Measurements of soiling and colour change using outdoor rephotography and image processing in Adobe Photoshop along the southern façade of the Ashmolean Museum, Oxford
Mary J. Thornbush, 2010. "Measurements of soiling and colour change using outdoor rephotography and image processing in Adobe Photoshop along the southern façade of the Ashmolean Museum, Oxford", Limestone in the Built Environment: Present-Day Challenges for the Preservation of the Past, B. J. Smith, M. Gomez-Heras, H. A. Viles, J. Cassar
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This paper builds on work using Adobe Photoshop as image-processing software to obtain (histogram-based) quantification of camera-captured images. The analysis tracks cross-temporal surface colour change (in 2005 and 2007) at the façade-scale of a limestone building (cleaned between 2006 and 2007) by means of digital photographs obtained in repeat photographic surveys taken under different outdoor lighting conditions (of a clear sky v. overcast). The relevance of the study is to contribute to further research using the integrated digital photography and image processing (IDIP) method in an outdoor setting (O-IDIP) with differing levels of light at a façade- (building) scale necessary for assessments of soiling affecting decisions of maintenance and restoration. Calibration was performed using spectrophotometric data (acquired in the winter of 2006). Findings show that the calibrated method is able to measure change before v. after the cleaning of the southern façade, which was darker in 2005 (with a lower level of lightness), especially at the east elevation. Lightness (surface darkening or blackening) is more affected by outdoor lighting conditions than chromatic values along green–red (a) and blue–yellow (b) channels of colour. These findings confirm that there is more error associated with soiling measurements at the façade-scale (in an outdoor setting).