Increasing awareness of the need to understand the mechanisms of deterioration prior to any conservation intervention has led inevitably to assessment of the impact of the surrounding environment. This is especially true for wall paintings, and other artefacts found in situ within historic monuments, where there is often a limited potential for the control of environmental parameters such as temperature, relative humidity and the exposure to solar radiation. Environmental monitoring of these parameters is aimed at the systematic gathering of information regarding both quantitative assessment and the determination of the patterns of change over time. Although much progress has been made in many areas of environmental monitoring, the difficulties in measuring sunlight and its exposure across building surfaces has limited its inclusion within the majority of monitoring installations.
Advances in digital imaging technology have proven extremely useful within conservation for examination, analysis and recording of both specific conditions and of dynamic processes of deterioration. Digital recording provides information in the form of images composed of a two-dimensional array of numerical data which can allow for extensive examination and manipulation without any permanent alteration to the original image. The following project involved the development and testing of a digital-image based system specifically aimed at the monitoring and measurement of solar radiation on painted architectural surfaces.
As a preliminary, the effects of solar radiation on wall paintings were reviewed, including photochemical reactions and indirect effects such as changes in temperature and relative humidity. Current methods employed for the measurement and monitoring of light levels in conservation were then assessed in order to establish criteria for a system which could record both quantitative values and their distribution across the surface. The selection of the final system÷comprising a colour single-CCD camera and manual zoom lens linked to a PC computer through an RGB frame grabber÷followed careful examination and comparison of the commercially available equipment based on the selection criteria.
Preliminary testing of the system was undertaken in the studio under controlled conditions at which time the various camera functions and recording methods were assessed, including the calibration of the recorded image values into incident lux levels. The system was then installed and tested for one-week periods on two sites with important wall paintings: the Chapter House at Westminster Abbey and Holy Trinity Church in Coventry, each of which presented a range of different conditions and constraints typifying many of the challenges of in-situ monitoring. Decisions regarding the set-up of the equipment and the monitoring interval were based on extensive preliminary examination of each site and the development of a site-specific risk assessment.
Post-acquisition examination and analysis of the image data utilised standard commercially available spreadsheet programmes and customised image analysis software, and presented a wide variety of techniques available for the visual assessment and interpretation of the image data. Specifically, software developed at the National Gallery for the VASARI project was used to carry out the calibration conversion of camera recordings into incident lux levels. Total incident lux-hour values were calculated based on data from a single day of recording at Westminster Abbey which indicated exposure values of approximately 6,500 to 7,500 lux hours. While these values are some 5 times greater than the amounts generally recommended for paintings in gallery spaces, they cannot be taken as absolute or indicative of exposure over the entire calendar year.
In addition to successfully demonstrating the feasibility and useful potential of digital-imaging equipment for on-site measurement and monitoring of solar radiation, the project highlighted a number of areas for further improvement within the selected system such as increased sensitivity of the CCD, the use of auto-iris lenses and a timer-based interval programme. The project also offers many avenues for further study including waveband specific monitoring and the combination of image recording and time-lapse video. Most importantly, it has become clear that increasing our knowledge of the characteristics of solar radiation and its interaction with painted surfaces through direct study is vital to establishing links between exposure and specific deterioration.