Is flash safe for collection objects?

Why the question matters

Every digitisation project in a museum, archive or library starts with the same concern: will the light used to photograph an object harm it? Paintings, manuscripts, textiles and historic photographs are sensitive to light, and conservators carefully ration how much each one receives. The good news is that decades of conservation research point in one clear direction: correctly filtered studio flash is one of the gentlest ways to light a precious object.

Light damage is a matter of total dose

Light fades colourants gradually and cumulatively. What counts is the total dose – illuminance multiplied by time, measured in lux hours. Conservators call this the reciprocity principle. As the Conservation Center for Art and Historic Artifacts explains, one hour at 100,000 lux has the same effect as 2,000 hours at 50 lux.

For a long time, people feared that the extremely intense, very short pulse of a flash might trigger different and more harmful chemistry. In 1995, David Saunders of the National Gallery in London put this to the test. In his study Photographic Flash: Threat or Nuisance? he exposed traditional organic pigments and Blue Wool standards both to gallery lighting and to repeated flash until every set had received the same total exposure. The fading was the same in every case. Modern flash units are simply not powerful enough to cause the feared "two-photon" processes – and the National Gallery's recommendations for photography consequently came to favour flash over continuous lighting.

A flash delivers remarkably little light

Because a flash only lights the object for the fraction of a second the camera needs, its contribution is tiny. In On the Illumination of Light-Sensitive Photographs (American Institute for Conservation, 2009), Mike Ware calculates that a correctly exposed flash copy shot at f/11 and ISO 100 exposes an object to roughly 0.1 lux hours. That is about ten seconds of display under strict museum lighting of 50 lux. The same shot with continuous studio lights delivers dozens of times more light, simply because the lamps stay on while the photographer composes, focuses and shoots.

Compare that with the annual light budget for highly sensitive objects such as watercolours or textiles, which is typically 150,000 lux hours per year. Even a session of a hundred flash exposures uses only around a hundredth of a percent of it.

Stefan Michalski of the Canadian Conservation Institute put it in perspective: a gallery lit at 50 lux would need around 3,600 amateur flashes a day just to increase fading by 10 percent (source).

Ware's analysis also shows something useful for professionals: the dose that reaches the object depends on aperture, ISO and magnification – not on the size of the flash generator. A more powerful pack does not expose the object to more light when the image is correctly exposed.

No heat on the object

A flash pulse lasts only milliseconds, so the object does not warm up. Tungsten and halogen lamps, by contrast, heat objects by radiation and convection throughout a session, which can stress paint layers, parchment or adhesives.

UV: the one factor that depends on your equipment

There is one important condition. A bare xenon flash tube is rich in ultraviolet radiation – around 300 microwatts per lumen, according to Ware. Museum guidelines set a maximum of 75 µW/lm and today aim for close to zero (Texas Historical Commission). Because UV is the most energetic part of the spectrum, it has to be removed at the source. As early as 1994, Johan Neevel of the Central Research Laboratory in Amsterdam measured twelve flash units and recommended UV filters that absorb at least 90 percent of the radiation between 300 and 400 nm (abstract).

Why only original broncolor flash tubes and protecting glasses

In broncolor lamps, UV is filtered by a dedicated coating. Depending on the lamp head, this coating sits either on the flash tube or on the protecting glass – the two are designed as a matched pair. The broncolor operating instructions are explicit: UV-coated protecting glasses or UV-coated flash tubes must be used as protection.

That is why we strongly recommend using only original broncolor flash tubes and original broncolor protecting glasses for cultural heritage work:

• A non-original tube or glass may lack the UV coating. Combine an uncoated tube with an uncoated glass and the lamp emits unfiltered xenon UV – around four times the museum limit.
• UV filter performance is rarely published and, as Ware notes, often taken on trust. Original parts are designed for exactly the lamp they are used in.
• Non-original parts can also shift the colour temperature and compromise colour-accurate documentation.
• Never photograph collection objects with a missing protecting glass, and replace a cracked or scratched glass with an original part first.

Best practice for digitisation sessions

• Use only original broncolor UV-coated flash tubes and protecting glasses.
• Dim or switch off the modelling lights between exposures, and set up on a stand-in object whenever possible. As Ware points out, set-up time can expose an object to more light than the photography itself.
• Avoid unnecessary test shots.
• Agree the photography dose with the conservator and count it against the object's annual light budget.
• Treat UV fluorescence imaging – for example with the broncolor UV attachment – as a deliberate UV exposure and keep it as short as possible.

Conclusion

Used with the right equipment, flash is not a threat to cultural heritage – it is the more conservative choice. It delivers less total light than continuous lighting, puts no heat on the object, and the coating on original broncolor flash tubes and protecting glasses removes the UV at the source. Discover our lighting solutions for cultural heritage photography.

Further reading

• David Saunders: Photographic Flash: Threat or Nuisance? National Gallery Technical Bulletin, Vol. 16, 1995
• Mike Ware: On the Illumination of Light-Sensitive Photographs. AIC Topics in Photographic Preservation, Vol. 13, 2009
• Stefan Michalski, Canadian Conservation Institute: Flash photography. Conservation DistList, 1996
• Johan G. Neevel: Exposure of objects of art and science to light from electronic flashguns or photocopiers. Central Research Laboratory, Amsterdam, 1994
• Terry T. Schaeffer: Effect of Light on Materials in Collections: Data on Photoflash and Related Sources. Getty Conservation Institute, 2001 (review in JAIC)
• CCAHA: Light Exposure for Artifacts on Exhibition, 2020