Non-destructive methods for chromatic investigation of building façades

Transcription

1 241 Non-destructive methods for chromatic investigation of building façades Enrico DASSORI Professor, Department of Civil, Environmental and Architectural Engineering, University of Genova, Via Montallegro 1, 16145, Genova, Italy Renata MORBIDUCCI Dr., Department of Civil, Environmental and Architectural Engineering, University of Genova, Via Montallegro 1, 16145, Genova, Italy ABSTRACT The chromatic characterization of surfaces is used in many research and production fields (quality control of industrial product, effect of color on human behavior, new design, building restoration,...). In building restoration field the colorimetric investigation of solid elements is a current subject in which several problematic aspects have to be analyzed: large area characterization, analysis of chromatic alteration due to weathering or/and degradation processes,... [1-2]. In the present work some non-destructive methods for chromatic investigation of ancient building façades are proposed: spectrophotometric and spectroradiometric surveys of small surfaces (from 1 to 10 mm 2 ); chromatic characterization of wide area by means of spectrophotometer, spectroradiometer and/or digital camera (almost from 10 to 100 cm 2 ); simultaneous use of color data by spectrophotometer, spectroradiometer and/or digital camera to verify the influence of natural different lights on the perceived colors. Keywords: chromatic characterization, non-destructive methods, ancient building façades 1. INTRODUCTION In architecture the external façade is one of most important part of an designer idea. This part is characterized by materials, shapes,... and colors. The choice of the colors of a surface influences a lot the final result of a architectural project. This choice is composed by a large subjective part and an important objective part that it implies the quantification of the color characteristics. But in architecture in many cases the quantification of an objective color characterization is based on qualitative/subjective methods [3-4]; it clearly is a contradiction. From the architectural project of a new construction to the restoration of an ancient building the use of quantitative methods for the chromatic characterization of surface could be very useful to give those information about the quantitative part of the objective color characterization. Starting from this observation in the present work the application of some instrumental methods for the chromatic characterization of surface are discussed [5].

2 242 Colour and Light in Architecture_First International Conference 2010_Proceedings An useful preview to define a quantitative method for the colors surface survey is the knowledge of the actual methodologies for the experimental color measuring in every application field. Many instruments are used in illuminating engineering, optics, imagine reproduction,..., but only several of these are (and can be) used for a chromatic surface survey; in the present work the following instruments are considered: spectrophotometer, spectroradiometer, videoluminancemeter and digital camera. In particular the last one is analyzed to verify the possibility to transform it in an optical measuring instrument. The digital management of color has as main aim: the reproduction of an image with different electronic devices, maintaining as much as possible the chromatic characteristics of the real subject. The problem is that every electronic device uses particular instruments and properties to reproduce (in the case of a monitor or a printer) or to read (in the case of a digital camera or a scanner) the colors of the analyzed scene. In other words, the specific digital device translates the RGB or CMYK coordinates in a particular numerical language and the meaning of the used values depend on the single device (device dependent). The ambiguity of this kind of values is avoided through the association of them with device independent colorimetric values, this process is named device characterization. In a traditional camera the exposed light film surface is lied on a plane in front of the shutter. In a digital camera the film is replaced by a CCD sensor connected to an electronic device and to a software able to record and reproduce digital images. The sensor surface is comparable to a photographic emulsion: the pixel matrix corresponds to the emulsion grain, but the accuracy of the pixel matrix is superior. At the end of the exposition the imprinted image into the sensor is transferred to an integrated circuit as an electric signal. This is the digitalization phase, a process that it transforms the image signal in a neat numerical sequence of bits. Then this sequence is elaborated by different electronic devices and software to reproduce the original image. This sequence of digital elaboration generates a very detailed and numerous data bank [7-13]. 2. CHROMATIC SURVEY METHOD FOR WALL SURFACE WITH DIGITAL CAMERA Every surface can change its coloring if the light conditions change. Many other factors (often uncontrollable) can modify the sensation and perception of the observed colors. In building restoration field the colorimetric investigation of solid elements is a current subject in which several problematic aspects have to be analyzed: large area characterization, analysis of chromatic alteration due to weathering or/and degradation processes,... [1-2]. In the present work some non-destructive methods for chromatic investigation of ancient building façades are referred/proposed/ discussed: spectrophotometric and spectroradiometric survey method for small surfaces (from 1 to 10 mm 2 ) and for wide area (almost from 10 to 100 cm 2 ) are developed and tested in real cases [14]; chromatic characterization of small and wide area by means of spectrophotometer and digital camera are proposed and shown here; simultaneous use of color data by spectrophotometer, spectroradiometer and/or digital camera to verify the influence of natural different lights on the perceived colors are developed and tested [14]. In particular the methodology to transform a digital camera in a optical measuring instrument is proposed, the validation phase of the new instrument is shown and the application of the proposed method to a real case is presented. The idea to transform a digital camera in an objective measuring instrument for the chromatic characterization of surfaces is based on a simple concept: in the digital camera the charge stored in the CCD, is proportional to its radiance

3 243 Colour and Light in Architecture_First International Conference 2010_Proceedings on the basis of a non linear relationship. In the photographic processing field the HDRI (High Dynamic Range Image) method (i.e. see [15] ) allows to determine this non linear relationship with a numerical process. The method is based on the numerical construction of a complex image using many single digital images and collecting, simultaneously, the radiance values of many exposures. This numerical methodology permits to evaluate the relation between the electric signal and the corresponding radiance (calibration phase of digital camera 1 ). The preliminary phase for the HDR image determination is the light natural condition evaluation of the place in which the photos are made. In the present work the light natural condition are deduced by the use of the combined information of the spectrophotometer and spectroradiometer data of the same place [14]. The validation phase of the proposed method consists in several laboratory experimental tests on specimens with known chromatic characteristics. Colorimetric measures are made by different instruments: - Digital cameras (Olympus Camedia C-720, Nikon Coolpix 4300, Sony Cyber-Shot Dsc-V1) - Videoluminancemeter (LMK98-2 Techno-Team) - Spectroradiometer (CS1000-a KONICA MINOLTA) - Spectrophotometer (CM-2600d KONICA MINOLTA) The colorimetric measures are made on 11 colored cards that they partially reproduce the GretagMacbeth Color Checker. The used cards are bigger than the GretagMacbeth s cards because the Spectroradiometer analysis needs large surfaces (Fig.1). The chromatic data by digital camera HDR images are compared with the measured data by the videoluminancemeter and spectroradiometer: - The luminance values are acquire by the videoluminancemeter for every corresponding pixel and several digital images for different exposures and constant aperture, that is the HDRI method application (Fig.1); then the average evaluated values are calculated; - For every colored card the trichromatic components (X, Y and Z), the Fig.1 Images by videoluminancemeter: (a) measuring method; (b) example of luminance digital survey. Fig.2 Images by spectroradiometer: (a) measuring method; (b) diagram of the x, y coordinates; (c) diagram of radiance distribution. 1 The Debevec algorithm [15] is used for the calibration of the digital cameras. The characteristic curves of every digital camera is deduced by the analysis of 1000 points in every single digital image.

4 244 Colour and Light in Architecture_First International Conference 2010_Proceedings luminance values (L), the chromatic coordinates (x and y) and the radiance values for every 5 nm wavelength step are evaluated by the spectroradiometer (Fig.2). Finally the comparable measures carried out by the different instruments are compared (Fig.3): the digital cameras images are processed by a software developed for the present work (hdrsurface) that it applies the Debevec s algorithm. The software evaluates the average values of X, Y and Z that they can be compared with the same coordinates evaluated by the spectroradiometer. The comparable analysis of the different measures shows small differences (few unit per cent). Negligible differences are also present through the measures carried out with the different digital camera. But the main result is that the large number of tests gives a good validation of the proposed method. The following phase is the application of the method to a wall surface in a real place. The external tests are carried out on an ancient building (XVI century) in the historical part of Genova (Fig.3). Fig.3 The ancient building on which the chromatic survey is carried out. Fig.4 Scheme of the different chromatic surveys and their different purposes.

5 245 Colour and Light in Architecture_First International Conference 2010_Proceedings The ancient construction is chosen for the simplicity and conservation of many parts of pictorial surface decorations. In particular the footing part of two façades is analyzed where the decorations are clearly visible and on which it is easy to carry out the direct chromatic surveys. The colorimetric analyses are carried out with spectrophotometer, spectroradiometer and digital cameras: - the spectrophotometer is used to obtain colorimetric data independent to the natural light spectrum of small areas. - the spectroradiometer is used to obtain colorimetric data dependent to the natural light spectrum of larger areas comparable with the data from the digital cameras (Fig.4). The position and orientation of the building supplies a suitable environment to perform the tests, because the façades footing is never lighted by direct light. Preliminary phase for the colorimetric survey is the sunshine analysis to chose the best days and the hours for the surveys (Sketchup, [16]). The measures are carried out in October and in the middle part of the morning (9-12 am), in these period the main component of light is diffused: these kind of light conditions could be considered constant on all surface of the footing. The surveys are carried out in 30 different points for 5 times for every point (150 surveys). The points are chosen with different chromatic macroscopic characteristics (i.e some yellows, some reds, some greens,...). The recorded data by means of the different instruments are processed to obtain the chromatic characteristics in terms of colorimetric coordinates and diagrams of reflectance/ radiance: the measures with spectroradiometer consider the same large surfaces (7 different areas) analyzed by means of the digital camera HDR images. These area also contain the zones analyzed by spectrophotometer. The spectrum of the natural light conditions during the tests is evaluated by means of the same method used during laboratory tests. The same spectrum is used for the numerical processing on the HDR images. The HDR images are simultaneously recorded with the spectroradiometer data to maintain the same natural light conditions and thus to can compare the corresponding measures. The digital cameras images are processed by the same software used for the laboratory tests (hdrsurface). Also in this case the comparable analysis shows negligible differences through the results of the different instruments (Fig.5): the maximum differences are of 3 per cent; besides these differences are homogeneous without discordant measures from the average values. Fig.5 Example of the comparable analysis of the results obtained by spectroradiometer and digital camera: component X measures.

6 246 Colour and Light in Architecture_First International Conference 2010_Proceedings 3. CONCLUSIONS The use of a digital camera as color measuring instrument is innovative for several reasons. First, it is to note the potentiality of the photographic methodology that it permits to do chromatic surveys with commercial instruments avoiding some important problems of the specific instruments like a spectroradiometer: quantity and quality of data for large e very small area, simplicity and rapidity of measuring, possibility to carry out measures of inaccessible place, lower price, minor volume and weight,.... Besides the previous advantages there is a problem to solve yet: the use of digital camera for chromatic survey needs at least the simultaneously use of a spectrophotometer to do direct measures on a surface. This operation is almost always possible, because the digital images consider large area when there are average homogeneous natural light conditions. The next step of the present research will be the study to develop a method for the determination of the radiance distribution in the different wavelength directly by the digital camera HDR images. REFERENCES 1. Musso S.F., Recupero e restauro degli edifici storici, Epc libri, Roma, Feliu M.J., Edreira M.C., Martin J., Calleja S. and Ortega P., Study of Various Interventions in the Façades of a Historical Building Methodology Proposal, Chromatic and Material Analysis, Color research and application Journal, 30, 5, 2005, pp Oleari C., Misurare il colore, Ulrico Hoepli, Milano, Clark A., Color Perception (in 3000 Words), in William Bechtel and George Graham (eds), A Companion to Cognitive Science, Blackwells, Hunt R. W. G., The reproduction of colour, John Wiley & Sons Inc, AA.VV., IESNA Lighting Handbook, 9th Edition, Illuminating Engineering Society of North America, NY, U.S.A, Withagen P.J., Groen F.C.A. and Schutte K., CCD color camera characterization for image measurements, IEEE Transactions on instrumentation and measurement, Vol.56, n 1, February Li W., Soto-Thompson M. and Gustafsson U., A new image calibration system in digital colposcopy, Optic Express, Vol.14, n 26, December Kim E. S., Lee S.H., Jang S.W. and Sohng K. I., Adaptive colorimetric characterization of camera for the variation of white balance, IEICE Transactions Electron, Vol.E88-C, n 11, November Fiorentin P., Iacomussi P. and Rossi G., Characterization and calibration of a CCD detector for light engineering, IEEE Transactions on instrumentation and measurement, Vol.54, n 1, February Bellia L., Cesarano A., Minichiello F. and Sibilio S., Setting up a CCD photometer for lighting research and design, Building and Environment, n 1, Barnard K. and Funt B., Camera characterization for color research, preprint of an article for Color Research and Application, Solli M., Andersson M., Lenz R. and Kruse B, Color Measurements with a Consumer Digital Camera Using Spectral Estimation Techniques, H. Kalviainen et al. (Eds.): SCIA 2005, LNCS 3540, Springer-Verlag Berlin Heidelberg, pp , Morbiducci R., Metodi non distruttivi per la caratterizzazione cromatica delle facciate di antichi edifici Proceedings of International conference Ar.Tec_07,Ancona, Italy, Novembre, 2007, Alinea Editrice, pp , vol. 2., ISBN Debevec P.E. and Malik J., Recovering High Dynamic Range Radiance Maps from photographs, Computer Science Division, University of California at Berkley, Berkley, AA.VV, Google Sketchup, vers , Google Inc., 2007.