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Carrots and the First UV-VIS Spectrophotometer

World War II generated a lot of interesting propaganda and a lot of interesting hardware. In this post, we’ll see a mix.

From 1939-1945, Great Britain was dark at night as a result of wartime blackouts that were designed to make it difficult for the Germans. “Carrots provide a good remedy for blackout blindness, which afflicts many a Briton, on these wartime nights and have the additional merit of improving the complexion, the British have been told officially. The Ministry of Agriculture in London declares that carrots are the best source of Vitamin A, which is healthy for healthy eyes. They should be eaten raw, as cooking tends to destroy the vitamin. “If we included a sufficient quantity of carrots in our diet,” said the ministry, “we should overcome the early prevalent maladay of blackout blindness.” Carrots are termed the best vegetable for improving the complexion, by Gaylord Hanser, Hollywood beauty expert, according to the Ministry.” [1]

Food was scarce, but there were plenty of carrots and carrots were inexpensive. The British government wanted Britons to eat more carrots. They generated interesting recipes to help facilitate it…even carrot fudge.

Perhaps more hopefully, the British government wanted the Germans to think that the reason British fighters were succeeding against German aircraft at night was…..carrots. They did not want the Germans to know about its new airborne RADAR, which was responsible for thwarting the nighttime raids. It’s doubtful that the German bought the story.

But this note is not really about carrots. Leading up to World War II, the US military was confronted with a young male population that was suffering from a nutritional deficit caused by The Great Depression. There was a big push for more research into vitamins and nutrition. Vitamin A is important to good health and it received a lot of attention.

Before 1942, the standard procedure for measuring the concentration of Vitamin A in a food supplement/sample was:

  1. Feed the sample to rats for three or four weeks
  2. Measure the growth in the length of the rats’ tails
  3. Develop a model for tail bone growth as a function of Vitamin A concentration

Dr. Arnold O. Beckman had a better solution. He said, “In 1940, no one at National Technical Laboratories had any extensive experience in spectrophotometry. The fact was recognized, however, that the amplifier of the Beckman pH meter was well suited for use with vacuum-type phototubes. The company began a spectrophotometer development program in early 1940, and the responsibility for this program was assigned to H. H. Cary. Consulting assistance was sought from recognized optical experts, but World War II was under way and experts were hard to find. Roger Hayward, a professional architect and amateur scientist with some optic experience from his association with the Mount Wilson Observatory, provided a needed link to monochromator technology. His genius for quickly translating ideas into useful sketches was partially responsible for the extreme rapidity with which the DU spectrophotometer was developed. Douglas Marlow provided proficiency in mechanical design.

The Beckman DU at the time of product launch.

The first instrument designed was a glass Fery prism instrument, but its performance was not considered suitable. A quartz prism Littrow design with a tangent-bar drive followed and was designated the Model Β. Of the two quartz Model Β instruments produced, one was sold to the Chemistry Department of the University of California of Los Angeles in February 1941, and the other is in the company’s historical museum. This instrument utilized a tangent-bar mechanism which provided a substantially linear wavelength scale. Unfortunately, the scale was too compressed, particularly in the ultraviolet region, and was replaced by a Model C with its innovative scroll drive, which was used in all subsequent Beckman quartz prism monochromators. Of the three Model C instruments produced, California Institute of Technology, Vita Foods Co., and Riverside Experiment Station each purchased one. The Caltech instrument was later returned to the company for its museum.” [2]

After Beckman’s DU, the process for Vitamin A concentration measurements became:

  1. Dissolve the nutritional sample in water
  2. Place it in the DU and measure absorption

Rats were spared, researchers were spared the three weeks for tail growth, and the military improved the nutrition of warfighters. Yes, carrots were tested in a DU!

It’s important to note that visible and IR spectrophotometers were already present in the marketplace by the time of the DU’s introduction. However, Dr. Beckman realized that biological samples required UV sources and optical systems. The DU was the first commercial UV-VIS device. This was a very big deal.

I’m hunting for a DU to inspect and perhaps refurbish.

[1] “Carrots Remedy for Blindness,” The Daily Colonist, Victoria BC, FEB 9, 1941.

[2] Beckman, A.O., Gallaway, W. S., Kaye, W., and Ulrich, W. F. “History of Spectrophotometry at Beckman Instruments, Inc.”. Analytical Chemistry, 49, pp 280A-300A (1977).

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Rittenhouse Beats Fraunhofer!

“By pursuing these experiments it is probable that new and interesting discoveries may be made respecting the properties of this wonderful substance, light….but want of leisure obliges me to quit the subject for the present… ” David Rittenhouse in a letter to Francis Hopkinson (1786).

“Want of leisure?” That is certainly understandable! Rittenhouse was the Treasurer of Pennsylvania and had other duties in the newly-formed USA. He would become the first Director of the US Mint in 1792. Hopkinson, a signer of the Declaration of Independence, caused Rittenhouse’s temporary loss of leisure when he sent him the initial letter (March 16, 1785) describing what he saw (diffraction) while looking through a silk handkerchief. But should Rittenhouse have taken the time off from his work on diffraction gratings?

David Rittenhouse

“It is difficult to point to another single device that has brought more
important experimental information to every field of science than the
diffraction grating. The physicist, the astronomer, the chemist, the
biologist, the metallurgist, all use it as a routine tool of unsurpassed
accuracy and precision, as a detector of atomic species to determine the characteristics of heavenly bodies and the presence of atmospheres in the planets, to study the structures of molecules and atoms, and to obtain a thousand and one items of information without which modern science would be greatly handicapped.” (J. Strong, J. Opt. Soc. Am. 50 (1148-1152), quoting G. R. Harrison), from Diffraction Grating Handbook, 5th edition, Christopher Palmer, Thermo RGL.

Big Diffraction Gratings at LLNL
NASA IRIS Telescope with Imaging Spectrograph, with grating, first light July 2013

It’s tough to say who discovered/invented the diffraction grating. Isaac Newton wrote in Opticks about “scratches made in polished plates of glass.” A contemporary of Newton, James Gregory, studied diffraction patterns produced by bird feathers. However, it looks like Rittenhouse in 1785/86 was the first to build and use a diffraction grating to make spectral measurements. Rittenhouse said, “…I made a square of parallel hairs about half an inch each way. And to have them nearly parallel and equidistant, I got a watchmaker to cut a very fine screw on two pieces of small brass wire. In the threads of these screws, 106 of which made one inch, the hairs were laid 50 or 60 in number…” That was one year before Fraunhofer’s birth and three decades before his experiments. But Fraunhofer did not take leisure; he finished the job!

Joseph von Fraunhofer

For an excellent summary of Rittenhouse’s experiments and results, see I. D. Bagbaya, “On the History of the Diffraction Grating,” Soviet Physics Uspekhi Vol. 15 No. 5, p. 660-661, March-April 1973.

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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.