Specific lighting levels

Regarding the duration and intensity of radiation, illumination of the objects should be limited to exhibition times only or indeed alternated. It should also be done in such a way that no ray falls directly on fragile objects. In certain cases (very fragile objects such as watercolours, textiles, drawings, manuscripts, etc.), it might be worth looking into the expediency of replacing the originals with copies or facsimiles.

Taken from:
"La lumière et la protection des objets et spécimens exposés dans les musées et galeries d'art". Op. Cit.

The recommended annual limits for exposure to light rays for the three categories of art objects for the main conservation institutions (ICOM, ICCROM, IES, Ministry of Culture, etc.) are:

Approximately:
50 lux x 8 hours per day x 300 days per year.
Taken from: "La lumière et la protection des objets et spécimens exposés dans les musées et galeries d'art". Op. Cit.

When it comes to choosing the light sources to illuminate a particular work of art, it is essential to take a series of considerations into account. Those considerations will determine the selection criteria that we use.

First, we know that the types of radiation responsible for deterioration in the objects we are studying are not all visible. Neither are they present in the same proportions, since they vary depending on the light source chosen.

Second, we know that studying the composition of the radiation emitted by the light source is essential in order to control its harmful effects on the object. Therefore, it simply is not enough to control the absence of the most harmful types of radiation in the lighting system chosen. In fact, it is more important to check that the light source does not exceed maximum lighting limits recommended for a particular object.

In order of priority, the selection criteria for light sources used to illuminate works of art are:

• The colour temperature (°K) and the spectral distribution of the light source, especially the wavelengths of ultraviolet radiation and infrared radiation.
• The average life expectancy (total operating hours) of the lights used.
• Lighting efficiency (lm/W) of the lights used to assess energy consumption and thermal loads in the environment.

Regarding new lighting systems, technological development has placed a whole host of lighting equipment on the market. Some existing products have been modified or improved, and some new systems have been invented. Such modifications, improvements or inventions are always viewed from the angle of applying them to works of art, thereby making their exhibition and public display compatible with their conservation.

Regarding discharge sources, it is now possible to regulate fluorescent flux. Being able to illuminate display cabinets by regulating the luminous flux between 75% and 90% is no longer a problem. This procedure also provides extra advantages: a reduction of dissipated heat, better chromatic reproduction of the sources using high-frequency ballast and frequency emissions that impede the development of a potential microbiological attack.

Regarding traditional incandescent systems, in the last few years new methods have begun to be used to channel and distribute light energy through systems that employ the physical characteristics of some materials. Those systems are called "Light Guides".

There are different types of light guide now on the market:

• Longitudinal emission, of which there are two types: hollow and fluid light guides. Both use the principle of optical diffusion, but it is very difficult to get a directional ray of light with them.
• Fibre-optic transport and emission systems (puntiform sources). These systems offer a high degree of manageability, but the disadvantage they have is that they cannot carry large luminous fluxes. The emission angle is concentrated. As a result, to achieve an acceptable degree of uniformity, display units or cabinets must be large and the number of optical fibres used must be very high. When lighting three-dimensional objects, it produces many annoying shadows and uniform illumination cannot be achieved.

As you can see, a much wider range of light sources is available to us, but opting for one system or another is difficult. Some will solve certain problems yet accentuate others.

A knowledge of lighting problems and a detailed study of conservation requirements has allowed a system to be developed that today represents the specific answer to such demands. That system is the "prismatic light guide".
The studies, research and experiments done by specialists from different sectors have led to the design of a product that offers the following advantages:

• It solves the conservation requirements of lighting since ultraviolet radiation is completely eliminated and the thermal load is reduced by more than 90%, thus making it a source that is practically cold. Consequently, works of art can be subjected to higher levels of lighting than those set out in international standards without damaging them.
• It eliminates dazzle, whether primary or secondary, because the emission angle can be controlled. That allows us to illuminate only the object in question and not the onlooker.
• The light source can be restricted in such a way that the display environment (display cabinets) is illuminated on the outside only, and the performance is higher than other lighting systems used so far.
• Finally, cold halogen light sources (dichroic) can be used, thereby eliminating up to two-thirds of total irradiated energy. That prevents the build-up of heat inside display cabinets and eliminates the phenomenon of localised dehumidification, which could harm some materials that need a perfectly stable thermohygrometric balance.

This type of lighting is based on the transportation of light energy through an optically stable material (PMMA, polymethyl methacrylate), which has the highest possible refraction index, in order to use the phenomenon of total internal reflection, thus obtaining high emission angles and a high level of lighting efficiency.
(Text taken from "La iluminación de un bien cultural" by Raniero Baglioni, Boletín del Instituto Andaluz del Patrimonio Histórico, No. 23, June 1998)

So, for the exhibition of works belonging to temporary exhibitions that are shown in our rooms, and for those belonging to the Collection, there are three main types of artificial light source or spotlight available:

1. Eclipse QR 111 by ERCO. Reference number 75 326 (50W / 12V - with a UV filter. Parabolic light by OSRAM) / adjustable intensity.
   
   
2. Eclipse QT 32 by ERCO. Reference number 77 750 (150W/m - with a silver aluminium reflector).
   
   
3. 500W halogen projector by CAR-IBE / it does not have a UV filter. With a 300W wolfram halogen lamp by OSRAM.
   
   In addition, as auxiliary means, we have two other models:
   
   
4. ighting using fluorescent tubes in transit zones and access to rooms: OSRAM L36W (11-860) LUMILUX PLUS (Tagesright-daylight / Germany).
   
   
5. Lighting in storerooms and the restoration workshop: PHILIPS 'TL'D 58W/93 or (90 lux). For natural colours.

The final type of lighting we have is of a natural character, through a curtain-wall that illuminates the hall and the access ramps to the rooms. This source of lighting must be filtered to prevent excessive ultraviolet exposure (it is worth recalling that ultraviolet emission through glass is up to 6 times more intense than the ultraviolet emission from a wolfram lamp).

The curtain wall currently comprises Climalit Glass (6+6+12+8) with the following technical specifications (%):

The ideal filter for absorbing ultraviolet radiation must not transmit radiation with wavelengths shorter than 400 nm. In turn, it must allow all radiation of the visible part of the spectrum to pass through it (ICE VE Perspex and other brand names such as Plexiglas 201 or Oroglas UF3 are the recommended acrylic filters, which can be defined generically as polymethyl methacrylates).