R E V I E W 33 night. For example, only one elevation of First Derry Presbyterian Church was illuminated – its main and most impressive elevation, which faces Magazine Street (see page 34). The illumination of key architectural features such as columns, arches and string courses may be sufficient rather than attempting to light the entire facade of the building. In the case of the Playhouse Theatre (see below) external lighting highlights the rhythms of the building’s composition while drawing attention to the details of the facade. The use of up-lighting was limited and well controlled to avoid light pollution. A number of other techniques were considered during the lighting trials but not ultimately used. These included back-lighting, which involves lighting the facade from within (for example by positioning lights inside a ground floor colonnade to silhouette the columns); and cross-lighting, which uses light sources directed across a building’s elevation from opposite sides. INTEGRATION OF EQUIPMENT All external fittings need to be unobtrusive, require minimum intervention during installation and must be easy to maintain without risk of damage to the building fabric. Painstaking work was needed to carefully weave wiring through and around historic structures to small well-concealed lighting sources. This was the case at First Derry Presbyterian Church, where the ashlar stonework did not lend itself to cabling and also at the Playhouse Theatre where pedestrians pass by in very close proximity to light fittings. The location of external light positions within the grounds of the cathedral was challenging given the close proximity of graves and tombstones right up to the cathedral walls. However, the presence of crenellated parapets at higher levels along the main roof structure provided ideal sites for light fittings and cable integration associated with illuminating the main cathedral roof and spire. LIGHT SPILL Light spill from adjoining buildings and surrounding areas always needs to be considered. More light is required to illuminate a building in a city environment than in a rural area where ambient light levels are lower. As was the case at the Apprentice Boys’ Memorial Hall (see page 32), it may be that existing lighting provision by statutory authorities will no longer be needed in proximity to a wellilluminated historic building. BUILDING SCALE AND SURFACE CONDITION As well as the existing ambient lighting, the reflectivity of the building surfaces must be taken into account. This will be affected by the colour of the building materials, and how clean their surfaces are. Buildings that are more reflective will require fewer, less powerful luminaires. In the case of the Guildhall, First Presbyterian Church and St Columb’s Cathedral the lighting designers had to take account of the fact that all three buildings had recently been the subject of substantial conservation works including facade cleaning. NATURAL ENVIRONMENT Historic buildings are often home to a variety of animals such as bats and swifts that can be affected by new lighting proposals. Wildlife habitat surveys were undertaken that considered breeding, hibernation, flight paths and points of access to ensure that the proposed lighting points did not have adverse effects either in installation or operation. LIGHT SOURCES Important considerations on the Walled City Lighting Project were the strength and type of light source, both of which are key to a successful scheme. Light source appearance can range from warm reddish tones through to cool blue tones. Colour rendering is described using the Colour Rendering Index, which ranges from 26Ra (poor, or least similar to natural daylight) to 90Ra (good). Proposed light sources must take into account the external finish of St Columb’s Cathedral (c1633) The Playhouse Theatre (c1877)
RkJQdWJsaXNoZXIy MjgyMjA=