Case Study City Museum Münster - LED Relighting

Spotlights & showcase retrofit

Category: Arts & Culture, Specials

LED Lighting in Museums - The Art of Lighting Art

The City Museum of Münster has a large inventory of different spotlights that has grown over the years. Due to the EU lamp ban, replacement lamps required for the spotlights with halogen lamps used to illuminate the exhibitions are no longer available. In addition, the high energy consumption and high maintenance costs due to the short life cycles of the analogue lamps were decisive for a project to convert to sustainable LED lighting.

The people in charge of the museum asked a question that is currently being discussed in many museums: to what extent can the existing spotlights, which in principle still do their job well, be retrofitted and inexpensively with so-called LED retrofit lamps without having to make serious compromises in light quality or functionality. In a compact project scope, KOBER lighting designers were commissioned to carry out a corresponding analysis.

The basis for the photometric and electrotechnical investigation was the following jointly developed requirement catalogue:

  • Very high demands on color rendering with CRI higher than 90
  • Similar light distributions and beam angles after the LED conversion
  • Clean light beams without streaks, artefacts or color-over-angle effects
  • 3,000K light color with a maximum of 2-3 SDMC deviation
  • Flicker-free dimming with on-board dimming or trailing edge dimming up to 50lx
  • Long service life and significant energy savings
  • Compliance with the conservation requirements regarding UV and IR damage

Initial research quickly showed that there are very few LED retrofit lamps on the market that are even suitable for the requirements of museum lighting. The majority of retrofit lamps are designed more for private use. Tests and trial lighting were then carried out in the original existing lights using selected retrofit LED lamps from well-known manufacturers.

The photos attached above show some of the test results, which are summarized in the following section:

Mechanical Compatibility
Almost all of the retrofit lamps tested fit well into the sockets of the original halogen lamps, so that the electrical connection was intact. Unfortunately, the LED technology requires a little more space for the electronics and cooling of the circuit boards than the analogue, original halogen lamps, which means that all retrofit lamps are longer at the front than the existing lamps and thus protrude from the spotlights, or the installation of accessories, filters or cover glasses was only possible to a very limited extent.

Color Over Angle
The Color Over Angle phenomenon does not only exist with LED retrofit lamps, but with LED light sources in general. The background to the "Color Over Angle" is the so-called Lambertian radiation of the luminous flux from the LED chips at a spatial angle of mostly 180 degrees, whereby the light rays hit the converting phosphor layer of the LED light source at different angles and thus the luminescence layer shows different light colors converts the coverage area. As a result, all but one of the tested LED lamps showed a color gradient from cool/bluish light to warm/brownish light at the edges from the inside to the outside of the light cone. This color gradient is particularly disruptive for a homogeneous illumination of large-scale paintings. For some applications, however, also acceptable to a certain extent.

Color location deviations
LED chips have a certain production spread in relation to the light color emitted. This deviation is evaluated with the so-called SDCM value. SDCM stands for Standard Deviation of Color Matching. Put simply, the SDCM value is a measure of how much the light from an LED deviates from the nominal color temperature or light color that is promised in the data sheet. Of course, especially when using many lights, the scattering of the color locus deviation from emitter to emitter must be as small as possible. Manufacturers of integral LED lights (i.e. e.g. spotlights that have been developed from scratch for LED technology) or LED lamps can counteract this deviation by using a higher quality, narrower selection of LED chips (so-called bins). In general, SDCM deviations of up to 2 step MacAdams ellipses are not or hardly perceptible to the human eye and correspond to the highest museum quality. However, the LED retrofit lamps tested were more likely to have a deviation of 3 to well over 5 SDCM for lamps of the same type and thus had very high and clearly visible production-related color location deviations. It turned out that the retrofit lamps can only meet the high museum requirements for certain applications.

Damage Factor
The damage factor of the light refers to the emitted UV and IR radiation, which can cause more or less severe damage to the exhibits. Paintings, photographs and material exhibits in particular have the highest conservation requirements. Unfortunately, it turned out that the manufacturer's information on the damage factor for the tested LED retrofit lamps was only rudimentary or that it was too high compared to high-quality integral LED spotlights. With high-quality, state-of-the-art LED spotlights, for example, damage factors can be achieved much better than with previous halogen lamps.

The dimming technology installed in the museum's existing spotlights is designed for dimming halogen lamps. The dimming of LED lamps requires other electrotechnical principles if they are to be flicker-free and you want to dim down far. Here, too, it was shown that the old spotlights fitted with modern LED retrofit lamps could not achieve satisfactory dimming results.

Conclusion of the investigation
The results show that the use of LED retrofit lamps in existing spotlights only makes sense under certain conditions with a few special lamps. With this approach, however, subsets of the existing lights can still be used, so that the otherwise sensible replacement of the analogue halogen spotlights with special LED museum spotlights with integrated LED technology does not have to take place all at once. Here it is important to find the right strategy of retrofitting and completely replacing the existing lighting for each case.

After analyzing the spotlight lighting, the museum Münster now uses the same procedure to convert the display case lighting to LED.

Further case studies here:
Museum Draiflessen Collection
Museum Folkwang
LED Museum Workshop