Natural and artificial lighting of works of art has always been a sector in which work has been done without sufficient data or specific experimentation applied to the field of the conservation of the works of art forming part of the heritage.
The high levels of lighting used in homes, offices and workplaces in accordance with American and European standards has created the habit of appreciating colours and shapes at luminance values ranging from 250 lux to 1000 lux. Shop windows have popularised a type of very complex and detailed lighting in order to accentuate and rank products by colour and shape. This type of light and lighting, which is very popular all over the world, has made us very demanding. Demands of this nature can easily be met in domestic, work or entertainment environments. However, that lighting culture is questionable when we want to contemplate a work of art in its original environment. Let's take the example of a work of art created to be admired on an altar, which is lit by a few candles and surrounded by the darkness and obscurity of the architecture.
Today it would be unthinkable to exhibit a work of art in semi-darkness even though we have the ability and physical sensitivity to adapt to the new reality in just a few seconds in order to appreciate the detail of an object. But the habit of seeing things under high levels of lighting is constantly vying with the ability to reduce the power of light sources, thereby causing discontent and an unreasonable predisposition among the general public in terms of understanding why a series of careful decisions have been taken about the type of lighting and the luminance levels chosen to illuminate materials as delicate as the ones we are dealing with.
A compromise between the expectations of the general public and the careful attitude of conservators and museum experts could be reached if there was a real possibility of eliminating the damaging effects of light. New theories and recent experiments in this field allow us to assert that it is indeed possible to reach such a compromise.
As we said earlier, light is necessary to see things. However, the power of degradation it exerts on some materials means that it must be controlled. And that is more imperative than aesthetic considerations, in which conservation criteria tend to be underestimated.
Light is considered to be a wave phenomenon and can be defined as a small portion of the electromagnetic vibration spectrum. And it is precisely that portion of the visible spectrum that is situated among high-frequency radiation - gamma rays, X-rays and UV-rays - and radiation with a wavelength longer than 760 nm, that is, infrared rays and Hertzian waves (radar, TV, FM and radio).
Going into more detail, the possible photochemical effects of light on a material can be expressed in terms of that material's susceptibility to damage and the energy absorbed by it. As a result, with a given level of lighting, the energy received by the material is inversely proportional to the wavelength. As the wavelength gets shorter, the energy associated with it becomes greater, as does the probability of photochemical deterioration. That means that ultraviolet light across the whole of its wavelength (near, far, etc.) is the type of radiation that has the greatest potential to cause damage. A rise in temperature of an object will affect the rate at which chemical reactions take place. That manifests itself in two different ways: the general agitation of molecules and atoms increases (physical effect), meaning that its kinetic energy rises, and consequently the speed of chemical reaction also increases (chemical effect), with a reduction in the humidity content of the material. That is why there is a need to study and evaluate the light radiation falling on an object to ensure that proper viewing of it does not interfere with its conservation in the medium-to-long term.
All visible and invisible radiation, whether natural or artificial, leads to deterioration in works of art: yellowing, drying, discoloration, destruction.
Some materials are more affected than others depending on their ability to absorb energy. As seen earlier, and putting to one side the different physical theories about the nature and propagation of light (quantum theory and electromagnetic theory), we can assert, using the words of Professor Feller, that: "light is energy. Any chemical change requires energy. Most surface colour changes get that energy from light".
Irrespective of the light source, both types of invisible radiation must be controlled so that infrared radiation (IR) does not raise the temperature (T) of the objects, especially in display cabinets, or affect the T and/or relative humidity (RH of the air), and ultraviolet radiation (UV) does not exceed 75 mW/lumen.
Many museum pieces can progressively deteriorate and end up being destroyed if they are constantly lit for public display. Partial destruction of the materials and colours can sometimes lead to aesthetic distortion that even confuses art experts. Some objects are not susceptible to the effects of light, some are moderately susceptible and others are so susceptible that their appearance changes after just a few months of being exposed to it.
Materials that are susceptible to the effects of light can be grouped as follows: