the proper relative width and thickness for refractive index 1.49. The required radius of curvature was then the desired focal length divided by the focal length for unit radius. The lens was made as wide as possible without introducing serious aberrations. This width was approximately 1.2 times the radius of curvature. The optimum focal length of such a lens was less than that of a narrow lens. For a narrow lens of unit radius and of thickness equal to the radius, the unit focal length was 2.37 cm, whereas for the wide lens it was 2.20 cm.

The slit, which was placed at the principal focus of the lens, was made in a number of ways. One method was to bring the light to a focus at the surface of the lens block or the photocell wall. The lens surface or photocell wall was then painted black and the paint cut away to form the slit. An alternate method made use of opaque sleeves placed over opposite ends of the photocell and so spaced as to form a slit.

The position of the slit along the axis of the fuze controlled the look-forward angle of the fuze, and the slit width controlled the angular width of the ring of sky seen by the fuze.

The earlier models of the photoelectric fuze utilized photocells with conical cathodes, whereas the later models used photocells with flat cathodes. The flat cathode enabled light from the area seen by the fuze to spread evenly over the entire cathode surface. This smoothed out inequalities in the emission from various parts of the cathode and provided a more uniform response to light from various directions.”(1)

There were problems with the optical fuze; sunblinding and sunfiring were significant. Solutions existed, including the development of active optical sensors and the dual-channel arrangement shown in the patent above, but the RF proximity fuze won the war-time race.

Photoelectric proximity fuze. Note the plastic toroid near the tip.

Millions of proximity fuzes were made for the war. “Manufacture of the battery-powered fuzes (both radio and photoelectric) was started in the latter part of 1942 by Westinghouse Electric and Manufacturing Company, Philco Radio and Television Corporation, General Electric Company, Emerson Radio and Phonograph Corporation, Julien P. Friez and Sons, Western Electric Company, and Rudolph Wurlitzer Company….a later generator-powered type were produced by Westinghouse, Emerson, Philco, General Electric, and Zenith Radio Corporation. (2)

As an aside, there were related articles appearing in consumer magazines before the war. E&M was new and attracted lots of attention. Here is short note in Popular Mechanics, August 1936.(3)

Phototubes were used in airplane locators, rapatronic cameras, supersonic rocket fuzes, and talking pictures. They were versatile devices.

References

1. For some background on proximity fuzes and optical proximity fuzes specifically, see “Summary, Photoelectric Fuzes and Miscellaneous Projects,” NDRC, 1946.

2. H. C. Thomson and Lida Mayo, “The Ordnance Department: Procurement and Supply,” p. 123-124, Library of Congress 60-60004, 1960.

3. Staff, “Twin Eyes Scan Skies for Planes,” Popular Mechanics, p. 167 August 1936.

4. F. A. Zupa, “The Optical Proximity Fuze,” Bell Laboratories Record, February 1947.