Whitmore presents at the “Cultural Heritage Conservation and Sustainable Development: Experience, Research, Innovation” conference in Paris, France

October 23, 2013

Better preservation through solid-state lighting?

Abstract: For many years the prospect of slowing light-induced damage with rapid light-dark cycling of exhibiting lighting has been examined. Taking advantage of the persistence of vision that allows the smooth transition of motion picture frame images, the idea is to display an object during a brief flash of light, then to turn the lights off - protecting the object - while the viewer’s eyes are still responding to the lit scene. As is well known now, though, such an approach using a shutter or chopper on a steady light source cannot offer any preservation advantage: if the light is on for only half the time, the apparent brightness of the illumination will be about half, following what is known as Talbot’s Law. So illumination with a rapidly pulsing light would appear dimmer - and an equivalent reduction of light-induced damage would be gained by simply lowering the intensity of a steady light - or the intensity of the pulsed lighting would have to be raised to compensate for the dark periods, thus losing whatever preservation advantages were gained by the cycling.

With the advent of solid-state lighting such as LEDs, however, much greater control and range of the on-off light cycles are possible. In particular, very rapid, intense bursts of light can be delivered. By delivering light in this way, one might be able to take advantage of a peculiarity of human vision, described as the Broca-Sulzer effect. According to this principle, the human eye will perceive a very brief burst of light as being somewhat brighter than it actually is, violating the Talbot relationship. This effect offers the possibility that cycling lights could be perceived as having equal brightness as a steady source while delivering less radiant power to the object. Tests performed by Japanese workers using red, green, and blue LEDs demonstrated that the Broca-Sulzer effect, and not the Talbot effect, seemed to be at work in observers’ perception of the flashing LEDs.

In our laboratory, we studied illumination using four different brands of LEDs, pulsed in very short bursts (1-2 milliseconds at a frequency of 60 Hz). The pulsed LED source appeared to observers to be steady and up to twice as bright as the measured intensity indicated, a demonstration of the Broca-Sulzer Effect. While the Japanese studies had indicated slightly different responses to different colored LED pulses, there was no apparent influence of an object color on the brightness enhancement seen with the pulsed white LEDs. Upon exposure to a pulsed LED the fading rate of a light-sensitive dosimeter was reduced by a factor of up to 1.4 to 2.0, while maintaining the same perceived brightness as under a steady illumination source.