# Why isn't flash photography permitted in museums? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [September 27, 2003, 12:33am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081 "2003-09-27T00:33:02Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![sford](https://avatars.discourse-cdn.com/v4/letter/s/edb3f5/32.png) [@sford](https://boards.straightdope.com/u/sford)\ **Post date:** [December 23, 2003, 6:35pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/41 "2003-12-23T18:35:55Z")
Regarding taking a flash picture through a window, I agree that in most cases the flash will not _help_ you. But if you hold the lens very close to the glass, there won’t be any glare. I’ve taken more than one flash picture out a bus window and while the risk of blurring is high, the risk of glare is low. I also have to say … I’ve been a relatively-non-participatory doper for quite a few years, and I was distressed when I read Dex’s statement, “Rather than playing games with mathematics, I have a better idea.” That statement, and the tone of that and several other posts sounds a lot like, “don’t confuse me with facts, my mind is made up.” Hand-waving can often get you in the ball park, but numbers will get you to home plate. (Gawd, did I just use a sports analogy?) True, the original article gave the _reasons_ (at least the stated reasons) that museums disallow flashes, but it is completely within TSD tradition to follow-up by questioning those reasons, especially since Veg appeared to accept the reasons in the last paragraph. Grg88 challanged the reasons, with an attempt at rigor over hand-waving, which is TSD at its best. I thought grg88’s analysis was a good attempt. He did the best he could with the data available and he tried to err on the conservative side. And he graciously accepted a not-entirely-graciously given sanity check. All this demonstrated what TSD is supposedly all about: the _pursuit_ of truth. Cecil may already be there; for the rest of us it’s an endless journey. Musicat: “Wow, talk about an exercise in futility … But if you just love to crunch numbers for their own sake…” Did you harass nerds like me in High School? Sorry if you only meant to be light-hearted, but ridiculing an honest attempt at rigor rubs me the wrong way. Like I said, hand-waving will get you on the battle field, but numbers will … aw hell. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [December 24, 2003, 12:33am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/42 "2003-12-24T00:33:28Z")
**sford** , my comment about not playing games with mathematics was because all the math being tossed around was based on non-substantiated assumptions. Assume the intensity of light is this, assume the period of flash is that, and you can prove anything you want. Numbers may get you to home plate (as you suggest), but only if the numbers themselves are based on reality. Numbers based on wild-ass guesses and “reasonable” assumptions are worthless. ---
**Author:** ![The\_PHB](https://avatars.discourse-cdn.com/v4/letter/t/df788c/32.png) [@The\_PHB](https://boards.straightdope.com/u/The_PHB)\ **Post date:** [December 24, 2003, 11:31am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/43 "2003-12-24T11:31:35Z")
Hear hear, **sford**! I too think **CK Dexter Haven** is out of order. What **grg88** and **Measure for Measure** were trying to do was not inherently unreasonable. Yes, the quantitative assumptions they’ve made are only assumptions, but are they _that_ bad? If Dex thinks they are, he should provide facts, rather than just dismiss them as “wild-ass guesses”. (In the UK, to use another sporting analogy, we’d call that playing the man instead of the ball.) It’s also important to note the relative size of the effects. The analysis to date \*seems \* to suggest that the effects of flashes are several orders of magnitude below the effects of ambient lighting. Assuming that the quantitative assumptions used by grg88 and MfM haven’t just been plucked out of thin air - they don’t look like it to me, and no other correspondents have suggested it either - then they’re unlikely to be out by that many orders of magnitude. Ergo, the conclusions (that the fffects of flash are small) are _likely_ to be sound. Of course this approach isn’t always useful. You’d be stupid to claim that the analysis supported your argument if the numbers come out close - because then the quality of the assumptions _does_ matter. But that doesn’t seem to be where we are in this case, and it’s disingenuous of Dex to suggest otherwise. PS Many organisations use aptitude for this sort of approximate numerical reasoning as a criterion in their recruitment selection procedures. I encountered it when I went to university (Cambridge, England) and it’s also used by the big consultancy firms such as KPMG. Sample questions: How many undertakers are there in the UK? How many Olympic swimming pools would the annual UK rainfall fill? ---
**Author:** ![l337\_h0b0](https://avatars.discourse-cdn.com/v4/letter/l/91b2a8/32.png) [@l337\_h0b0](https://boards.straightdope.com/u/l337_h0b0)\ **Post date:** [December 29, 2003, 5:40am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/44 "2003-12-29T05:40:30Z")
d00dz after 3 years of study in school takeing Fine Art and skimming most of our post here I got this 1. most of you sem to be Sicience and Photogoraphy types looking at what the Light and heat dose to the paint 2. Museums LIke to keep there enviroment stabe tempratue light and sound even vibration WHY ?? JUST CUZ how would you like to go though there and get a photo flash in the face from every Camera wielding fool ?? the enviroment in the gallery can be JUST as much apart of the work you are viewing that is why there normaly ketp nuteral in tone ( ie off whites and earth tones) 1. a gallery can control the quantity and quality of the light in the space that they provide BUT they can not control the amount that the camera flash will provide and with some delicate works ( old masters) they want to perserve the image as much as they can it is basicly a balanceing act 2. it also gives then an excuse to remove someone who is being unruley with a camera ( not that I have ever seen this happen ) ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 29, 2003, 3:57pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/45 "2003-12-29T15:57:33Z")
> [@](#): > > \*Originally posted by sford \* > > > [@](#): > > > > _Originally posted by Musicat_ > > Wow, talk about an exercise in futility … But if you just love to crunch numbers for their own sake… > > Did you harass nerds like me in High School? Sorry if you only meant to be light-hearted, but ridiculing an honest attempt at rigor rubs me the wrong way. Like I said, hand-waving will get you on the battle field, but numbers will … aw hell. I am not trying to be light-hearted, I am not hand-waving, and if I harassed nerds in HS, I would have to harass myself. As **Dex** has tried to point out, a collection of assumptions can make for such an inaccurate computation that it may not be worth the electrons used to compute it in the first place. Typical formulas for this kind of math are often (this is a crude example, I’m only trying to show the principles) like this: Given A as the amount of ambient light, D the number of days the museum is open per year, F the number of flashes expected per day, I the intensity of each flash, U the duration, C the relative value of the color, T the distance from the painting, etc… This might lead to a formula like: A \* D \* F \* I \* U \* C \*(T[sup]2[/sup]), that is, variables like this often _multiply_. And so do errors, which means they accumulate faster than if they were _added._. If only one number has a valid range of plus/minus 30%, the result also has a valid range of +/- 30%. And if other numbers are also estimates, each one decreases the validity of the result as well. If the answer is expected to be valid within plus or minus 45,000%, have we really learned anything? Most of your numbers are assumptions or estimates, to wit (these are all exact quotes from your posts, italics mine): [ul][li]here’s a table that _probably brackets_ a museum’s lighting conduitions[_]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.[_]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[_]Duration of flash…lets _guess_ 1/4 millisecond.[_]an average flash level[_]_assuming_ the museum is open 10 hourswe _assume_ one flash per second…with a _typical_ electronic flash duration[/ul]So to sum up, and to paraphrase Darrell Huff in [How To Lie With Statistics](http://www.amazon.com/exec/obidos/tg/detail/-/0393310728/qid=1072712745/sr=1-1/ref=sr_1_1/104-7877055-2270301?v=glance&s=books), **there’s a nice feeling of exactness to that mathematical result, but it’s all bluff.** [/li] Or, in my words, an exercise in futility, at least if you expect the result to prove anything. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [December 29, 2003, 4:19pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/46 "2003-12-29T16:19:14Z")
Ahhhh, **Musicat** , anyone citing Darrell Huff wins applause from me! … and I think _How to Lie with Statistics_ should be required reading for every educated person! ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 29, 2003, 4:42pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/47 "2003-12-29T16:42:27Z")
[slight hijack] **Dex,** I happen to have a 1954 copy of Huff’s book. None of Huff’s points have ever gone out of date, but it is interesting to see what changes have been made in the newer editions – for no other reason than political correctness. [/slight hijack] ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [December 29, 2003, 10:23pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/48 "2003-12-29T22:23:21Z")
[continued hijack] Yep, I have a 1954 copy, and I think the newer editions are just reprints, with no changes. The numbers are a little “off” (he talks about $5,000 a year as a very very nice salary), but the points he makes are still absolutely valid… [/continued hijack] ---
**Author:** ![5cents](https://avatars.discourse-cdn.com/v4/letter/5/46a35a/32.png) [@5cents](https://boards.straightdope.com/u/5cents)\ **Post date:** [December 30, 2003, 1:46am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/49 "2003-12-30T01:46:37Z")
> [@](#): > > \*Originally posted by Musicat \* > **Most of your numbers are assumptions or estimates** While I’m not the original poster, I do have a degree in mathematics, and I do understand statistics. I don’t have the Huff book, but I did take a course that covered the same material, but with considerably more depth (taught by a professor emeritus and former School of Mathematics dean). To me, the numbers were valid assumptions, and either reasonable (open 10 hours, flash duration), conservative (3 feet from painting, f16, 100 speed film) or very conservative (1 flash per second). The things you point out, like _typical_ flash duration really aren’t numbers you can quibble with - flashes are constrained by camera size, which constrains battery size, which constrains flash tube size and power usage. Give everybody walking into the museum a $3000 camera setup with a “wedding flash” (you know, the type used at wedding, which are normally powered by 8 D cells in a belt pack) and you might have an argument. The numbers given would tend to find an upper bound. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 30, 2003, 2:56am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/50 "2003-12-30T02:56:28Z")
**5cents,** I have no quibble with the individual starting numbers supplied by you or others in this thread as working estimates. They seem reasonable to me, and I do have some photographic experience. If you could say that all pictures are taken exactly 3 feet from a painting, all flashes are of 1/100 sec duration, exactly 15 people per day take pictures of a particular painting, the exact number of days the museum is open per year is 320, the exact number of years we are considering is 100, the rate of natural deterioration of red oil pigment made between 1850 and 1860 is exactly 5.4% loss per 3.9 lumens per second, etc. – well, I hope you get my drift. Under these conditions, if we can be reasonably sure of incorporating all variables (and that itself is not certain!), we _might_ be able to come up with a pretty good idea of what flash photography does to portraits. But we have none of these constants; we only have variables _with a very wide range of validity._ Is the picture under the skylight? To one side? Are flashes more powerful than last decade? If so, how much? Does pigment deteriorate faster under different humidity conditions? If so, what are those conditions in the museum? How do you know how many tourists will be bringing their cameras? Taking pictures? More tourists this year than last? Etc… If you feel 3 feet is a typical distance, would you accept the range of 2-4 feet? Then we have just multiplied the range of error by a _large factor,_ since the intensity of light diminishes with the cube of the distance. What if there are twice as many visitors than you estimate? Then we have multiplied the already large error factor by 2. What if the museum is open only 5 days a week instead of 7 as you assumed? Then we must multiply the result so far by 5/7 (and no, we can’t assume that the errors cancel each other out). Before we get very far into our calculations, we have accumulated so much gross error that the computation is silly beyond words if you expect it to have any practical validity. In short, the range of likely scenarios is so great that it is mathematically impossible to compute a number _with even the slightest exactness_ that represents the amount of effect flash photography has on a painting in a musuem. Personally, I think the prohibition of flash is because museum directors don’t want to take any chances and they rule the roost. To further illustrate, I would like to quote more extensively from Darrell Huff’s book, and I highly recommend it. It is short and engaging reading, and although the examples are dated, the principles espoused are as vital as ever. Huff says, referring to Engels and Marx’ _Das Capital_ economic calculations (most ellipses are Huff’s): > [@](#): > > Karl Marx was not above achieving a spurious air of precision… In figuring the “rate of surplus-value” in a mill he began with a splendid collection of assumptions, guesses, and round numbers: “We assume the waste to be 6%…the raw material…costs in round numbers ₤342… the 10,000 spindles…cost, we will assume ₤1 per spindle…The wear and tear we put at 10%…The rent of the building we suppose to be ₤300…” He says, “The above data, which may be relied upon, were given me by a Manchester spinner.” > > From these approximations Marx calculates that: “The rate of surplus-value is therefore 80/52 = 153 11/13%.” For a ten-hour day this gives him “necessary labour = 3 31/33 hours and surplus-labour = 6 2/33.” > > There’s a nice feeling of exactness to that two thirty-thirds of an hour, but it’s all bluff. And **5cents** , Huff’s book is not an extensive treatise about _how to use_ statistics, but _how to lie_ with statistics, and how to recognize when you are being conned. Dunno about yours, but in the college stats classes I took, this was given pretty short shrift. ---
**Author:** ![5cents](https://avatars.discourse-cdn.com/v4/letter/5/46a35a/32.png) [@5cents](https://boards.straightdope.com/u/5cents)\ **Post date:** [December 30, 2003, 4:54am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/51 "2003-12-30T04:54:13Z")
> [@](#): > > \*Originally posted by Musicat \* > **But we have none of these constants; we only have variables _with a very wide range of validity._** Right, so take a worst case and make the calculation. You now have an upper bound on the potential damage. Just take the “3 feet away” number. It would have to be a pretty small painting to get it all in under 3 feet away. I’ve taken photography courses at a well-respected fine arts college, and the first thing you are taught it to move closer to the subject. Untrained photographers tend to take in a huge amount of background in addition to whatever it is that they want to photograph. You don’t see many cameras in US art museums, but go to the Louvre. A lot of the picture taking is from 10 feet away. 3 feet seems to me to be an extremely conservate estimate. Flashes if anything have become less powerful. Small is good, and there’s only so much battery power you can pack into a small package. High-speed film helps make up for this loss. The typical film used today is 400 speed, 20 years ago it was 100 speed. As for heat and humidity, I haven’t seen cameras wandering around museums all by themselves. Typically they are attached to a human, and humans constantly give off heat and humidity, so much so that the occasional additional contribution of a little xenon tube is negligible. Take two unventilated rooms, one with 100 humans, the other with 100 cameras. Let the cameras flash all they want, and just have the humans wander around slowly. Want to take a wild guess which room will be hotter and more humid at the end of the hour? > [@](#): > > \*\*Huff’s book is not an extensive treatise about _how to use_ statistics, but _how to lie_ with statistics, and how to recognize when you are being conned. Dunno about yours, but in the college stats classes I took, this was given pretty short shrift. \*\* You should have gone to a better college :). The course I was referring to was called “Uses and Abuses of Statistics”, and was a full credit semester course, taught by somebody who had way too much experience but liked to teach this class (he was about 75 at the time he taught me, former math dean, former stats & act sci chair, prof emeritus). I seriously doubt that museum folks have done the math. I do believe that flashes are a nuisance in a museum, and saying “no flashes in museum, to protect our artifacts” achieves this, and pegs the blame on some 500 year old canvas that you can’t argue with. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [December 30, 2003, 3:29pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/52 "2003-12-30T15:29:41Z")
Museums are not just oil paintings, but include fabrics that are already hundreds of years old and may have already been exposed to significant sunlight. Is the effect of sunlight linear, or does it accumulate exponentially? Everyone’s calculations to date in this thread have been based on the underlying assumption that the accumulated effect is linear. I am not at all convinced that the impact of six days of exposure to sunlight is simply six times one day’s exposure. For instance, if X days exposure reduced the colors by 5%, it seems to me that the deterioration process is exponential rather than linear. That alone would make all these calculations useless. And note that we have unanswered the question of WHAT part of the light spectrum causes fading. Are flashes more harmful or less harmful than direct sunlight? How do different materials and media (cloth or canvas, oil paint or water colour, covered with glass or not, etc) react to various bits of light. Does a glass case magnify or dilute the effect of the flash? If the flash isn’t aimed directly at the item, but at a nearby item, is there a residual impact? In short, the seeming quantification that folks have been offering are interesting theoretical exercises in completely hypothetical fluff. Without concrete experimentation, there’s no way to know. And experimentation with modern fabrics won’t do because the dyes and materials would be different from (say) Renaissance tapestries. I can’t imagine any museum willing to experiment. So, in short, you’re stuck with the museum’s fear that flash photography MIGHT cause things to fade, with no concrete proof possible. Live with it. ---
**Author:** ![grg88](https://avatars.discourse-cdn.com/v4/letter/g/bbe5ce/32.png) [@grg88](https://boards.straightdope.com/u/grg88)\ **Post date:** [December 30, 2003, 3:57pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/53 "2003-12-30T15:57:16Z")
Here we see the huge divide between scientists and common folk. Scientists have about 200 years of experience in developing the scientific method, a large part of which is: Dont go with your gut feelings, use logic, measure, replicate, use math. It’s been determined that the “word arguments” presented by the moderator and others, while superficially convincing, are not very useful in determining the truth. In fact, almost anything else is better, even rough quantitative estimates. Scientists have a couple of centuries of good and bad examples, where they’ve both gone far astray by interjecting too much wishful thinking, and on the good side, where there have been great breakthrus when they stick to the scientific method, no ,atter how counter-intuitive it sometimes seems. How one “feels” about light exposure really does not help us get at the truth. What is helpful is for somebody to collect data, make reasonable and conservative estimates, and give references so others can verify the data. Hey, that sounds like what I and others have done! Scientists are perfectly open to hear reasoned arguments about the data. But repeated fuzzy and illogical appeals with words really are not going to help us find the truth. I could go on and on with line by line rebuttals, but real work calls. Regards, grg88 ---
**Author:** ![5cents](https://avatars.discourse-cdn.com/v4/letter/5/46a35a/32.png) [@5cents](https://boards.straightdope.com/u/5cents)\ **Post date:** [December 30, 2003, 5:28pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/54 "2003-12-30T17:28:29Z")
> [@](#): > > \*Originally posted by C K Dexter Haven \* > **For instance, if X days exposure reduced the colors by 5%, it seems to me that the deterioration process is exponential rather than linear.** Sure, absolutely, it is exponential. Exponential _decay_. To assume linear is to be more conservative. For instance, say, like you do, that X days exposure reduced the colors 5%. Now the piece has 95% of the original color. Expose it to another X days and lose another 5%, and you have a piece with 90.25% of the original color. This is exponential decay, and it less than linear. > [@](#): > > \*\*So, in short, you’re stuck with the museum’s fear that flash photography MIGHT cause things to fade, with no concrete proof possible. Live with it. \*\* UV light causes fading, that is well known. Infrared causes heating, that is also well known. In the grand scheme of things, flashes add an insignificant amount of UV and infrared light. I fully support museums restricting the use of flash photography, as it is very annoying to me when I’m trying to look at something and flashes are going off all around me. However the reason usually given is not the most significant reason, but rather it is the reason least likely to cause the potential-flash-photographer to take offense. If flash photography were a significant contributor to decay, the Louvre collection would be a smouldering heap of ash right now. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [December 31, 2003, 3:02am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/55 "2003-12-31T03:02:06Z")
**grg88** , I think you’ve got it exactly backwards. The moderator is the one saying that the word arguments are nonsense… even if the word arguments are hidden behind lots of numbers. The estimates and guesses that you and others have given are the word arguments. Where is the experimentation? Where is the data? There is none, there are only vague estimates of what people THINK that the effects of flashes are. You’re pointing your finger in the wrong direction. There is nothing scientific in the razzle-dazzle that’s been presented. My complaint is exactly that these numbers are irrelevant and meaningless without substantitive data. And **5cents** , I’d take it the other way. If the amount of damage done is 5% per X days of sunlight, then the exponential 1.05[sup]n[/sup] is much worse than the linear 1+.05n. Whether you think that the damage from flashes is significant or not is beside the point. The museums think that there is signficiant risk of damage, and that’s sufficient. ---
**Author:** ![5cents](https://avatars.discourse-cdn.com/v4/letter/5/46a35a/32.png) [@5cents](https://boards.straightdope.com/u/5cents)\ **Post date:** [December 31, 2003, 3:42am UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/56 "2003-12-31T03:42:45Z")
> [@](#): > > \*Originally posted by C K Dexter Haven \* > \*\*And **5cents** , I’d take it the other way. If the amount of damage done is 5% per X days of sunlight, then the exponential 1.05[sup]n[/sup] is much worse than the linear 1+.05n. > \*\* I’m well aware of what exponential growth is, but please, if you insist that there is even the possibility of exponential growth occuring, give me a hint as to what is growing? Fading is classic exponential decay. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [December 31, 2003, 12:44pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/57 "2003-12-31T12:44:15Z")
Sorry, my initial example and the use of 5% was wrongly focused, I was writing and thinking too quickly. My point was the following: in all the numerical examples that have been offered, there is an underlying assumption that one flash per minute (or whatever the rate is that is being assumed) is the same as 60 flashes per hour, times an 8 hour day. I’m challenging that assumption. That is, n flashes sequentially is the same impact as n x one flash. But that’s an underlying assumption – what if the impact is exponential, that n sequential flashes accumulates exponentially and is the same impact as 2sup flashes? In that case, the flash impact would not be as trivial as the hypothetical calculations imply. My point is that these number games are just number games, unless someone provides a scientific experiment with data that shows some substance to them. ---
**Author:** ![grg88](https://avatars.discourse-cdn.com/v4/letter/g/bbe5ce/32.png) [@grg88](https://boards.straightdope.com/u/grg88)\ **Post date:** [December 31, 2003, 1:53pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/58 "2003-12-31T13:53:36Z")
Well, your exponential theory is a bit unlikely. If it were true, then: (1) The ratio of fading from the light that’s been hitting the painting over the last say 200 years versus the light from the flash would be on the order of e to the 200th power over e to the f power, (where f is the nuber of years of flash). Now e to the 200th power is a HUGE number, around 7 times 10^86: 7,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 So if there was an exponential factor, 200 years of fading would have left a pure white canvas, whiter than any other white, even the silly white costume Marlon Brando wore in the first Superman movie. Whiter than Clorox white. Whiter than Queen Elizabeth, dipped in whitewash, then rolled in bicarbonate of soda (not that I’d suggest doing so). (2) If photochemical reactions like fading had an exponential component, however small, you’d have to posit some kind of chemical memory effect. Photographers and especially astronomers would have discovered this long ago. The only thing they’ve found is a “reciprocity failure” at very short duration pulses, which kinda is the opposite of the exponential theory. Very short bursts of light have a smaller than expected effect. This effect is down in the millisecond range, and flattens out to normal quickly thereafter. Nothing of note happens over a longer time range. To do so would require the chemicals to have some very strange intermediate energy state, which is quite unlikely and contrary to theory and experience. Photochemical reactions have been extensively studied for over 140 years. It’s unlikely that one can dream up a new theory out of whole cloth that hasnt been already investigated to death. Regards, grg88 ---
**Author:** ![5cents](https://avatars.discourse-cdn.com/v4/letter/5/46a35a/32.png) [@5cents](https://boards.straightdope.com/u/5cents)\ **Post date:** [December 31, 2003, 2:19pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/59 "2003-12-31T14:19:59Z")
> [@](#): > > \*Originally posted by C K Dexter Haven \* > **But that’s an underlying assumption – what if the impact is exponential, that n sequential flashes accumulates exponentially and is the same impact as 2sup flashes?** This makes no sense. If n flashes has the same impact as 2sup[/sup] flashes, then what you are really saying is that there is no loss for n+1, n+2, … 2sup[/sup] flashes. I know what you are trying to get at, but the contortions don’t work. Take something and stick it on a sunny window. Measure how many (days/weeks/months) until it has faded significantly, 80%, 90%, whatever. Now let it sit in the sun some more, say for an equal number of days. If your theory of exponential growth in fading pans out, the object will have long ago lost all traces of what was there. If you like empirical evidence, go to the Louvre. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [December 31, 2003, 2:23pm UTC](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081/60 "2003-12-31T14:23:14Z")
**Dex** , why would the effect of multiple exposures to light be anything _but_ additive? If, as you speculate, the cumulative effect is more severe than an additive function (i.e., exponential), then if you turn lights off at night, you _increase_ the light damage, as that would create multiple light events (each day, in other words) instead of a single non-interrupted event. That doesn’t seem reasonable at all. What seems more reasonable is that damage is simply a function of how many photons of what energy level hit a particular object, cumulatively. This cite doesn’t really address your objection directly, but [in this technical bulletin](http://www.nedcc.org/plam3/tleaf24.htm), the equivalence of different lighting levels is discussed, to wit: > [@](#): > > It is essential to understand that light damage is cumulative, and that lower levels of illumination will mean less damage over the long term. Another important concept in controlling visible light is the law of reciprocity. This says that limited exposure to a high-intensity light will produce the same amount of damage as long exposure to a low-intensity light. For example, exposure to 100 lux for 5 hours would cause the same amount of damage as exposure to 50 lux for 10 hours. [Previous page](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081.md?page=2) [Next page](https://boards.straightdope.com/t/why-isnt-flash-photography-permitted-in-museums/204081.md?page=4)