V&A’s Medieval & Renaissance Galleries. The Renaissance City 1350-1600 © V&A images. Credit: www.alanwilliamsphotography.com

New advances in daylighting

Stephen Cannon-Brookes, 17.01.2011
Museums are taking a renewed interest in allowing natural light into galleries. Stephen Cannon-Brookes looks at advances in predicting daylight levels
With pressure to reduce energy consumption and maximise the passive performance of buildings, there is renewed interest in the potential for using more daylight in galleries. This is reinforcing the view that the presence of daylight offers psychological benefits to visitors.

Recent projects, such as the Medieval & Renaissance Galleries at the Victoria and Albert Museum (V&A), have shown that daylight can be used responsibly and with less need for complex and often unreliable systems of active control, such as motorised louvres or blinds. The technique that has allowed daylight to be used with more confidence is not based on building technology but on predictive modelling of daylight in buildings.

Contrary to many sources of guidance on museum lighting, it is not possible to accurately predict the daylighting performance of interiors using analysis tools available, such as the “daylight factor”. This has left a substantial gap between the aspirations of designers and building owners and the low levels of daylight targets proposed by Gary Thomson in the 1960s and now widely adopted in museums and galleries.

These targets are understood in terms of recommended maximum annual lighting dosages, which serve to relate current display practice with the expected life of light-sensitive materials. These dosages are quoted in terms of lux hours of exposure and it is up to those responsible for collections to decide on the duration/levels of illumination in order to avoid over exposure.

Finding a balance between the two is considerably easier with electric lighting, which can tuned and remains largely static, but is a far more difficult task with a varying light source such as daylight. Use of the dosage recommendations allows a range of daylight levels to be acceptable and – given the experience of trying to control daylight to fixed levels over the past 30 years – most institutions now allow daylight to fluctuate in galleries.


CLIMATE-BASED DAYLIGHT MODELLING

The challenge for designers has, until recently, been to predict how much daylight will be present in an interior without either monitoring for a year in existing buildings or creating full-size mock-ups. With the development of climate-based daylight modelling (CBDM) there is now a means to predict dosage.

CBDM is computer-based analysis of daylight in interiors that uses realistic sky and sun conditions to accurately simulate the amount and distribution of light. This type of modelling is founded on standardised climate files and is location specific. It enables prediction of:

• Absolute values of illuminance (overall light levels) and luminance (light levels within a specific area)
• Time varying and cumulative light levels i.e. dosage
• Duration of periods when daylight levels exceed or does not achieve user-determined targets
• Field of view luminance, which is related to brightness of surfaces
• Prediction of photosensor response

Taken to together this permits a “holistic” evaluation of daylighting combined with solar shading, which has not been possible with earlier techniques. It will greatly support the design of interiors where daylight is admitted and will encourage the development of daylighting control strategies based on passive low-risk solutions.

IMPLICATIONS

The use of CBDM will allow the daylighting design of interiors for the display of light-sensitive objects to develop beyond the current mixture of simplification and inferred results. Reliable prediction of daylight dosage will raise confidence that galleries are not over-lit and re-establish the intuitive link between appearance and performance.

It will also mean that greater care is needed in the selection of finishes to walls, floor and ceiling and encourage designers to work more actively to improve the passive performance of daylighting. This should reduce reliance on active systems, which many museums have such painful experience of, and by implication risk of over-exposure.

With this technique in place, the design focus when using daylight in display environments is likely to switch away from exposure control to the visual appearance of displays and issues such as avoidance of reflections and glare, a preoccupation for designers before the introduction of electric lighting.

Click here to read the State Hermitage Museum in St Petersburg daylighting case study

DAYLIGHTING

Stephen Cannon-Brookes is director of Cannon-Brookes Lighting & Design. Click here to email Stephen Cannon-Brookes. Alternatively, he can be contacted on +44 (0) 208 964 8300.

For more information on daylighting, you can also contact John Mardaljevic, a reader in daylight modelling at the Institute of Energy and Sustainable Development at De Montfort University on +44 (0) 116 257 7972. Click here to email him.

Comments

Sort by: Most recent - Most liked
07.10.2015, 11:43
Dear Stephen Cannon Smith

Is it possible to view the diagrams referred to in your 2000 paper - Using Daylight in Display's?

Nick Baker