Daylight is fundamental for visual comfort and indoor climate comfort in buildings. Interiors with plenty of daylight appear to have a positive influence on the occupants’ environment.
However, in some cases daylight surplus might cause problems for specific buildings. It is in the case of historical monuments where light causes damage of light-sensitive materials and artefacts.
In this case, a light control strategy must be considered. The article is focused on evaluation of potential light damage in an architecturally valuable interior.
Light environment was analysed in a hall of a historical protected monument of the chateau Hluboka located on the river Vltava in Czechia.
Development History Of Chateau & Morning Salon
The historical monument was originally founded as a castle in the middle of 13th century on the west bank of the river Vltava as a royal residence. In the 16th century it was rebuilt into the Renaissance chateau.
The modernized chateau architectural style was influenced by Windsor Castle in England because the then owner, Prince Jan Adolf II Schwarzenberg, was an admirer of England culture.
The reconstruction exterior and interior works started in 1840 based on the plans by architect Franz Beer and were finished in 1871 with Damasius Deworetzky, designer of the chateau’s magnificent interiors. The chateau is surrounded with a green park in the English style, and sculptural scenery.
Today the chateau is listed among historical buildings kept by the Czech National Heritage Institute. A part of the chateau is open for public tours. The assessment of the microclimate environment of specific interiors was part of a project supported from the program for applied research and development of national and cultural identity (NAKI) in 2011.
After a time-consuming and financially demanding construction and restoration works preceded by a meticulous archive research, nine of the guest rooms were restored to their original appearance.
The chateau interior is decorated with woodcarving panels on the walls and ceiling soffits. The most valuable interior decoration is in the eastern residential hall. The hall, located in the eastern wing of the chateau complex, has valuable interiors with original furniture, unique walls and ceiling panelling and wallpapers.
It also contains valuable paintings. High levels and/or extended exposure to light can irreversibly damage sensitive materials. It is, therefore, important to assess whether this can be considered a problem in particular rooms.
On the other side, the residential hall is one of the biggest rooms of the first floor with the main windows south east in orientation.
Because of the orientation for solar radiation access in morning time, the hall is called the Morning Salon. There are three bay windows in the front peripheral wall and two glazed doors at the sidewalls close to terraces.
The salon has preserved original interior decoration from the 19th century such as wooden parquet floors and wall and ceiling panelling of carved chestnut, stylish furniture with high-quality textiles and valuable oil paintings.
The ceiling is faced with wooden coffered soffit, which is combined with valuable coloured wallpapers.
The Morning Salon has been selected for long-term indoor climate comfort controlling and daylight illuminance assessment. The main task is to evaluate the influence of the hall daylighting on light-sensitive materials and artefacts.
The eastern wing of the chateau is opened for public tours. It means that this part of the chateau is frequently visited. The Morning Salon has a specific location in the chateau. It is situated on the first floor in the east wing.
It is illuminated through side-lit bay windows and two glazed doors in side walls. Windows are not shaded by the upper floor overhangs. The internal space of the salon is overheated and exposed to high daylight illuminance because of excessive solar gains through the windows. Especially in mornings, irradiance and illuminance in the salon is very high.
The external wall with windows is opposite the internal wall with wood panelling and paintings. Windows have their original shading of external wooden lamellae shutters, and new internal curtains and paper-folded blinds, as well as textile roller blinds.
The use of internal blinds and curtains is the commonly recommended solution. However, the blinds are not permanently activated. For that reason, some windows may remain with the binds up.
Light Control Requirements
The interior illuminance level monitored in the salon was compared to the standard requirements for maximal daylight exposition of historically and architecturally valuable interiors.
Light as a form of energy has a potential to change materials. This energy causes changes, either through radiant heating or photochemical action.
Photochemical changes are irreversible and cannot be rectified by conservation treatment.
Light exposure to such light-sensitive materials as textiles and tapestries, paintings and interior decorations in historical buildings could result into their deterioration or even permanent damage.
Many factors influence the light exposition in practical situations, such as light sensitivity and life span of the displays, permitted time for the display and demands for the visibility and visual aesthetic effects.
The fundamental dilemma in lighting objects is visibility versus vulnerability. In many cases, indoor illumination on conservation illuminance levels cause poor viewing conditions.
Reciprocity law refers to the principle that an object exposed to low light level for extended periods of time will incur as much damage as if exposed to high light levels for brief periods.
Low illuminance levels over a long period of time can have similar effects as intensive light affection for a short period. That is why valuable light-sensitive artefacts have permitted exposition time and limited illuminance level.
Light level and time of the exposition is 50 lux as a minimum for lighting requirements determined for visibility to humans.
The maximal permitted illuminance level for the high light-responsive materials (as silk and highly fugitive colorants, old newspapers, etc.) is 50 lux.
This level is accepted as a lighting standard for museums. It is the minimum illuminance level required for most people to be able to observe the artwork. Values in lux hours are recommended, not only lux illuminance values.
The requirement for exposure of highly light-responsive materials and artworks is maximal illuminance. This relates to the acceptable amount of deterioration to the level of illuminance on the work and to its duration.
Light-sensitive artefacts can be exposed to 50 lux but in the overall annual time limit total light exposition does not exceed 150,000 lux hours.
The light level of 50 lux is recommended for adequate visibility. It means that in some cases a room with daylight shadings and artificial lighting dimming control is lit for example for 10 lux, but for visitors 50 lux is activated.
The mentioned 150,000 lux hour is a limit for the new responsivity category; it means that the object cannot be on continuous display.
Lower sensitive materials can be exposed to 200 lux, which corresponds to 600,000 lux hours per year. Also, the spectrum of light is important for the lighting control strategy.
Therefore, the above limits of illumination values are considered with the exclusion of UV radiation. However, the UV radiation needs to be taken into consideration in the conservation.
High-energy light spectrum near UV and visible light, mainly in its blue spectral band, has a negative impact on deterioration of light-sensitive materials.
Monitoring of indoor climate in the Morning Salon has been carried out for many years. Indoor temperature and relative humidity, as well as illuminance and UV radiation, is controlled.
Measurement sensors are located on the mantelpiece of the fireplace opposite the wall with windows, in the middle of the salon. Despite of the lighting control strategy, visible bleaching deficiencies and frail changes on furniture upholstery were found in the salon.
For this reason, continual daily illuminance control measurements were taken in period from July to October 2013. Illuminance was measured with a Hanwell ML4000 light sensor.
Illuminance data processing brought important information. Light exposure monitoring in October 2013 showed extremely high light exposition of about 80,000 lux hours.
Reasonable explanation of such high values seems to be negligent solar shading activation by the staff. This was confirmed by a test using Light Check light dosimeters, and after a 90-min time exposition very high values were found at the window jambs.
High light exposition might be very negative for light-sensitive materials and cause serious and irreversible change and deterioration of quality of the materials such as paintings or upholstery. This finding has led to demands for detailed study of the daylight level in the Morning Salon under different conditions in the annual profile assessment.
Complex measurements using data-loggers with light, UV and temperature sensors ELSEC 765C were done in May 2016. The purpose of the measurement was to find illuminance in two different places of the salon compared to the exterior illuminance.
Results of the continual measurements of illuminance (lux × 1000), UV radiation power and temperature are visible from the graphs that illuminance and temperature increase in the morning.
Higher illuminance rise is from about 9:00. Afternoon illuminance is lower because of the salon windows’ south-east orientation. The UV radiation power is greatest early in the morning at sun rise and in late afternoon during the sun set. It exceeds the limits of maximal permitted standards 75 μW/lm better 10 μW/lm.
It points to the fact that the Morning Salon windows should have solar shadings activated all the time in the morning and in the afternoon to protect fading and light damage of the salon interior.
Daylight Influence On Architecture
There is no architecture without light and there can be no building where the presence of natural light, either in part or as a whole, will not benefit those who use it.
The statement is absolutely truthful, and it represents one of topical ideas of building design. Buildings with plenty of daylight positively influence occupants’ well-being in an indoor environment.
However, in ancient and historical heritage buildings light exposition must be reduced as much as possible to maintain sufficient visibility but minimise damage of light-sensitive materials.
It appears reasonable that historical monuments and architecturally valuable interiors should have a controlled daylighting strategy.
The results presented above of measurements and daylight simulations show that light intensity level in the Morning Salon in many cases exceeds the illuminance permitted for conservation of light-sensitive materials.
The daylight illuminance is so high that it always exceeds the 50-lux limit. It is obvious from all simulation outputs. It means that large interior areas are at potential risk of light degradation, mainly in summer and transitional time.
Simulations for the 21 December also show high illuminance distribution for winter time. Mainly, the wall with paintings and textiles on historical furniture located in the middle of the salon are affected. Ceiling wallpapers are less damaged than expected.
It appears that mornings represent the period of the salon interior’s damage. The most sensitive for light damage are oil paintings on the wall opposite the windows, but original wall panelling and upholstery and textiles are also damaged.
The salon interior and furnishing need to be protected from the high light intensity and solar gain.
Excessive direct solar radiation causes glare discomfort and overheating of indoor climate. It is recommended that strict shading control is maintained.
As any automatic shading systems are not permitted in such a historically valuable place, the full-time activation of the rollers and shading blinds is recommended in mornings for the whole year, especially during summer.
Full-time activation of the shading blinds, especially in the morning because of the southeast orientation of the Morning Salon is also recommended.
Especially at sun rise and then until about 10:00 or 11:00 the Morning Salon should be protected by blinds at 100 percent full blind shading.
The Salon should be controlled by at least 75 percent shading activation at noon. In the afternoon, the shading can be reduced, bout the illuminance should be controlled all the time.
If possible, some ultraviolet filters which are known for conservation are recommended. Special protective coatings could be applied to protect the paintings against ultraviolet radiation bleaching.
If the historical value of glass panels allows application of special shading systems, these might be completed with transparent spectrally selective glazing.
Special glass is used for reduction of transmission of ultraviolet and infrared ranges of solar radiation into the interior. Switchable glazing could solve demands for reduction of daylight transmission in time of intensive solar shining and transparency for diffusive skylight conditions.
Nevertheless, selected glazing must not influence the architectural style of the historical monument. In special cases, advanced daylight guiding and redirecting systems can also be applied. All these special system applications must be consulted over with architects and historical monument preservation authorities.
Artificial lighting should also be controlled. Luminaires should contain light sources to supply light intensity for the interior visual scene.
Light sources free of the ultraviolet and infrared parts of radiation are preferred. A neutral white colour of artificial light seems to be convenient for the lighting the colourful interior.
Dimming of lamps is recommended in the response to the daylight access for energy efficiency and glare elimination.
Luminaires should be movable, to allow flexibility in the artificial lighting scenes in a mixture of direct and indirect light sources to provide uniform lighting with accentuating light spots directed on the artefacts and display items with respect to the protection against damage of light-sensitive materials for optimal balance between exposure and damage.