By Patricia Daukantas
Is flash photography destroying the most hallowed documents of the United States? The National Archives says it may be doing so. And it got my scientific curiosity going.
Specifically, the National Archives, the agency responsible for all U.S. government documents, announced this week that, effective Feb. 24, it was banning all photography inside its exhibit hall in Washington, D.C. Flash photography has been banned for years, but too many visitors couldn’t be bothered to switch off the electronic flash on their automatic cameras, so now nobody will able to take pictures.
My initial reaction was to say, “Bah, this is just more homeland security.” But then I got to thinking about the science of all those teeny flashes of light.
According to the news reports and blogs, National Archives officials said that the documents on exhibit are still experiencing 50,000 photographic flashes per year, and the light and ultraviolet radiation from those flashes can cause the ink on the documents to fade.
Suppose that each flash lasts 1/1000 of a second, which seems to be the typical duration of the light impulse in a modern electronic camera flash unit. So 50,000 flashes × 0.001 s/flash = 50 seconds of exposure to this very bright light per year. (And, of course, the National Archives would like its documents to be readable for many, many years in the future.)
Looking at the (flash) light
Next question: How can we quantify the brightness of this light? How much light are the documents receiving during these 50 seconds of exposure per year?
Figuring out “how much light” is not so easy, in part because electronic flash units are not described (on Web sites, anyway) in units or terminology that connects to the basic equations we learned in physics class. I looked at the online specs for a couple of high-end standalone flash units from famous manufacturers and found that the output is given in terms of “guide number.” This guide number is the product of the distance from the flash to the photographic subject and the f-number of the camera’s aperture. (It’s like “The Inverse-Square Law for Dummies.”) So, if the flash has a guide number of 40 meters, you can get good lighting if the subject is 20 m away and you set the aperture at f/4, or if the subject is 10 m away and you set your lens to f/8.
Yeah, but … then you get into a lot of factors that are dependent on each camera’s optics, such as focal length. I don’t care about all that, nor do I want to worry what ISO speed (film or digital equivalent) these 50,000 tourists are using. I’m interested in the amount of light that falls on the documents, not the amount that reaches the camera. And I’m also interested in the spectral properties of the light that hits the documents.
What is a camera flash unit, anyway? Nowadays, it contains one or two glass tubes filled with xenon, and a high-voltage electrical discharge through the gas creates the arc that gives off the burst of light. Xenon is the gas of choice because at certain current densities, its emission looks more like a continuum and less like a discrete spectrum. Most flash units are designed to give off light with a color temperature of about 5,500 K, which resembles normal daylight – and the response of color photographic film.
Now, if you remember your Planck’s law and Wien’s law, you’ll recall that the black-body curve for T = 5,500 K has a bit of emission in the near-ultraviolet region. Roughly speaking, our Sun (T = 5,778 K) gives off 12 percent of its radiation below λ = 400 nm, so as a first (back-of-the-envelope) approximation, we could use this percentage for the electronic flash unit too.
OK, so we need the total amount of radiation given off by the flash unit. But manufacturers tend to leave any mention of energy, power, lumens, irradiance, luminosity, etc. out of their product specifications. I managed to find a couple of Web pages (here and here) that gave a rough idea of a cheap camera flashlamp’s energy output: on the order of a few joules.
Putting the flash data together
Next, we put all this information together for a back-of-the-envelope calculation, even though we need to make a few big assumptions too.
Let’s say that each hypothetical unit gives off 5 J (we might as well make it simple) per flash. Since the light burst is of such short duration, that’s 5,000 W of radiant flux. I’m also going to assume that the entire output of the flash is directed toward the subject of the photograph – although the flash tube is three-dimensional, it’s backed by a reflector – and that the flash is emitting from a 1-cm2 area (0.0001 m2). So, at the front surface of the flash unit – I’ll assume it’s a plastic lens 1 mm from the flash itself – you’ve got an irradiance is 5 × 107 W/m2.
The final wild card is the distance from the flash unit to the objects being photographed. The one time I went to the exhibit hall at the National Archives’ headquarters – some years before I started writing for OPN – the public couldn’t get too close to the Declaration of Independence and the U.S. Constitution; the distance was maybe 5 m (I’ll say for those convenient numbers again). The line of visitors also snaked past a display case that contained a number of interesting original documents and letters from U.S. history, so I’ll say the viewing distance to those papers is only 1 m.
So, thanks to our inverse-square law, the irradiance at the documents in the close-up display case is 50 W/m2 and the irradiance at the Declaration is 2 W/m2. If you assume that 12 percent of that irradiance is in the ultraviolet, then the nearby documents are getting hit with 6 W/m2 of UV and the Declaration gets 0.24 W/m2 of UV.
But wait! There’s glass and ink!
All this analysis fails to take into account two major factors: how well the glass display cases transmit UV light and how strongly light (of any wavelength) affects the inks, parchment and papers that constitute all these documents.
We know that the Declaration, the U.S. Constitution and the Bill of Rights are mounted, according to the National Archives, underneath 9.5-mm-thick “laminated, tempered float glass that includes an anti-reflective coating.” I don’t know how well that type of glass transmits UV-A and UV-B rays, but I do know that the original, signed Declaration now at the Archives was badly mistreated during the 19th century – including 35 years’ worth of exposure to sunlight.
Here’s where you, the reader, come in.
Please comment! I’d love to hear from you. I would especially appreciate hearing about any student projects or class discussions that resulted from this blog entry.
Double-check my calculations. Did I make any obvious mistakes?
Evaluate my assumptions about the properties of the flash unit and the light it emits. Are they reasonable?
Add in any knowledge you have about the transmission properties of glass and the effects of light on 18th-century ink and parchment.
Finally: Is the amount of light that the documents get from electronic camera flashes really enough to accelerate aging and fading? Or are those 50 seconds’ worth of bright light insignificant when compared with the dimmer room light that falls on the documents for thousands of hours annually?