January 27: Exposure

27January 27: Today I’ve been making copies of artworks for artists, a process requiring accuracy in reproducing colours and tones of the original and rendering them with calibration that will enable a faithful reproduction by a printer who has not seen the original to match it.

Without electronic flash and a reliable light meter, the job would be much more difficult, especially on large format transparency material which is what I used before the advent of digital imaging.

It just happens, as is so often the case in writing this blog, that I have found two inventors connected to this date who contributed those two essentials of photography.


Just last year in 2016 on January 27, German inventor Artur Fischer died in Waldachtal. To him we owe the invention of flash synchronisation which he patented in 1949. Previously, it would be necessary to open the shutter before manually firing the flash, which restricted the use of flash only to dark conditions.

Alfred Watkins Collage of black and white photographs, published as frontispiece of Alfred Watkins, Early British Trackways, 1921

Born this day in 1855 was the British eccentric Alfred Watkins, beekeeper and discoverer of ‘ley lines‘ (straight lines supposedly connecting ancient monuments), who gave us what is generally recognised as the first light meter, which he patented in 1890.

Watkins Meter Company photographic light meter, made in Hereford, England from 1890 patent.

Pictured above, it worked by exposing a sliver of photography ‘printing-out’ paper, so named because it is coated with an emulsion that requires no development and can make prints outside the darkroom. As the light darkened the paper, the photographer would watch and time it until it matched the tone of the grey circle surrounding the slit. Instructions with the meter noted that “The standard for the actinometer is that it darkens to its standard tint in best light (that is unclouded mid-day summer sunshine in England) in two seconds.” Then, after determining, on a chart, the sensitivity (now called ISO) of the film being used, the camera operator rotated the dial to match that number, then read off the shutter and aperture combinations given and selected one with which to take their picture. Simple but effective!


Marietta Blau at the Institute for Radium Research in Vienna, circa 1925

However, another, much more important inventor died on this date in 1970, in poverty and alone, naming only her concierge as her ‘next of kin’. She was Austrian physicist Marietta Blau, born in 1894.

The investigation of cosmic rays opened up the field of particle physics and it was photographic emulsion that played a crucial role in the progress of atomic physics.

It was Blau who discovered that film could record the passage of high-energy nuclear particles and events, including reactions induced by cosmic radiation.

Marietta Blau: Cosmic decays caught in film emulsion

At first sight, a nuclear emulsion plate seems to be as simple to use as Watkins Bee Meter. It is a photographic plate with a particularly thick emulsion layer and with a very uniform grain size. After exposing and developing the plate, single particle tracks can be observed and measured using a microscope. The emulsions are designed to trap the tracks of passing charged entities. Being able to see and measure nuclei, protons, electrons and other objects launched the field of particle physics. Though simple in concept, in scientific practice, making the film sensitive to minimally ionizing particles, and learning how to store, process, dry and ultimately analyze the spreading maze of tracks on film was a problem that took years and the efforts of many to resolve.

What is remarkable, almost difficult to conceive, about this nuclear emulsion technique is that the dimensions of the particles themselves are at the atomic level and what we see on the plates is their effect on the emulsion, not, of course, the particles themselves. There was very low chance of a successful observation due to the constraint that the nuclear event might come from in any direction in space, but had to coincide with the thin emulsion plane in order to register. From the 1950s cloud chambers (invented in 1913; there is much overlap in the technologies), themselves  superseded by other technologies including bubble chambers, spark chambers, drift chambers and silicon detectors greatly increased the sensitive volume and gave much greater chance of success.

Blau was among the first women to study physics at the University of Vienna. In 1923 she joined the Radium Institute in Vienna where, surprisingly, she and other workers, most were women, worked for no money. Between 1923 and 1937, Blau’s contributions were essential in every aspect of emulsion development and use. In 1925 she successfully distinguished the tracks of alpha particles, fast protons, and background events in commercial emulsions. By 1927 she could gauge  proton energies by measuring the distances between the exposed photographic grains in the tracks they left. Then in order to record the long tracks of fast protons more accurately, she worked with British manufacturer Ilford to thicken the emulsion on its commercial film. She found that she could employ grain size, latent image retention, and  development conditions to trace  alpha-particle and fast-proton tracks.

Tracks left by high-speed protons on a sheet of photographic film are shown at left. The dotted horizontal lines were made by protons. The “star” was made when an atom disintegrated in the photographic emulsion. (Brookhaven Lab photo)

Beginning in 1932 Blau used hydrogen-rich emulsions to determined neutron energies by measuring the tracks of recoiling protons. In 1936 she had the idea of exposing stacks of photographic plates for several months at an elevation of 2300 metres to study cosmic radiation.

She and her assistants expected to find the tracks of extraterrestrial protons and neutrons as they traversed the stacked plates, but to their surprise they also discovered several starburst patterns.  These could only have been formed by cosmic particles explosively disintegrating heavy nuclei in the emulsion, a discovery that, in 1937, created a sensation among nuclear and cosmic-ray physicists worldwide. Nuclear emulsions had proven themselves in reliably recording and quantifying rare high-energy nuclear events, thus paving the way for further research in particle physics.

Passport photo of Marietta Blau issued in Mexico c.1944.

Marietta Blau, who was Jewish, fled her homeland following Hitler’s Anschluss in March 1938 because of her Jewish descent. It caused a severe break in her scientific career. She first went to Oslo. Then, through Albert Einstein’s intercession, she obtained a teaching position at the Polytechnical Institute in Mexico City. But since conditions in Mexico made research extremely difficult for her, she seized an opportunity to move to the United States in 1944. She taught her nuclear emulsion method to Cecil Powell, who took it over and transformed it during the 1940s into a cottage industry, with female “scanners” and an international team of physicists and chemists.

From Powell’s laboratory in Bristol, England, the method migrated, with Blau, to American accelerator centers at Berkeley and Brookhaven. Emulsions were superseded by the industrial‐scale bubble chambers of the 1950s and 1960s, though Blau’s last PhD student worked from 1960 to 1964 analysing an experiment done at CERN in which emulsions were exposed to a beam of protons. This was one of a large number of experiments at CERN at this time that used emulsions.

Marietta Blau died from illness that may have been aggravated by exposure to the radioactive materials she handled (though she was a heavy smoker), and was almost forgotten as a pioneer in the field of nuclear science. Though the great Erwin Schrödinger himself twice nominated Blau for a Nobel Prize, she remained amongst numbers of women (and some men) in science who have been overlooked, ignored, plagiarised or discriminated against, just as was Rosalind Franklin who was responsible for detecting the the double-helix structure of DNA with X-Ray crystallography, a technique which also used photographic emulsions and their reaction to invisible radiation.

Rosalind Franklin (May 1952) an X-ray photograph of B-DNA (called Photo 51)

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