Light sources in exhibition design – part 1

Light sources in exhibition design | Part 1

Artificial lighting plays a crucial role in exhibition design. In the following articles, I will describe the most important light sources, sorting them by the technology used to produce the light flux, and trying to explain what makes a light source more or less suitable for some specific application related to exhibition design.
I will consider six types of lighting sources:

–        Halogen lamps including dichroic lamps and PARs
–        Fluorescent lamps (linear and compact)
–        Metal halide lamps
–        LEDs
–        Electroluminescent lights
–        Fiber-optic systems

Introduction
The choice of one type of lamp over another depends on many factors. Lighting a Ferrari car is hardly comparable to lighting a Van Gogh painting. Nevertheless, there are families of lamps more suitable for some applications than others. For example, fluorescent lamps are usually unfit to illuminate paintings due to their peculiar spectral distribution, while incandescent lamps (including halogen lamps) are far from ideal when reduced power consumption should be taken into consideration. To cut a long story short, there are no fixed rules on lighting, other than collecting as much data as possible, clearly defining your and your client’s objectives, and eventually take the correct decisions and adopting the most appropriate design strategy. To help such a process, I provide here an overview of all the most diffused types of lamps adopted in exhibition design, describing their characteristics, their pros and cons, and their most appropriate applications.
Nevertheless, I must point out that there is no such thing as a “perfect” light source; designers and architects should look for the “most suitable” solution for their specific needs, instead. For example, I often read about some kind of lighting system that is suspected to “potentially damage” the artworks on view in a renowned museum. This is not a correct point of view, in my opinion. The simple action of illuminating an artwork always changes it to some extent, if you don’t want light (natural or artificial) to accelerate the fall of a fragile piece of art down the hill of entropy, you should keep it in a sealed lead container. But, if someone decides to publicly display it, the only correct approach is to provide the best possible lighting conditions, including conservation as a fundamental design factor.

A preliminary note: the CRI rating
In this article, I will use terms such as “color accuracy”, “color rendition” and the like.
I should point out that, since the early ’30s, the CIE – International Commission on Illumination had developed a tool, the Color rendering index (CRI), aimed to measure how well a light source renders colors.
The CRI, which is frequently used by manufacturers to rate the color performances of their lights, compares the color rendition of a light source to that of an ideal black body with the same color (or Light Color Temperature, to be precise) and provides a rating ranging from very bad (0 and even negative numbers) to perfect (100). A light source with a CRI of 85 or more is considered suitable for installation in museums and art exhibitions.

I don’t use much CRI in this article; even if CRI could be a useful tool in home lighting, I don’t deem it acceptable in serious exhibition lighting for, at least, three reasons.

1) CRI compares the spectral emission curve of a light source to that of a black body with the same color temperature; lights that have a continuous spectral emission, such as most incandescent bulbs, usually get a CRI of 100 – since they can almost always be assimilated to perfect blackbodies – independently from their color temperature. Although a continuous spectral emission is a key factor in the color rendition of a light source, nevertheless, also color temperature matters. For example, as far as I know, there are no incandescent bulbs with the same color temperature as the reference daylight source, the Sun.

2) The CRI method is rather old and many of its theoretical basis and measuring procedures are widely considered obsolete, today, especially for light sources based on relatively new technologies, such as LED.

3) Technically, the CRI of a light source is measured on 8 samples of different colors (Light greyish red, Dark greyish-yellow, Strong yellow-green, Moderate yellowish-green, Light bluish green, Light blue, Light violet, and Light reddish-purple) with 6 additional colors used for comparative purposes. Therefore, a 100 CRI light, which would perform perfectly with those samples, could not be as good when different colors are involved. Although this limitation can be considered of marginal importance in domestic lighting, it could become a problem in exhibition lighting, especially in extremely color-critical situations, such as the lighting of, say, an egg tempera painting on wood. 

Incandescent halogen
Halogen lamps (or Tungsten halogen) are actually a sub-family of incandescent lamps since they also produce light by the photon emission from a hot filament (usually made in tungsten); but due to the addition of halogen gas inside the bulb, they reach a higher luminous efficacy and longer working life, compared to traditional incandescent bulbs.

Traditionally, Halogen lamps have been widely used in exhibitions because of the chromatic quality of their light; they have typically a color temperature between 2800 and 3000 Kelvin degrees, and a “natural” spectral distribution (which means they produce a quite continuous spectrum) that make them ideal to illuminate paintings, sculptures, woodworks, textiles, musical instruments and virtually every object which requires a high chromatic accuracy. I must point out that on the market other types of lamps are available claiming to produce a “warm white” light (usually corresponding to a color temperature around 3000K), nevertheless, since their spectrum is often different from that of a halogen lamp, they can produce an unnatural “greenish” tint (due to emission peaks at specific wavelengths).

Halogen and fluorescent lamp spectra compared. Note the emission peaks at specific wavelengths typical of fluorescent lamps

Halogen lamps are available in a broad range of different powers, usually between 75 and 10,000 Watts, with a typical luminous efficacy of 20 lumens per Watt; the theoretical working life of a halogen bulb is  2000 hours on average, but in real life, it could be much shorter. Virtually all linear halogen lamps require a 110 or 230 volts supply, while dichroic lamps could have different requirements (see the next chapter). A useful feature of many light fixtures based on halogen bulbs is the capability to support a dimmer, providing fine-tuning of the light intensity. Since halogen bulbs are always a relevant source of UV radiation, they often require a filter, usually made of quartz glass. Another precaution is to keep the halogen bulbs at a sufficient distance from the “target” objects, as well as to prevent people to touch them since their surface temperature can easily exceed 600°.

When natural colors and flexibility are necessary, halogen lamps are often the best solution, although often replaced by LED lamps, which are somehow more practical but usually at the cost of slightly lower lighting quality.

A 300W wide-beam linear halogen spotlight used to illuminate a large graphic panel

Halogen Dichroic lamps and PARs
Dichroic lamps are a sub-set of halogen lamps and offer the same light quality; in dichroic lamps, a halogen bulb is coupled with a small glass or aluminum reflector, obtaining a sort of compact directional light with a beam usually comprised between 10 and 60 degrees, depending on the model. To be precise, only lights provided with a dichroic glass reflector can truly be called dichroic, but the name is commonly used for all similar solutions.  Halogen dichroic lamps are available with an electric power ranging from 20 to 50 Watts for 230/110 and, more often, a 12 V power supply. They are widely used for accent lighting on relatively small objects, such as paintings and sculptures, and are often mounted on rails in large galleries. Lamps that are particularly suitable for exhibition purposes, such as Philips Mastercolour or  GE ConstantColor, are always equipped with UV filters and provide a very well-balanced spectral distribution. Dichroic-based fixtures are often smaller and less intrusive than other solutions, including linear halogen-based ones. A minor drawback in permanent exhibitions is the need for a higher number of lamps, if compared to more powerful linear halogen, and consequently a much higher bulb replacement rate.

PAR (Parabolic Anodized Reflector) halogens are substantially similar to dichroic lamps, but are often larger, with a different reflector, and available also with higher electric (and lighting) power (typically from 30W up to 300W and more); their application range is substantially the same of dichroic lamps.

Track-mounted directional lights, based on dichroic lamps, in an art museum (the Musée Chagall in Nice) and a Science one (the Muse in Trento)

Directional lighting based on dichroic lamps used to spotlight musical instruments, tall light fixtures are equipped with a satin glass diffuser, also functioning as an additional UV filter

Track-mounted PAR lamps used to spotlight Roman findings in an archaeological museum (the San Lorenzo Museum in Cremona)

Light sources for exhibition design – part 2
Light sources for exhibition design – part 3