LAPD’s challenge on this project was to reflect sunlight down into a 3- storey atrium in London. Therefore we ensured that as long as the sun was out, it would be reflected on to the basement floor 20 metres below, even on the shortest day of the year.
To continually reflect sunlight requires an intelligent sun-tracking mirror, called a Heliostat. These large mirrors continually reflect the sun towards an object or area, provided there is no cloud cover.
It was important to conduct investigations as to the feasibility of achieving uninterrupted sunlight throughout the day and year. For the site in question LAPD contacted the Met Office, comparing sunlight hours in various Heliostat sites across Europe.
The next stage was to investigate any obstructions around the site in three ways:
Firstly to check all building elevations of the proposed site. This would ensure the surrounding buildings would not obstruct the sun at its lowest elevation.
Following this, LAPD conducted site visits to view the existing surrounding buildings from the rooftop location. This ensured that none of the towers in the vicinity would have an effect on the possibility of sunlight reaching the site location.
Finally, checks with the local council established the locations proposed high-rise buildings at planning stage in the area.
The Feasibility Stage:
A heliostat needs a target in the form of a secondary mirror to direct light to areas inside the atrium. This can be one single mirror or multiple mirrors to light multiple locations. The need to capture all of the light from the primary mirror drives the sizing of the secondary array. The more mirrors used the smaller they become to stay within the maximum size limit.
Instead of creating single patches of light on the basement floor LAPD wanted some of the mirrors to produce the effect of dappled light through trees. Therefore, mirrors with multiple facets achieved this effect.
To ensure the convincing delivery of the dappled light effect, LAPD requested construction of mock up mirrors. These allowed LAPD to vary the angles of every facet within the mirrors to create a footprint of light of a suitable size with a random spread of ‘dapple’. We shone light from these test mirrors, at the correct offset, on to LAPD’s office building. The square mirrors reflected shapes of light very similar to the dapple of sunlight through trees. Finally, the manufactured mirrors replicated these angles.
We arranged the secondary mirrors in such a way that they had 9 mirrors in total. 5 of these are faceted to reflect dappled light across different areas of the floor.
LAPD wanted the other mirrors to be flat because they had an alternative purpose within the atrium.