For several decades it has been widely believed that the intense illumination from photographic flashguns will damage delicate art and documents. Curators, journalists, art-lovers and museum directors have been telling each other this for years, and many gallery visitors concur. I knew a distinguished museum curator who hung his personal collection of 18th and 19th century English watercolours on the walls of his bright south-facing living room; he was certain that just one flash from a camera would ruin them.
I want to look again at the quantitative evidence behind this widespread belief, and try to re-evaluate the risks, taking into consideration the widespread use of small digital cameras. I will be working in international scientific units as far as possible: SI and MKS units such as metre, second, joule and lux. Where possible I will equate them with units such as Guide Numbers (GNs) and footcandles which are more familiar to most photographers.
The early use of flash for night-time and indoor photography was definitely alarming. Originally a spoonful of magnesium metal powder was ignited on an open tray and it exploded with a short flash of bright light, some sparks and some smoke. Not something that anyone would want to use near a work of art! By the 1930s newspaper cameramen were using glass bulbs with magnesium filaments inside - we have seen them in old newsreel films. Sometimes the bulbs burst, scattering hot sparks. Even if the bulb did not burst it was hot enough immediately after use to be a fire hazard if not disposed of properly. Many curators would not let them be used in galleries. After World War II miniature flash bulbs were being used by amateur photographers and even these occasionally malfunctioned.
The ability to generate very brief but intense pulses of light by discharging electricity through xenon gas inside a glass or quartz tube was known in the 1930s (Edgerton, 1979) but it was not until the 1970s that compact inexpensive electronically controlled flash units (erroneously called 'strobes' by some photographers) were widely available to amateur photographers. As their use spread, museum professionals became concerned. The brief duration of the flash, from less than 1 millisecond (1/1000 second) to a few milliseconds, allowed photographers to 'freeze' movement - a journalistic phrase that was sometimes misinterpreted literally. A friend of mine was once admonished by a museum attendant, who said that the light was so bright that it could freeze an object, and this sudden cold shock would be damaging to a delicate wooden exhibit! The brightness of the peak intensity of the xenon flash and uncertainty about the ultraviolet (UV) energy also frightened curators, and soon there was general agreement that use of these electronic flashguns should not be permitted in museum and fine art galleries.
By the 1990s conservation experts were defining the safe gallery lighting limits for works of art. In Europe, at least, the conservators were using the international M.K.S. units such as the lux in their publications. The lux is a measure of illumination brightness, is defined as one lumen per square metre, and is approximately equivalent to 0.093 foot-candles, the older non-metric unit of brightness. Colby (1993) suggested that works with very fugitive colours should not be put on public display for more than 12,000 lux hours per annum (equivalent to about 4 weeks exhibition at 75 lux, which is quite a low light level: about 7 footcandles). Documents with pigments that are less light-sensitive should be limited to 100 lux for a maximum of 10 to 20 weeks in every year. A detailed classification of various inks, pigments and papers is included in her work. Mansfield (1993) referred to gallery standards of 800, 200 and 50 lux for insensitive items, general paintings, and photosensitive items respectively (the equivalents in footcandles are: 74, 18.6 and 4.6). For comparison, full daylight out of doors, shaded from direct light from the sun, ranges from 10,000 to 25,000 lux. The policy at the National Gallery, London, is that most paintings should be lit at 200 lux, with a total annual exposure not exceeding 600,000 lux hours. The major international museums and galleries are usually careful about the spectral content of the gallery lighting, and in many cases the UV is removed by filters over the light sources; even the natural daylight may be filtered. Small museums are seldom able to control their illuminance spectrum.
A few experimental studies investigated the bleaching effects of electronic flash. Hanlan (1970) exposed a number of pigments to a series of intense flashes from a powerful 4500 footcandle second studio unit, that seems not to have had the ultraviolet (UV) filtered out. In SI units that is nearly 50,000 lux seconds (lux新). Hanlan found that most pigments were unaffected. Rose Red, Hansa Yellow and some blue wool standards showed detectable changes. A publication by Young (1982) considered several light sources and recommended that an electronic flash system should not exceed 1400 joules in power when photographing works of art. This would also have been a powerful studio flash unit. Saunders (1995) commented that the unit used by Hanlan was 75 times more powerful than the 'hammerhead' flashguns used in his own experiments (see below).
In 1994 Evans used photographic equations to estimate what the total quantity of light from an amateur camera flash might be equivalent to, in terms of the day-by-day exposure to the ambient lighting in a gallery (see the Technical Note below, for details of the arithmetic). He made the assumption that a typical gallery visitor might have a camera loaded with 100 ASA/ISO film, and in the accessory shoe there would be a medium sized flash with a full-power guide number (GN) rating of about 20 metres. The calculations showed that taking a flash photograph of something at 2.5 metres (about 8 feet), exposed it to about the same quantity of light that it was receiving every second in a gallery of paintings, and about every 4 seconds if it was in a dimly lit 50 lux gallery. In other words, the equations suggest that during an 8 hour day the exhibits are getting the same quantity of light-stress from very dim gallery lighting as from about 7000 'typical' photo-flashes. The equivalent in a 200 lux gallery for paintings is nearly 30,000 'typical' flashes per day. The conclusion is that if the number of photo-flashes does not exceed a few hundred per day, the extra light stress over and above the ambient gallery lighting should be negligible. Because the intensity of the light varies with the square of the distance between flash and object, the threat could become significant if an abnormally high usage of flash was occurring at very close distances from photosensitive materials.
At the National Gallery in London David Saunders (1995) was concerned that at high flash intensities photosensitive pigments might not be affected in such a proportional way, but might suffer additional non-linear damage due to sequential 'two-photon' effects. He set up some practical trials to test this. For these he calculated that gallery illuminance at 200 lux would cause a cumulative light stress over one year about equal to that from 3.6 million 600 lux second (lux新) electronic flashes. He regarded 600 lux新 as being typical of a single exposure from a modest amateur flashgun. It equates to a GN 20 flashgun about 1.2 to 1.5 metres (say 4 to 5 feet) from the object (see Technical Note, below). For the trials he exposed a range of 25 watercolour pigments and standard blue wool samples to more than one million flashes from Mecablitz 45 CT-1 'hammerhead' flashguns. These large units, rated GN 45, were commonly used by wedding photographers and other professionals. The output of one of the flashguns was unfiltered and the other unit had a UV filter to block the ultraviolet end of the spectrum. In both cases their outputs were reduced to 650 lux新 per flash - approximately equivalent to a GN 20 unit between 1.1 and 1.4 metres from the pigments (see Technical Note). The flashguns were fired at the pigments every 7 seconds, and the total light quantity measured. At the end of a trial lasting many months it was found that these flashes had produced some change in the pigments. After more than 1 million of these test flashes, totalling 200,000 lux hours of exposure, about half the pigments showed changes just visible to the eye. The remaining pigments showed changes too small to detect with the eye, but measurable by photometry. In the vast majority of pigments there was no more change from UV-filtered xenon flash than from the same quantity of gallery lighting. When there was no UV filter the change was about 10-15% greater than from the equivalent quantity of gallery light. When the trial continued to a total of 350,000 lux hours most of the pigments showed small visible changes, with the very fugitive blue wool standard #1 and litmus pigment showing considerable changes. There was still no significant difference between the effects of the UV-filtered xenon flashgun (nearly 2 million flashes) and the equivalent number of hours on the wall of a 200 lux controlled-light gallery. If the ultraviolet was not filtered out the flash caused a little more damage than expected from simple light exposure calculations. In practice almost all small camera-mounted flashguns now incorporate a correction filter to bring the xenon light balance close to natural daylight. These filters also remove most of the UV wavelengths which conservators fear (see below).
These trials showed that 'fugitive' pigments deteriorated while on the walls of a controlled-light gallery at about the same rate as if a modest 'hotshoe' flashgun was fired at them every 4 seconds from a distance of about 4 feet. Saunders' results for pigment damage agree quite well with Evans' calculation that one flash from a flashgun of similar power, at a distance of 2.5 metres, was equivalent to about one second of gallery light. Others, such as Michalski (1966), Vitale (1998) and Peter Amsden (2005) have reached similar conclusions. In view of Saunders' work the National Gallery issued rules for professional photographers in 1999. These included: "electronic flash is permitted and is preferred to the use of photographic lamps, since the overall exposure of the paintings to light is greatly reduced. The maximum exposure for any painting in the field of the flash is 1250 lux seconds per frame; equivalent to f22 at 100 ASA. A supplementary ultraviolet filter must be fitted to the flash unit". This is equivalent to one flash from a GN 58 flashgun at about 2.5 metres, with the camera lens set to f/22. Now (in 2010) flashguns that can be fitted to a hotshoe have powers that range from GN 20 to about GN 58.
The ultraviolet component. The wavelengths of visible light lie between about 400 nanometres (nm) at the violet end of the spectrum to just beyond 700 nm at the red end (see, for example, the entries for 'Color vision' and 'Sunlight' in Wikipedia). When a pulse of high voltage electricity is sent through a transparent tube filled with xenon gas, a brief pulse of bright light is generated. The spectrum of this light typically covers the whole visible spectrum, from violet to red, and also extends into the ultraviolet (UV) and the infrared (IR). Curators could rightly be concerned if a flash used for photography might have UV components that could cause damage to works of art or documents. In practice, the UV output from a camera flashgun is negligible. Some types of xenon flash tubes do emit significant amounts of UV, but these are normally specialist tubes, made from transparent fused quartz/silica that can pass wavelengths as short as 200 nm, which lies in the damaging UV C region. These tubes are used for scientific or industrial purposes where the UV component is needed. The xenon tubes used in photographers' flashguns are made of glass. Glass is transparent to wavelengths longer than 350 nm (the very near UV) but blocks 90% at 300 nm (which is the middle of the 'near UV' or UV A band). Therefore these flashguns cannot emit the damaging UV C and UV B radiation at all, and much of the less dangerous UV A band is blocked (see, for example, the entry for 'Ultraviolet' in Wikipedia). At low gas pressure and low current densities xenon emits a broken spectrum of discrete wavelengths in the visible as well as in the UV and IR bands. At higher gas pressures and higher currents the spectrum becomes almost continuous and is very suitable as a photographic light source. It can be arranged to closely resemble average daylight, and is therefore suited to photography with colour films balanced for daylight - about 5600 degrees K colour temperature. The efficiency of the tube, in terms of light output for electrical energy used, is better when the tube is driven at a higher colour temperature of about 6000 - 6600 K, with wavelengths 290 - 820 nm (Edgerton, 1979, Klipstein, 2002). This shifts the balance towards the blue end of the spectrum, somewhat increasing the near UV component, though the output falls steeply at wavelengths shorter than 300 nm (Edgerton, 1979). In the 1970s it was recognized that this light was too blue for accurate colour photography (Edgerton, 1979) and that a pale yellow filter was needed for photography. By the 1980s it was becoming standard practice for manufacturers of photographic flashguns to incorporate such a filter to achieve correct daylight 'white balance' (Blaker, 1989). This filter will also reduce the near UV component to something resembling natural daylight and all the major makes of camera-mountable flashguns now incorporate such a filter.
Small digital cameras are now in widespread use. Some of these 'point-and-shoot' cameras are used in 'auto' mode by people who do not know how to turn off the flash that automatically fires in low light. This results in a lot of accidental flashes in galleries that have previously tolerated photography without flash. The increasing use of these compact digital cameras is now the main cause of concern to curators and conservators.
In some cases the authorities have reacted by prohibiting all photography in their galleries, ostensibly because they say, eg: "50,000 camera flashes a year are damaging the exhibits". The same reason has been given for banning the photographing of Pharaonic Egyptian relics which have been exposed to tropical sunlight on and off for over 3000 years.
How much damage might these compact camera flashes cause? Most of them incorporate a small xenon flash, much less powerful than detachable units designed to be fitted to a hotshoe. The flash built into a digital compact has, typically, a GN value of about 6 to 9 (though some manufacturers are rather coy about revealing the GN rating). If one extends the calculations used by Saunders and by Evans to these little units, then one finds that if one of these is fired at full power at about 2.5 metres (ca 8 feet), it exposes the object to about the same quantity of light as that falling on it every one-eighth of a second in a 200 lux (ca 18.6 footcandle) gallery, or every half second in a dark 50 lux (ca 4.6 footcandle) gallery. Is it worth getting steamed up about such a tiny extra quantity of light, as far as pigment fading is concerned? Several photographers have already suggested that any trifling damage done by a few hundred of these little flashes in a day could be fully offset by closing the gallery and turning off the lights a few minutes early. A ban would be justified in rare cases, where large numbers of photographers might be taking many flash photographs very close to something that could reasonably be considered photosensitive. The more advanced (and expensive) cameras used by serious photographers also have a built-in flash facility. The flash units fitted in digital single lens reflex (DSLR) cameras have guide numbers in the range GN 10 to 14 - somewhat more powerful than those built into the small cameras. However, these DSLR and similar advanced cameras can now take photographs at such high ISO sensitivity settings that their users seldom need to use flash. Does the ban on photography in some galleries really reflect a genuine, though misplaced, fear of light damage, or is it part of a hidden general anti-camera attitude by some administrators?
Nevertheless, there may be other reasons why museum and art gallery directors and curators might want to prohibit electronic flash photography in their museums or in individual galleries:
There is no doubt that repeated flashes from other visitors' cameras are disturbing. They are specially annoying to art lovers who wish to contemplate a work without distractions. They are also a real nuisance to everyone in a dimly lit gallery, where photosensitive artefacts such as old textiles or historic documents might be on display under minimal lighting. Even a low powered flash is stressful to the dark-adapted eye. It is a matter of common courtesy to avoid using electronic flash in these galleries if other visitors are present.
Sometimes an institution will claim that it must prohibit photography because of copyright rules. Copyright laws vary from one country to another, and are notoriously difficult to interpret. In some cases a museum or art gallery might be using the copyright argument as a smokescreen to hide a general desire to prevent visitors from taking photographs. In other cases the very vagueness of copyright laws might lead the administration to take an ultra cautious approach for fear of legal consequences. There is so much uncertainty over specific situations that, until these situations are defined by a legal challenge in court, one can understand the caution of some curators and directors. Even the 'fair use' conventions regarding the use of photocopy machines has its grey areas of uncertainty. I know of at least one museum where photography is allowed for personal use only, on condition that the images are never published in any way. This museum not only forbids publication for commercial purposes, but also forbids publication on personal web pages such as social networking sites on the internet. Now that the World Wide Web makes it possible to show an image all over the world, this adds an additional dimension to copyright laws that are framed primarily country-by-country.
I offer this suggestion: in many cases copyright difficulties might be overcome by requiring all who wished to take photographs to get a permit at the entry desk. The issuing of the permit could require the photographer to sign an agreement with some conditions. These could include an acknowledgement that the permit conferred no copyright to the photographer, and that all copyrights remained with the artist/owner/institution, as appropriate. Any photographs taken would be for the personal private use of the photographer and not for publication or reproduction in any form. I have seen this used effectively in special exhibition galleries. Many museums and galleries issue photography permits on payment of a fee, and it has been suggested that such a fee adds a level of legal force to the conditions set. The level of the fee could be set to something approximately equal to or somewhat exceeding any loss of profit (not total sales) that might be anticipated in the museum shop, as discussed below. Whether this strategy would be effective under the laws of different countries is something I have no knowledge of. I do not intend to add anything further on this matter, which is only marginally relevant to the use of photographic flash in museums and galleries. As photographers, we do need a legally authoritative review of the various copyright laws and the ways in which they might affect a person's wishes to photograph exhibits for their private use, and also an institution's rights to restrict them.
There have been many allegations that some galleries prohibit photography in order to boost sales in their shop. Some shop managers might think this way, but I find it hard to take it seriously. I would not regard a snapshot taken under less than ideal circumstances an adequate substitute for a professionally photographed poster, greetings card or colour transparency sold in the shop. Photographs taken with on-camera flash are often spoiled by the reflection of the flash when the object is behind glass. The red/infrared autofocus beam from other people's digital cameras also can spoil one's own 'available light' photographs if they reflect from glass in front of the object during one's exposure. If shop sales are poor, that is probably due to the high prices charged by some museums and art galleries for their images, plus the poor keeping quality of many commercial colour transparencies. Some galleries, however, employ a professional photographer on the staff. In such cases a ban is understandably imposed in order to channel all photography through the in-house facilities.
In some institutions it has been found that flashes from cameras can trigger gallery alarm systems. It is then reasonable to control the use of electronic flash.
There have been suggestions that some institutions are fearful that any kind of photography by visitors might bring a security risk, ranging from criminals planning a theft to terrorism. The weakness of this position has been pointed out on several photography forums and web-sites.
There are therefore some plausible reasons why a museum or gallery might decide to ban the use of photographic flash. However, to prohibit the use of flash on the grounds that it will harm the exhibits is the least plausible reason of all. If a visitor, attempting to use flash photography in a gallery, was ejected by a gallery attendant on the grounds that the visitor was endangering the exhibits, I would not be surprised if that visitor might reasonably consider bringing legal action for assault against the institution or attendant, because the alleged grounds for ejection were based on a demonstrably false premise.
I would suggest that if museum and art gallery administrators decide for reasons of their own to forbid electronic flash specifically, or photography in general, then they should simply advertise that the procedure is prohibited, without adding any explanation. Once one attempts to justify one's reasons for a prohibition, one is open to legitimate arguments against it.
To take a successful photograph when relying primarily on light from a simple flashgun, one sets only the lens aperture (f-stop) on the camera. The shutter time is irrelevant, provided that it is long enough for the shutter to be fully open when the brief flash fires. The value of the f-stop can be estimated from knowing: (a) the power of the flashgun, rated as a Guide Number (GN), (b) the distance of the flashgun from the object being photographed, and (c) the sensitivity ('speed') of the film or digital sensor. The guide number is normally provided by the manufacturer. The numerical value will depend on whether the distance is measured in feet or metres. It is usual to rate the GN for a film or sensor speed of 100 ISO/ASA.
If the GN is quoted for metres, and one is using a film or sensor sensitivity of ISO 100, then:
numerical value of the f-stop = GN/metres
From this equation one can calculate that when a GN 20 flashgun is used at full power, 2.5 metres from the object, one needs to set the aperture to f/8 for a film speed of ISO 100.
If one is using a more sensitive film, or higher ISO value for the digital sensor, then the lens aperture must be smaller, ie: a larger numerical value for the f-stop. This calculation applies primarily when the flashgun is being used at its full output power (eg: set to 'Manual'). In practice, most modern flashguns incorporate a sensor that automatically reduces the power of the flash when necessary, eg: if the ISO speed is high, the aperture is wide or the distance short.
The equation can be rearranged to:
GN = (flashgun to object distance) x (numerical value of the f-stop lens aperture)
Available light photography:
If one chooses to take the same photograph as above, but using the gallery lighting instead of flash, one would usually use an exposure meter (hand-held or built into the camera) to find the correct combination of shutter speed and f-stop. Assume that one still has one's camera set to f/8 and the film or sensor at ISO 100. If one is in a correctly-lit gallery for paintings, the meter will probably suggest an exposure of about one second. How can this be known? Because there is a standard photographic equation, used in the calibration of light meters and for other purposes. One arrangement of the equation is:
(f-stop)2 = (ISO speed) x (illuminance) x (exposure time) x (constant)
which rearranges to:
exposure time = (f-stop)2/(ISO x lux x constant)
The brightness of the lighting (illuminance) in a gallery for paintings should be set by the technical staff to be 200 lux. The 1974 international photographic standard, ISO 2720, suggested a value of 0.003 for the constant in this arrangement of the equation, when calibrating hemispheric incident-light meters. Putting these values into the equation gives, for f/8 and ISO 100:
time = 82/(100 x 200 x 0.003)
which gives an exposure time of just over 1 second.
Since 1974 other values have been used for the constant. Tables published by Kodak in 1999 (AM-105KIC) imply a constant between 0.0047 and 0.0048. Other professional photographers have used 0.0045 (eg: D Kerr uses the reciprocal of 224. See: http://www.openphotographyforums.com/forums/). Whichever constant is chosen the results will differ by less than one f-stop from one extreme to the other. Depending on which constant one uses, the equation gives exposure times of 0.71 to 1.07 second in a 200 lux gallery, and 2.8 to 4.3 seconds in a dark 50 lux gallery.
In Conclusion: The same ISO and f-stop values have been used for both the ambient light photographs taken at these shutter speeds and the one taken with flash. The photographic equations imply that all these photographs should show correct exposures. Therefore one can infer that the quantity of light received by the object from this flash is similar to the quantity of ambient light falling on it about every second in a 200 lux gallery, or about every 4 seconds, in a 50 lux galleries.
Estimating equivalent Guide Numbers from SI units in lux seconds (lux新)
Saunders expressed his exposures in lux seconds in order to equate them with gallery illuminance and to work in SI units. To convert his results into terms of flashgun Guide Numbers one can use the equation:
GN = distance in metres x [square root](ISO 100 x lux新 x constant)
where again the constant will be about 0.003 to 0.0048 for these SI units. This equation is derived by substitution from two of the preceding equations.
All these equations are based on a largely empirical value for the constant, defined in ISO 2720 and later documents (see ANSI PH2.7 - 1986, and see the Wikipedia entries for 'Light Meter' and 'APEX system'). The calculations are estimates, and ignore relatively minor factors such as the angle of incident illumination and reciprocity failure.
Large xenon flash units designed for studio use often have their power rated in joules (Watt seconds). It is not possible to convert joules to GNs or lux新 reliably because too much depends on the geometry of any reflector in the studio units. In most cases the beamed light output from these studio units will be at least 40 times more than from a modest accessory-shoe flashgun.
Amsden, Peter (ca 2005): "The facilities for camera users in archives and public record offices." (http://www.btinternet.com/~amsden/archives/camerasinarchives.html)
Blaker, Alfred A. (1989): "Handbook for scientific photography." Second edition. Focal Press: Boston/London: 1989.
Colby, Karen M. (1993): "A suggested exhibition/exposure policy for works of art on paper". (http://www.lightresource.com/policy1.html)
Edgerton, Harold E. (1979): "Electronic flash, strobe." Second edition. MIT Press: Cambridge MA: 1979.
Evans, M.H. (1994): "Photography: can gallery exhibits be harmed by visitors using photographic flash?" Museum Management and Curatorship, vol 13, pp 104-106.
Hanlan, J.F. (1979): "The effects of electronic photographic lamps on the materials of works of art". Museum News, vol 48, pp 33-41. Cited by Saunders, 1995.
Klipstein, Don (ca 2002): "Continuously and apparently continuously operating xenon lamps." (http://members.misty.com/don/contxe.html). I am grateful to Don Klipstein for much additional information.
Mansfield, K.P. (1993) "Environmental news". Museums Journal, vol 93, p 41.
Michalski, Stefan (1996): "The effect of light from flashbulbs and copiers". DistList discussion, August 1996 (http://cool.conservation-us.org/byorg/abbey/an/an20/an20-6/an20-607.html)
Saunders, D. (1995): "Photographic flash: threat or nuisance?" National Gallery Technical Bulletin, vol 16, pp 66-72. (http://www.ingentaconnect.com/content/ng/ngtb/1995/00000016/00000001/art00005) Summaries of Dr. Saunders' findings were published in New Scientist, 25 March 1995 (http://www.newscientist.com/article/mg14519701.500-old-masters-will-not-fade-in-a-flash.html) and by Jim Henderson in The Master Photographer, August 1995.
Vitale, T. (1998) "Light Levels Used in Modern Flatbed Scanners" RLG DIGI News Oct. 15th 1998 Vol 2 No. 5 ISSN 1093-5371 cited by Amsden.
Young, P. (1982): "Ask the photographer: precautions when photographing works of art". International Bulletin for Photographic Documentation of the Visual Arts, vol 9, p 8. Cited by Evans, 1994.
I am specially grateful to Peter Amsden, Jim Henderson, Doug Kerr, Don Klipstein, Ned Levi and David Saunders for their very helpful comments and advice and also to members of photography forums on the internet. None of them necessarily agrees with the views expressed in this article, nor are they responsible for any errors.
The conclusions and opinions that I have presented here are entirely my own, and errors are likely also to be my own. The substance of this article has no approval by any institution, college or department. I shall be pleased to have comments from photographers, curators and others.
Martin H. Evans B.Sc., Ph.D.
3rd September 2010.
This essay is still in draft status and is being modified frequently. I welcome informed comment and constructive criticisms.
e-mail: mhe1000 'at' hermes.cam.ac.uk (edit as necessary).
Some useful web-sites for information about photography in museums, flash photography, etc:
10 tips for compelling museum photography. Consumer Traveler web-site: http://www.tripso.com/columns/10-tips-for-compelling-museum-photography/
The Art Law Blog. Comments on copyright in art: http://theartlawblog.blogspot.com/2007/11/museum-photography-revisited.html
Blackwell, Ben: Light exposure to sensitive artworks during digital photography. [First published in the Museum Computer Network journal Spectra, November 2000] http://www.benblackwell.com/lighting.pdf
Blackwell, Ben: Addendum - recent developments in lighting and scan back technology. http://www.benblackwell.com/Addendum.pdf
Color temperature (Wikipedia page) http://en.wikipedia.org/wiki/Color_temperature
Color vision (Wikipedia page) http://en.wikipedia.org/wiki/Color_vision
Flash Photography ~ History & Ilford Flashguns. http://www.photomemorabilia.co.uk/Ilford/Flash_History.html
Flash Photography with Canon EOS Cameras (3 part monograph by N.K. Guy). http://photonotes.org/articles/eos-flash/index.html
Horn, Berthold: The Facts of Light. (maths of the physics of light, with SI units). http://dspace.mit.edu/handle/1721.1/41123
Illuminance and lux units (Wikipedia page). http://en.wikipedia.org/wiki/Lux
Kerr, Douglas A.: APEX additive system of photographic exposure. http://dougkerr.net/Pumpkin/articles/APEX.pdf
Kodak AM105KIC: Estimating luminance and illuminance. http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.shtml
Light meter (Wikipedia page): calibration, ISO 2720:1974, maths etc. http://en.wikipedia.org/wiki/Light_meter
Museum Photography with digital cameras. (Digicam web-site). http://www.digicamhelp.com/how-to/places-events/museums/
Museum Photography: Ten Tips. (NSL photography blog). http://www.nslphotographyblog.com/2009/06/museum-photography-ten-tips.html
Museum photography (Wikiversity page). http://en.wikiversity.org/wiki/Museum_photography
Museum photography. (Wikimedia Commons list of regulations at different museums). http://commons.wikimedia.org/wiki/Commons:WikiProject_Arts/Museum_photography
Museum Photo Policies Should Be as Open as Possible. http://museumtwo.blogspot.com/2009/08/museum-photo-policies-should-be-as-open.html
Practical Guide to Museum and Heritage Site Photography. (Heritage-Key web-site). http://heritage-key.com/world/capturing-images-worth-thousand-words-practical-guide-museum-and-heritage-site-photography
Photography and the law (Wikipedia page). http://en.wikipedia.org/wiki/Photography_and_the_law
Photometry (optics) and visible light (Wikipedia page). http://en.wikipedia.org/wiki/Photometry_(optics)
The Pumpkin. A large selection of papers by an engineer, Douglas A. Kerr, on photographic topics etc. At: http://dougkerr.net/Pumpkin/index.htm
Radiometry and photometry FAQ. http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm
Sunlight (Wikipedia page). http://en.wikipedia.org/wiki/Sunlight
Ultraviolet (Wikipedia page). http://en.wikipedia.org/wiki/Ultraviolet