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Safety on site: Light level survey in Iron Age roundhouse

Thursday, 22 March, 2012
Steve Burrow
Penny Hill
Number of Participants / Visitors / Audience: 
This survey was designed to assess light levels within the Moel y Gerddi roundhouse, in order to understand better how the house functioned as a visitor experience, and also to provide a baseline for study of light levels in the new, Bryn Eryr roundhouses due to be built in 2013.

The Moel y Gerddi roundhouse at St Fagans was constructed by Peter Reynolds in 1992. It was built in a woodland clearing surrounded by high trees at a distance of 12m to the south and southwest, extending to 24m to the west. To the north the house is 2m from the Moel y Gaer roundhouse. The immediate landscape is more open to the east.

The Moel y Gerddi house has wattle and daub walls and a thatch roof, with the only points for light ingress being the two opposing door ways on the northeast and southwest sides (100cm wide x 130cm high) – neither of which have doors – and in the 20cm gap between the tops of the walls, and the overhanging thatch. Artificial light is provided by a fire lit in a central hearth, although this is managed to provide heat rather than bright flame. On warm days the fire is not lit at all. The roof space is smoke blackened, the walls are whitewashed, and the floor is maintained as bare compacted earth.

The interior of the house is kept relatively open on its north side. The only furniture is a line of benches arranged between the inner ring of posts. The south side is segmented into a series of bays delineated by wattle panels, some are used for storage, others for display of items. These wattle panels block the light from the doorways making the bays very dark inside.

Throughout its life Moel y Gerddi has been used as a school classroom during term time with an interpreter leading classes from a seat beside the hearth while the children sit on the benches. At other times the house is accessed by general visitors some of whom stand and some of whom make use of the benches. Most visitors enter and leave via the southwest doorway, although both are always open.

The darkness of the house’s interior is often commented upon, particularly on bright days when the contrast between the interior and exterior light levels is especially pronounced. And in the past re-enactment groups have noted the difficulty of performing basic tasks like food preparation when visitors are blocking the light from the door ways. In addition, the wattle panels that run from the inner post ring to the outer wall on the south side of the house serve to block light from entering the storage and display bays. This has led to these areas being poorly maintained and becoming an unintended dumping ground for infrequently used items and general rubbish.

Weather conditions
The survey was conducted over one hour from 14:00 on 22 March 2012. No fire was lit in order to provide a “worst-case scenario” for light levels inside the house. Throughout the survey the sky was overcast, providing around 8,000 lux.

Equipment and analysis
The survey was conducted using a Velleman DVM401 4 in 1 digital multimeter. This multimeter included a basic lux meter with a recording range from 0 – 20,000 lux and an accuracy of +/-5% of readings.

Readings were taken at regular intervals throughout the roundhouse with each reading being repeated twice. Both sets of readings were intended to simulate light levels experienced by an individual while sat reading or carrying out basic work. The first set recorded light levels experienced while facing the centre of the roundhouse and concentrating on material held on the lap. The second were taken with the lux meter positioned to capture the maximum amount of light from whichever doorway was closest, as though the individual had deliberately positioned themselves so their work caught the most light. These two sets of readings are titled here “lap work” and “maximum working light” respectively.

Data was recorded manually onto a plan of the roundhouse and was then digitised into ESRI’s ArcView 3. Subsequent visualisations of the data were also produced in ArcView.

The results of the two surveys (lap work and maximum working light) are presented with a lux scale which reflects ambient light levels.

Night (0 – 1 lux)
Twilight (10 lux)
Dark day (100 lux)
Overcast winter’s day (5,000 lux)
Full daylight (10,000 lux)
Sunlight (100,000 lux)

Fig 1. Light level survey showing maximum values which could be obtained from each location (scale in lux).
The funnelling of available light through the doorways and towards the centre of the house is apparent in the illustration, and just 2m into the inside of the house, light levels have dropped to the equivalent of “twilight”. Around the sides of the house light levels are lower still, with the readings on the south side (which is mostly used for storage) being close to 0 lux. The north side, which has less furniture and white walls, is slightly lighter, but even here readings fall as low as 1 lux in places.

Fig 2. Light level survey showing values which could be obtained for "lap work" at each location (scale in lux).
The fall off in light levels away from the doorways is even more apparent when the light levels available for lap work are considered. Away from the doorways, light levels are generally 10 lux or lower, with almost the entire south side of the house reading 1 lux or lower.

Experience has shown that it is perfectly possible to perform manual tasks within the house (for example working with a drop spindle) despite these light levels, so there is no reason to doubt that the house was functional from an Iron Age perspective. It should also be borne in mind that the house would also have benefitted from fire and possibly lamp light.

Nonetheless light levels are far below the working levels recommended by the CIBSE (Chartered Institution of Building Services Engineers), who note that areas where reading and writing is undertaken should be lit to 500 lux, and where less demanding visual tasks are performed, eg, filing, 300 lux is acceptable. None of these tasks are relevant to a prehistoric house, but the levels provide an indication of how much darker the roundhouse is than are our 21st century environments.

The darkness is of course a part of the visitor experience, but it does have implications for the value of the house as a display space. Small and detailed displays are unlikely to be appreciated, even after the eyes have become accustomed to the dark. The extent to which visitors acclimatise will also vary according to age and other health factors.


Maximum light levels at each location in the...
Light levels for "lap work" at each location...