March 24, 2020 was Edmond Becquerel’s 200th birthday. He was the first person ever to make a color photograph, in 1848 (above), although no one, including Becquerel himself, had known exactly why it had worked. It’s been debated for many years, with no satisfactory explanation.
But a few days ago, we found out in the esteemed chemical journal Angewandte Chemie that a research team led by Dr. Victor de Seauve and Dr. Marie‐Angélique Languille, on behalf of the Muséum national d'Histoire naturelle in Paris, has used some modern spectroscopic techniques to finally figure out how he did it.
When Becquerel was 19, he invented the photovoltaic (solar) cell, demonstrating for the first time that light could make an electric current flow. We wouldn’t have solar power today if it weren’t for him. I’m quite sure he is better known for that than for the first color photograph. In fact, today the European Commission awards the Becquerel Prize for advances in photovoltaics.
In 1848, at the age of 28, Becquerel made the images you see above in the main diary picture (among others, most of which have been lost). They show the sun’s visible spectrum, or sunlight shone through a prism, projected onto a surface, and made permanent. Well, sort of permanent. You can’t display his photos out in the open because exposure to light will gradually fade them. Nevertheless, they are permanent enough to be considered the world’s first color photographs.
I thought for sure there would have been a Google Doodle for Becquerel’s 200th, but on that day Google celebrated bánh mì. Well, OK, I admit bánh mì is pretty good and … I … wouldn’t mind some right now…
Hey, is that fresh cilantro they’re putting on there? Mmmmm, Vietnamese takeout….. must get when pandemic is over….
Errr, anyway… Another angle, where the hues show up a little better:
This is what Becquerel was capturing, essentially sunlight through a prism:
Just for completeness (and possibly a nifty wallpaper for whatever screen you happen to be using), here is the entire solar spectrum in all its glory:
If we want to credit the inventor of the type of color photography that survives today, we should look to the brilliant James Clerk Maxwell, who took this truly permanent color photograph of a tartan ribbon in 1861 with the help of expert photographer Thomas Sutton:
Maxwell was interested in color theory, or how we perceive all the colors we do. He wanted to show that the Young-Helmholtz theory (which says our eyes have receptors that individually prefer red, green, and blue light) could be applied to making an image, solidifying the basis for that line of thought.
So he took three black-and-white slide photographs of a tartan ribbon, but through a red, a green, and a blue filter. Then he projected his images with red, green, and blue light, overlapping each other, up onto a screen to show that he could reconstruct the colors we see using combinations of just these three colors. Color prints have also been made using Maxwell’s original slides. While you’re quarantining, you can see what those looked like in a display case on a virtual tour of Maxwell’s birthplace (go to Exhibition Room One, just off the main stairwell).
There are some truly wonderful galleries of early color photographs made this way that are amazing to see, featuring the work of:
Sarah Angelina Acland
Louis Ducos du Hauron
Sergey Prokudin-Gorsky
Fortunately the Library of Congress has preserved much of the Prokudin-Gorsky collection, an eye-opening survey of pre-Revolutionary Russia, and we can see the actual three exposures through blue, green, and red filters:
Then the composite, this one made digitally from the glass negatives. If you were printing it, you’d get the most brilliant red, green, and blue pigments you could find. You want that jazzy painted-on Technicolor look. I like that the edges show each color individually from the parts where the slides didn’t quite overlap, so you can see the component colors that were used:
Here is a different implementation, from the same slides. This one de-emphasizes the red. The concrete looks more real, but the flowers in the pattern have lost some of their brilliance. Was that the right thing to do? What do you think?
So the three-color method is necessarily an interpretation of reality that can be tuned as you like.
But what Becquerel did is not subject to any interpretation. It’s a direct production of color, made only by the light that strikes it. No filters needed, and only a single (albeit long) exposure.
Becquerel used silver halide salt for the surface of his photographic plate. But so had Daguerre and all the other early black-and-white photographers. So, why did Becquerel get color when the others didn’t?
“Silver halide” means silver iodide, silver bromide, or silver chloride, which all act very similarly. All of these salts appear white, but they turn gray when light strikes them, because some silver metal forms.
Take silver chloride (which Becquerel used) as an example. In the salt form, all the silver atoms have donated one electron to a chlorine atom:
Atoms like to have their outermost shells full of electrons. Silver has only one electron out there, while chlorine needs one more to fill its outer shell, so these two make a happy pair. Let’s use shorthand now to make this simpler:
When light comes along, its energy can knock an electron off of a chlorine atom, and then two chlorine atoms that have been impacted this way can pair up to form chlorine gas (Cl2):
Of course, chlorine gas will float away, so we’re left with extra metallic silver, which appears gray. That’s how light usually works with silver halide films. (It’s basically how your photochromic lenses turn dark in sunlight, too.)
Daguerre had made silver iodide salt by wafting iodine gas in the vicinity of his silver plate, but Becquerel did it quite differently. He ran a current through a solution of hydrochloric acid (HCl), with one of the electrodes being his silver plate. That forced some chlorine atoms into the silver atoms to give a thin layer of silver chloride.
But Becquerel’s particular method apparently also did something else. It’s pointed out by de Seauve and Languille that you have to do it just right to get it to work; “details … are critical.”
After a very thorough analysis of colored plates produced by Becquerel’s method, they showed that the colors don’t arise because any pigments form, or because any tiny structures produce color by interference patterns, like a butterfly’s wing can do.
Rather, something about his treatment gives rise to tiny silver nanoparticles on the film, dispersed around within the usual silver chloride:
These nanoparticles are just the right size to absorb visible light by an odd property called surface plasmon resonance. The wavelength of light they absorb depends pretty sensitively on their size and orientation, and as they absorb the energy from that light, electrons get excited and start moving around and reacting with things, eventually altering the particles’ size and shape so that they can’t absorb light at that wavelength anymore.
So — if we expose an area of Becquerel’s film with, say red light, it absorbs all the red it can, but eventually the particles that are the right size to absorb red light get altered and can’t absorb red anymore. So going forward, this little area’s ability to absorb red light is lost. Now, when you view this area under white light, it absorbs all the other colors much better than it absorbs red, and when the light is reflected back to you, it appears red.
I always like to point out that these early discoveries that might seem obscure are very much with us today. Becquerel’s solar-cell concept obviously is, but this nanoparticle absorption thing? You bet…
There are whole areas like plasmonic photocatalysis and biosensors that take advantage of the phenomenon Becquerel fortuitously revealed. But I’ll just pick one fun example: zapping bacteria! Here’s E. coli on a titanium dioxide surface intentionally laced with silver nanoparticles, before and after exposure to light:
It’s very fitting that researchers at the Muséum national d'Histoire naturelle are the ones who solved the mystery of Becquerel’s achievement. It must have been a matter of pride to the institution, because its chair of applied physics was once … Edmond Becquerel. I think he’d be pleased.