If flash bulbs were still in use your argument would have some validity. It’s the year 2003 and if your sole source of income were from the sale of flashbulbs you would starve to death. If ultraviolet is such a threat then why do I see skylights and windows in museums? I find it difficult to believe that those who come up with these rules have ever passed a science class.
Other than that I completely agree.
Welcome to the SDMB, Newtons_Lawyer. Link to article referred:
Why isn’t flash photography permitted in museums?
A link to the column in question is helpful. Why isn’t flash photography permitted in museums?
In fact, modern electronic xenon photoflashes emit more UV than the old magnesium-oxygen flashbulbs did, FYI.
Oddly enough, flash photography seemed to be just fine in the Louvre this summer.
Surprised me, to say the least.
The skylights and windows in recently built museums usually have special ultraviolet-blocking glass. Older museums that have been renovated in recent decades often have the original glass replaced or a UV-filtering film applied over the existing glass. Even ordinary window glass blocks a large part of the ultraviolet. Cecil Adams on Can you tan through glass?
HEATING OF THE PAINTING FROM FLASHBULBS?
Ok, let’s do the math.
Let’s assume a few not too far off numbers:
The ambient light in a museum: 1/10th of full sunlight.
Number of flashes per second: Let’s assume a very popular painting: say 1 flash per second.
Intensity of flash: Let’s be generous and assume full sunlight.
Except electronic flash puts out much less infrared than the sun, about 1/5th as much. So while the flash brightness might seem as bright as sunlight, the actual energy transferred is much much less.
That gives us an average flash level of .00025 * 1/5 of full sunlight,
or expressed another way, .0005 of ambient sunlight, that’s five hundreths of one percent of one percent.
It’s frowned upon http://www.louvre.fr/anglais/infos/securite.htm (see article 26).
Besides, given the characteristics of the typical compact-camera flash, you’ll get a better shot with the flash off. Use high-speed film and keep the camera steady (a monopod helps, but isn’t strictly necessary).
<< That gives us an average flash level of .00025 * 1/5 of full sunlight,
or expressed another way, .0005 of ambient sunlight, that’s five hundreths of one percent of one percent. >>
Multiplied by, say 50 visitors a day, that’s .025, times let’s say 300 days per year that the museum is open, we’re at 7.5, times ten years and we’re at 75 times the effect of ambient sunlight.
And this is taking your assumptions, and ignoring Q.E.D.'s statement that electronic flashes have more UV, not less, than the old flashes.
<< That gives us an average flash level of .00025 * 1/5 of full sunlight,
or expressed another way, .0005 of ambient sunlight, that’s five hundreths of one percent of one percent. >>
Multiplied by, say 50 visitors a day,…
No no no, no need to multiply any further,
I’m already assuming one flash per second during sunlight hours.
<< No no no, no need to multiply any further,
I’m already assuming one flash per second during sunlight hours. >>
Then I don’t follow your calculations at all. Where does that magical .00025 come from?
If you are assuming one flash per second, that’s roughly 200,000 flashes per day (assuming the museum is open ten hours). If a flash is 1/5 as powerful as sunlight for the same time period, and if each flash takes 1/10th of a second, that’s the equivalent of an additional 2000 seconds or about 5 or 6 minutes of extra sunlight each day. Multiply that by six days a week, times 50 weeks a year, and it still starts to add up to LOTS of extra sunlight.
And, let’s remember that most museums do NOT want sunlight on the paintings, either.
Rather than playing games with mathematics, I have a better idea. Take a brightly colored fabric. Tear it in half. Stick one half in a window, where it gets direct sunlight for a few hours a day. Put the other elsewhere in the room, where it does not get direct sunlight but does get some reflected light. Leave them there for six months. Then compare the two pieces, and report to us whether you thing additional sunlight is harmful to colors.
In short: you can do all the fancy math you want with numbers and assumptions that you’ve made up. The people who work with this stuff have seen the impact of additional light on fabrics, paintings, and so forth, and they don’t want any more light on their pieces than they can possibly get away with.
Ok, I shouldnt have skipped the intermediate steps.
Let’s go thru this slowly:
If :
we assume one flash per second,
at the same brightness as full sunlight,
with a typical electronic flash duration, of 0.25 milli (thousandth) of a second:
That’s a blast of full sunlight for 0.00025 seconds, every second.
Another way of looking at it, taking the reciprocal of that number,
we see we are adding 1 part in 4000 of full sunlight.
If we further assume the painting is in a somewhat dim gallery,
getting only 1/10th of full sunlight, then we are adding 1 part in 400, or 0.25%.
Now WHAT we are adding is not quite like sunlight-- a typical xenon flashtube puts out, by comparison, very little (about 20%) infrared, so there’s not going to be much heating of the painting from the flash. The flashtube puts out more blue rays than sunlight, but since this is undesireable, most cameras have a color correcting filter over the flash to balance out the colors a bit better. So the amount of harmful higher-energy rays is also reduced
One might conclude, fromt he above facts, that this amount of added light is going to be insignificant.
>Rather than playing games with mathematics, I have a better idea. Take a brightly colored fabric. Tear it in half. Stick one half in a window, where it gets direct sunlight for a few hours a day. Put the other elsewhere in the room, where it does not get direct sunlight but does get some reflected light. Leave them there for six months. Then compare the two pieces, and report to us whether you thing additional sunlight is harmful to colors.
???
That was nowhere near the question under discussion. It’s well known that light bleaches pigments. That was never in doubt.
I just showed, by rough quantitative calculation, that the INCREASED amount of light hitting the canvas is negligible.
I suspect the old restriction on flash photos was more related to the tendency of the old flashbulbs to (rarely) burst, or to be inappropriately discarded. Nobody wanted to put a hot, used flashbulb in their pocket, so they tended to get “forgotten” on floors, benches, windowsills, etc.
Regards,
grg88
Ok, I read it. It’s all your basic Liberal-Arts person babble-- all soft arm-waving analysis, not a smidgen of a hard verifiable fact in there. Pls let me know where in ther there’s even a mention of the basic measures relating to heat or light transfer: watts, candelas, Joules, Angstroms, calories…
Without a few of these basic numbers you can’t begin to estiamte what’s happening to the artwork.
What is hard and verifiable is that one flash per second isnt going to add more than 0.25% of incident light. Once one has this figure in mind, one can start comparing that with the other variables, such as incident light, humidity, temperature, smoke, etc… in order to get a holistic picture of the challenges to the art.
Particularly laughable is the business about “heating of the artwork”. As I noted before, a xenon flashtube is quite poor in the infrared emissions. I suspect a full analysis of the heating from flashbulbs is going to be down in the thousandths of a degree. Meanwhile each person radiates about 150 watts, continuously; and considerably more if Tyra Banks is in the room.
If they really want to protect the art from heat damage, they should turn up the AC and turn down the supermodels.
Not to be a scientific snob about this, but the answers from the museum folks are just what one would expect from people that care deeply about the artworks, but havent the beginning of a clue as to how to estimate the magnitude of what’s going on.
I suspect the flash restrictions are more aimed at getting people to buy the expensive set of photos in the museum gift shop,
or perhaps more charitably, in order to create a more sedate atmosphere, without all the annoying flashes.
I’m with you, grg88, for your calculations seem to indicate a negligible effect on art work, even at one flash per second every hour, every day, which by itself, seems unlikely to happen.
So either art museum directors are ignorant of simple physics, or they are taking a “Pascal’s Wager” kind of attitude. We have established a negligible effect but not a zero effect. Museums know that[ul][]Some light is damaging, and []flash is not necessary for patrons to enjoy or even take photos (they can always use a tripod and a time exposure).[/ul]So why take the slightest chance?
That’s not a somewhat dim gallery. Artificial lighting is a heck of a lot dimmer than sunlight, and the only reason most folks don’t notice is that our eyes adapt so well. I’ll see if I can dig up numbers and cites later.
I still agree with you, by the way, that flashes probably add a negligible power to the amount from ambient lighting and from human bodies; I just don’t agree that it’s negligible to the same order of magnitude. And there’s also the issue of UV, and flashes might add an amount of UV comparable to other sources (many rooms in the museum don’t even have windows, and normal incandescent lights only produce a very small amount of UV).
Weird… they say it’s a bad idea, but then didn’t post any obvious signs(at least anywhere I went) saying not to do it.
Since Straight Dope Staff Veg hasn’t been real active in the last few years, are there any volunteers to investigate further? Speculation or assumptions about how strong flashes are or how long they last or whatever are, obviously, merely speculation.
If there’s a volunteer who would like to do some in-depth research, please email me.
BTW, please note that Veg answered the question absolutely correctly. He’s answered the question of “Why isn’t flash photography allowed in museums,” by telling us what the museum experts say. If they said it wasn’t allowed because the pink elephants didn’t like it, that would be the reason. The non-existence of pink elephants would be irrelevant (no puns permitted.)
The follow-up question that is being proposed is: are the museum experts correct in their statements? That’s a different question.
And please note that no amount of investigation is likely to change their policies. Even if you conclusively prove that zillions of flashes per second does no harm, you probably won’t have changed their policies or their minds. (Kind of like the “why is marijuana still illegal” question.)
Ok, here’s a detailed analysis, with references.
First we need to figure out how bright the lighting in a museum is.
I couldnt find a reference for this, but here’s a table that probably brackets a museum’s lighting conduitions:
from: http://ergo.human.cornell.edu/studentdownloads/DEA350pdfs/light.pdf
Typical field
Condition (cd m-2)
30 subdued indoor lighting
60 display only workplaces
120 typical office
240 bright indoor office
480 very bright, precision indoor
960 usual outdoors
1920 bright afternoon
I’d estimate your typical museum might fall between “subdued indoor lighting” and “display only workplaces”
(whatever that might be). So let’s estimate about 40 candelas per square meter for museum lighting.
We’ll also take note of the “usual outdoors” value of 960, let’s round it up to 1000 candelas/sq meter.
Now we need to figure out how bright a typical flash is. My bright idea was to
go to Kodak and see how their camera aperture settings compare between flash and bright sunlight.
From: http://wwwru.kodak.com/global/en/consumer/products/techInfo/e7006/e7006.shtml#daylight%20exposure:
The exposure guide for good-old Plus-X film (ASA 125 speed)
Daylight
Use the exposures in the table below for frontlighted subjects from 2 hours after sunrise to 2 hours before sunset.
Lighting Conditions Shutter Speed (Second) Lens Opening
Bright or Hazy Sun on Light Sand or Snow 1/125 f/22
Bright or Hazy Sun (Distinct Shadows) 1/125 f/16*
Weak, Hazy Sun (Soft Shadows) 1/125 f/11
Cloudy Bright (No Shadows) 1/125 f/8
Heavy Overcast or Open Shade** 1/125 f/5.6
*Use f/8 at 1/125 for backlighted close-up subjects.
**Subject shaded from the sun but lighted by a large area of clear sky.
Electronic Flash
Use the guide numbers in the table below as a starting point for your equipment.
To determine the lens opening, divide the guide number by the flash-to-subject distance.
Unit Output BCPS* Guide Number
For Distances in Feet For Distances in Metres
350 45 14
500 55 17
700 65 20
1000 80 24
1400 95 29
2000 110 33
2800 130 40
4000 160 50
5600 190 60
8000 220 65
*BCPS=beam candlepower seconds.
Lots of numbers there but the only most relevant ones are for “bright or hazy sun”
which I’d estimate corresponds best to the first table’s “usual outdoors”.
That’s a lens setting of f/16; and the guide number for a typical flash,
let’s pick a number from the middle of the table, say 1400, as it’s darn close to a round guide number of 30.
Now to map guide number to f-stop, we divide the guide number by the flash-to-painting distance.
I suspect you’d typically have to stand at least 3 feet away to get a typical sized painting to fill your viewfinder
frame, using a typical 50mm lens. So we divide 30 by 3 and get an f-stop of f/10 for flash.
Soooo, if outdors requires a f/16 lens setting, and a flash from 3 feet requires f/10
that’s a ratio of 1.6, but f/stops are the square-root of the intensity ratio,
so we take 1.6 times 1.6 and get an intensity ratio of 2.56, let’s call it 2.5
By this calculation a middle-of-the-road flash is about 2.5 times weaker than “bright or hazy sun”.
f/10 is a bigger lens opening than f/16).
Next we need to figure out how long a flash duration is.
Going to http://www.hammatsu.com, maker of xenon flashes and other optical gear, we find that it mostly depends on
how big a capacitor the flash discharges. Let’s pick a middle-of-the-road value from their
graph, say 500 microseconds. That’s half of a thousandth of a second.
Next we should compensate for “reciprocity failure”, that’s where short bursts of light have less photochemical effect,
on film or pigments, than continuous light, but not to worry, that’s already been
built into the Kodak exposure tables.
Soo putting the numbers together, a flash a second is 1/2000th of “bright or hazy sun” divided by 2.5,
or 1/5000th, or 0.02% of “bright or hazy sun”. Museum lighting is about 4% of the same.
So flash is 4% divided by 0.02% or 200 times weaker than museum lighting. If I got the decimal points right.
But were not quite done yet. From the hammatsu graphs we see that
sunlight has about 5 times the infrared of flash,
while a naked flashtube has more UV. But most flashtubes have a mildly yellowish lens
in order to improve color balance, which subdues the blues and UV rays. No figures
on this, but an educated guess might be: the painting is going to get 5 times less infrared
(the heating business the museum guy seemed to be really worried about), but perhaps
about the same blue and UV, all filters considered, reflection from walls,
ambient heat, visitor heat NOT considered (and those are non-negligible contributions).
So in regard to HEATING of the painting, it looks like it’s going to get about 200 times 5 times
less heating from flashes as it does from ambient light, plus a lot from ambient. And about 200 times
less UV from flash than from ambient light.
These are of course rough numbers, but even if our errors add up in the worst
possible direction, say by a factor of 5 off, flash lighting seems to be a negligible
factor.
I suspect the annoyance factor of flashes going off, losing
museum gift shiop sales, and (in olden days), the flashbulb litter left around
were much more significant factors in banning flash photography in museums.
Wow, talk about an exercise in futility. There are so many assumptions in those figures, grg88, and they all multiply. So even your best estimate could be off by a few orders of magnitude.
And as Dex says, you’re not likely to change any policies no matter what unless you become a museum director yourself.
But if you just love to crunch numbers for their own sake…
grg88, are you still assuming 1 flash per second for this calculation? I have a hard time believing your numbers. If one flash per second accounts for less than 1/200 of the power of ambient light, that would mean that a 1/200 second exposure photo without flash will be just as bright as a flash photo. This obviously is false - if it were true, you wouldn’t need a flash to begin with. From my experience, in a typical museum using a typical consumer camera (say F/4, ISO 100), a 1-second exposure is not enough to get a good photo. This would mean that heating from a camera flash going off every second is greater than the heating from ambient light. (That ignores reciprocity failure, but IIRC that’s at most a factor of 2 or 3 difference.)