Colour of Stone Slabs under Different Standard Illuminations

PP Periodica Polytechnica Civil Engineering 61(1), pp. 66–74, 2017 DOI: 10.3311/PPci.9038 Creative Commons Attribution b research article Colour of Stone Slabs under Different Standard Illuminations Ákos Antal1*, Péter Görög2, Ádám László Veres1, Petra Balla1 Ákos Török2 Received 20-01-2016; accepted 08-03-2016 Abstract Spectral measurements have been carried out on polished stone slabs to show the accuracy of theoretical colorimetric calculations on perceptual differences of color appearance under various standard illuminations. Effects of the different spectral characteristics of the light source on colorization of the samples have been also studied to analyze the changes of color sensation on the samples in connection with the changes on the type of the illuminating source. Solar light (Standard Illuminant D65), basic incandescent source (Standard Illuminant A) and a basic luorescent source (Standard Illuminant F11) have been tested on Tardos red limestone, Siklós green limestone, Carrara white and gray marbles and diorite samples. Colorizing effects of the examined illuminations are compared to Standard Illuminant E – the theoretically perfect white source. Keywords colour appearance, colour difference, limestone, marble, diorite 1 Department of Mechatronics, Optics and Engineering Informatics Budapest University of Technology and Economics H-1521 Budapest, P.O.B. 91, Hungary 2 Department of Engineering Geology and Geotechnics Budapest University of Technology and Economics H-1521 Budapest, P.O.B. 91, Hungary * Corresponding author, e-mail: antalakos@antalakos.hu 66 Period. Polytech. Civil Eng. 1 Introduction Stone slabs are frequently used for cladding outside and inside buildings. From architectural point of view the most important parameters for design of natural stone cladding are the optical properties of stone slabs. The appearance and optical properties can be controlled by naked eye according to Eurocode EN1469. More recently, Zamora-Mestre et al. [1] introduced a new method for the optical quality control, which measures the optical appearance of the slabs by image processing software. It helps to avoid the visual anomalies of the stone slabs, but the appearance of the cladding depends on the actual light conditions. The external cladding looks different under sunshine and in cloudy weather conditions. When the cladding is placed inside a building the artiicial lighting of the interior should be synchronized with the optical properties of the stone slabs. The present paper describes the optical appearance of different stone slabs under different lighting conditions. The main goal of this study is to give an advice how to select a stone slab based on colour difference or chroma when different types and different intensity of illuminations are applied. The spectroscopy can be used for studying the variations in colour of stone slabs under different artiicial lights. It is also a useful tool to measure the grade of weathering as it is described for granitic rocks by Nagano and Nakashima [2]. Different types of limestones, marble and diorite are commonly used for cladding both for indoor and outdoor. Natural and aesthetic appearance and outstanding durability of these stones make them perfect to create durable and spectacular coating even on large surface areas. However, appearance is strongly related to the applied illumination. From sunlight to the wide range of luorescent and incandescent sources, the spectral power distribution of the incoming light varies between wide boarders – and consequently the colour sensation provided by the cladding material can vary heavily under different illuminations. Furthermore, analysing these effects is not always easy and straightforward. Only complex theoretical colorimetric calculations can be applied during the design phase of an object, real “on-site” measurements are usually problematic before the Á. Antal, P. Görög, Á. L. Veres, P. Balla, Á. Török implementation. Regarding to the heavily theoretical nature of these calculations, it is reasonable to show the accuracy of them. Therefore simulated environments were created for the samples, to describe the theoretically considered conditions in real life and compare the two different sets of data – theoretical versus direct, “life-like” measurements. It is also reasonable to compare the stability of the chromatic characteristic of the samples under different illuminations. Thus, the differences in colorization and chroma against the theoretically perfect white illumination (Illuminant E) were compared. These calculations can show which sample has the most illuminant independent chromatic characteristic, i.e. which stone slab visual appearance is the most similar under differently illuminated environments. 2 Methodology 2.1 Measurement setup Colorimetric calculations were made based on the spectral intensity distribution of the light relected from the measured surfaces., Several types of colour coordinates were calculated – directly from the XYZ coordinates, or – the ones represented in the corresponding colour spaces – e.g. in CIE L*a*b* [3]. Spectral relectance measurements were made on all of the samples to get their “relative” – illumination independent – relection spectra. This raw data gives the possibility to calculate the appearance of the samples under different standard illuminations in a theoretical way. A Konica Minolta CM-2500d handheld spectrometer was used to collect the absolute spectral data. The measurements were carried out with a 2° wide aperture, in 45° measurement coniguration, and the data was processed with the scattering components included. The “absolute” spectral relectance of the samples under different standard illuminations [4] was also measured. The aim was to record the spectral data directly relected from the surfaces – to simulate the behaviour of the samples during a standard visual inspection, and to make a comparison between theoretically calculated values and the results of the corresponding real measurements. A Gretag Machbet Colour Box was used to produce D65 (similar to solar light), Illuminant A (typical incandescent source) and TL84 (typical luorescent source) [5] spectra (Fig. 1). The reference spectral data for both illuminations were recorded on a BaSO4 covered white etalon. Colour of Stone Slabs under Different Illuminations Fig. 1 Measured spectra of the applied sources These illuminations represent a good general approximation for the usual outdoor light, and the two commonly used indoor light source families [6]. Standardized forms of their spectral characteristics are also available – these data were used for the theoretical calculation (Fig. 2). Fig. 2 Standardized spectra of the applied illuminations A Konica Minolta CS-1000 spectroradiometer camera was used to record the relected spectral data. This device uses a calibrated standard photography objective lens to collect the incoming light from the measurement area (1° wide) and therefore makes it possible to adjust the exact position and relative size of it. The inspected samples were placed inside the Colour box on a rotatable holder. The illumination came from the topside of the box, and the rotatable holder was set to ensure 45° angle of incidence. The spectroradiometer camera was placed 120 cm away from the centre of the sample, and the height of the instrument was set to be on the same level with the measured specimen. The target-instrument distance – consequently the measurement area of the CS-1000 instrument – was selected to cover a reasonably large region on the measured sample surface with the measurement area (Fig. 3). 2017 61 1 67 Fig. 3 Measurement area Ten measurements were carried out under both illuminations on all of the samples – with constant repositioning of the measurement area. The collected spectral data were used to calculate x and y, and L* a* b* values for further analysis. 2.2 Calibration of the instruments Two different types of spectral measurements were carried out: one for the pure relectance spectra of the samples; and one for the “life-like” absolute relectance, which included the characteristics of the illuminating source as well. These two methods required different calibration processes. The illumination independent relection spectra measurement is a relative measurement, which means the measured spectral data is given from nanometer to nanometer as a percentage of the instrument’s built-in illuminating source’s spectral characteristic. To record the pure spectra of the source as a reference, a white calibration is required from time to time. To calibrate the instrument, a nearly-perfectly relective white reference surface is needed with very lat spectral characteristic through the whole measurement range. This is usually a small BaSO4 covered disk, which is a standard accessory of the instrument. Handheld spectrophotometers like the Konica Minolta CM2500d are usually also able to do a dark calibration, which is for eliminating the dark noise of their detector. In our case, both white and dark calibrations were done before the measurements – to get the best possible accuracy. The absolute relectance measurement however does not require such calibrations. Since this process is a direct measurement the light relected directly from the measured surface contains all of the necessary information – no further processing required. 68 Period. Polytech. Civil Eng. The accuracy of the instrument is guarantied with a calibration to a standard source. With the Konica Minolta CS-1000, two different type of calibration is possible – wavelength calibration, and level calibration. The former is to adjust the measurement data (taken with the use of the standard source) alongside the wavelength axis to match with the data preloaded (practically based on the documentation of the standard source). The same stands for the level calibration, but in this case the intensity values are matched to calibrate the data on the vertical axis. We used the CS-1000 in absolute mode with factory calibration, with which the vertical axis represented spectral 2 radiance in [ W/m sr nm ]. However, the absolute radiometric values were available, we did not use such data for any kind of calculations – just to represent the measured spectra. Since color data is independent from the intensity of the illumination it is indifferent from the colorimetric point of view in our case. The software calculates X, Y and Z coordinates directly, and we used these data instead of calculating these values from the spectral data manually. This method gives a more life-like comparison between fully calculated and purely measured data. 2.3 Description of the applied colour systems The quantitative representation of colour data is always a challenge – due to the subjective nature of human colour vision. However, there are few perceptually evident fundaments, which help to create generally usable systems to mathematically characterize the colour sensations. Some of these systems are standardized by the International Commission on Illumination (CIE), and commonly used in colour science. Most of the commonly used colour spaces are based on the CIE 1969 trichromatic system also known as the reined CIE XYZ space for 10° observer – which is an adjusted version of the irst standardized approximation of colour perception. This system is based on the trichromatic behaviour of colour vision, and formed with a series of colour matching experiments, where the participants had to adjust the intensity of three perceptually independent monochromatic light sources until the sense of the mixture of the three primary colours are match with a dedicated monochromatic light. With the repentance of this match – from one monochromatic stimulus to another – through the complete visible spectrum, three sets of colour matching functions [7] where presented – r ( λ ) , g ( λ ) and b ( λ ) (Fig. 4). Since r ( λ ) , g ( λ ) and b ( λ ) functions have negative values at some section of the visible spectra, it is reasonable to transform them to positive values only. This transformation provides x ( λ ) , y ( λ ) and z ( λ ) functions (Fig. 5). Á. Antal, P. Görög, Á. L. Veres, P. Balla, Á. Török Fig. 4 r ( λ ) , g ( λ ) and b ( λ ) CMFs Fig. 6 xy chromaticity diagram Fig. 5 x ( λ ) , y ( λ ) and z ( λ ) CMFs As it is described before under subchapter 2.3, X, Y and Z reined tristimulus values are calculated as the integral of the corresponding colour matching functions – x ( λ ) , y ( λ ) and z ( λ ) – multiplied by the spectral intensity distribution of the light stimulus to inspect. However, X, Y and Z values represents a 3D space and describes a dedicated stimulus completely, it is not easy to imagine, or show the colour data directly with them. X, Y and Z are absolute values, containing not only colour related, but intensity information as well – which we are usually not interested in. To get a better option to represent colours, it is reasonable to give a normalized version of the tristimulus values – the x, y and z chromaticity coordinates. x= X X +Y + Z (1) x= Y X +Y + Z (2) z= Z X +Y + Z (3) The x and y coordinates forms the chromaticity diagram of the CIE 1969 trichromatic system, which is unequivocal enough to represent colour data (Fig. 6). Colour of Stone Slabs under Different Illuminations Since the XYZ space is a very basic approximation of the colour vision – uses only the trichromatic phenomenon, as a hooking point to the neural basis of vision – there are a few derived colour systems developed during the last century. One of these reined colour spaces is the CIE L*a*b* space. CIE L*a*b* is based on the XYZ space and can be created with a linear transformation of it – as it is shown before under subchapter 2.3. CIE L*a*b* can be adjusted to represent colours under different illuminations more accurately than XYZ. Furthermore, the white point of the space for a dedicated illumination always takes place at the origin. L*a*b* is a more advanced model from the psychophysical point of view as well. Beside the trichromat nature of human colour vision, L*a*b* represents a deeper level of neurology of the human visual system – the opponent channels. L* is refers to lightness, a* represents red colour stimuli towards its positive direction and green colour stimuli towards the negative direction, the positive direction of the b* coordinate points towards yellow stimuli, and the negative side points to the direction of blue stimuli (Fig. 7). Fig. 7 L*a*b* color space 2017 61 1 69 There is another important difference between XYZ and L*a*b*. Just like most of the colour spaces, XYZ and L*a*b* are represent visible colours as a Euclidian space – however, an ideal colour space would be a Riemann-like one. This phenomenon entails that both XYZ and L*a*b* are nonlinear spaces from the practical point of view [8]. The main aim of the transformation to L*a*b* is to decreases the nonlinearities of the colour space compared to XZY, therefore distances between two different point of the L*a*b* space represents real perceptual colour differences much better. Consequently, it is reasonable to calculate ΔE colour differences [9] in L*a*b* spaces – to predict, analyse or simply show quantitative differences between different colour stimuli. to use to get white object data. Illuminant E is the theoretically perfect white source, with even characteristics on all wavelengths; therefore Xn, Yn and Zn values are both equals to 1. L∗ = 116 f (Y / Yn ) − 16 (9) a ∗ = 200  f ( X / X n ) − f (Y / Yn )  (10) b∗ = 200  f (Y / Yn ) − f ( Z / Z n )  (11) Where 1/ 3 f ( X / Xn ) = ( X / Xn ) if 2.4 Calculating colour data The measured relative spectral data contains only the relectance characteristic of the sample. To get exact colorimetric data under a speciied illumination a spectral characteristic of the light source used should be taken into consideration. To do so, the spectral power distributions (SPD) of the source and the sample have to be combined in the following way Φ λ ( λ ) = Rs ( λ ) Els ( λ ) (4) Where Els ( λ ) is the SPD of the selected standard illumination and Rs ( λ ) is the SPD of the investigated sample. Once the calculated absolute relectance spectra is available, X, Y and Z tristimulus values for the CIE 1969 10° standard colorimetric observer can be calculated with the following steps ( X / X n ) > ( 24 / 116) (12) 3 1/ 3 f ( X / X n ) = (841 / 108)( X / X n ) + 16 / 116 if f ( X / X n ) ≤ ( 24 / 116 ) 3 (13) 1/ 3 f (Y / Yn ) = (Y / Yn ) if (14) (Y / Yn ) > ( 24 / 116) 3 1/ 3 f (Y / Yn ) = (841 / 108)(Y / Yn ) + 16 / 116 if f (Y / Yn ) ≤ ( 24 / 116 ) (15) 3 1/ 3 f ( Z / Zn ) = ( Z / Zn ) if ( Z / Z n ) > ( 24 / 116) (16) 3 1/ 3  x ( λ )  0.341080 0.189145 0.387529   r ( λ )        y ( λ )  = 0.139058 0.837460 0.073160  g ( λ )  (5)   0 0.039553 0.026200  b ( λ )     z ( λ )   X = 683.6∫ 780 nm 380 nm Y = 683.6∫ 780 nm Z = 683.6∫ 780 nm 380 nm 380 nm Φλ (λ ) x (λ ) d λ (6) Φλ (λ ) y (λ ) d λ (7) Φλ (λ ) z (λ ) d λ (8) if f ( Z / Z n ) ≤ ( 24 / 116 ) Period. Polytech. Civil Eng. (17) 3 In the L*a*b* space it is possible to calculate chromatic and colorization differences as Euclidian distances between two points – representing two different colour sensations. The equations for ΔE (total colour difference) and ΔC (Chroma difference) are the following. ∆E ∗ = Where r ( λ ) , g ( λ ) and b ( λ ) are the experimental colour matching functions (CMFs), ( ) , y ( λ ) and z ( λ ) are the CMFs of the CIE 1969 10° standard colorimetric observer. In this way, both the directly measured and a calculated X, Y and Z values are available. To calculate the L*a*b* coordinates, the Xn, Yn and Zn coordinates for the speciied white object is also needed. To analyse all of the investigated illuminations alongside each other, Illuminant E is the perfect choice 70 f ( Z / Z n ) = (841 / 108)( Z / Z n ) + 16 / 116 ∗ 2 ∗ 2 ∗ 2 ( ∆L ) + ( ∆a ) + ( ∆b ) ∆C ∗ = C1∗ − C2∗ = ∗ 2 1 ∗ 2 1 (a ) + (b ) − ∗ 2 2 (18) ∗ 2 2 (a ) + (b ) (19) 3 Materials Stone slabs of ive different rock types were investigated: Tardos red limestone, Siklós green limestone, a white and a grey marble from Carrara, and grey diorite from Italy. All studied stone slabs were polished, having the same surface inish (Fig. 8). Á. Antal, P. Görög, Á. L. Veres, P. Balla, Á. Török The Tardos red limestone is a very common dimension stone in the Carpathian Region. This Jurassic limestone has been exploited from the Roman period in Gerecse Mountains, North Hungary having various shades of red colour (Fig. 8, S29, S30, S31). It was commonly termed as “red marble” of Hungary [10]. The red colour and aesthetics made this stone as one of the favorite dimension stones of Roman period and medieval kings of Hungary [11]. One example is the red “marble” fountain in the Castle of Visegrád which was made from this stone during the reign of King Matthias [11]. It has been also used at other sites such as the cathedral of Nitra [12]. The Siklós “green” limestone is quarried in the Villány Mountains, South Hungary. It represents thick bedded cemented limestones of Middle Triassic Zuhánya Limestone Formation (Fig 8. S 1377). This popular limestone shows great variety in colour, the most common types are the brownish-grey, grey slightly greenish ones. There are irregular mottles in the matrix having slightly lighter colour as a rule, i.e. pale yellow to pinkish mottles also occur. Besides the mottles of centimetre to 45 cm in size, brachiopods are also common [13]. Dolomitization occurs in micritic mottles and also along issures representing several phases of dolomite formation [14]. The physical properties of the Siklós green limestone were described in [13]. The uniaxial compressive strength is 80 MPa, but after 25 freezethaw cycles it is reduced by 37%. Hence, it is no frost resistant, but can be used only for inner cladding. The Carrara Marble one of the most famous dimension and ornamental stone of the world. In the medieval architecture mainly it was used in Europe in the Italian region, but nowadays it can be found all over the world [15]. Carrara marble is still used in large quantities in modern buildings (in Mekka at mosques, Opera House in Oslo). Rosso et al. [16] investigated the energy-eficiently of white marble used as facing stones on the facade of a new building in New York. The properties of the Carrara marble were described and studied in details. Karaca et al. [17] studied the micro-fabric; the grain properties and the grain boundaries of different types of marbles. Cardani and Meda [18] measured the lextural strength of the Carrara marble, while Leiss and Weiss [19] evaluated the fabric anisotropy and the thermal induced degradation. In this paper two different types of Carrara marbles were studied a white one (Fig. 8, S56) and a grey one (Fig. 8, S55). The white one is micro-crystalline with fairly homogenous micro-fabric. The grey marble of Carrara was described in detail by means of petrographic methods by Borghi [20]. The diorite is an intermediate plutonic rock, with prevailing minerals of plagioclase and pyroxene. The studied slab represents a dark grey variety, which has the darkest colour compared to the other specimens (Fig. 8, S861). Colour of Stone Slabs under Different Illuminations Fig. 8 Investigated stone slabs (S29-S31: Tardos red limestone; S32: white Carrara marble; S55-S56: grey Carrara marble; S861: diorite; S1377: Siklós greenlimestone) The samples can be divided into two different groups based on their visual appearance. The measured relectance spectra of the stones coincide with this grouping. The spectra of the three red limestones are nearly identical. The greenish grey limestone has a bit different relection characteristic, but clearly its between the other three samples (Fig. 9). Fig. 9 Relection spectra of red Tardos limestones (S29, S30, S31) and greenish grey Zuhánya limestone (S1377) The white marble from Carrara has a much higher relexion value than the grey marble, but the spectra of the grey Carrara marble is more lat. Diorite is much less relective than the other samples, but its lat characteristic clearly its into this group (Fig. 10). 2017 61 1 71 Fig. 10 Relection spectra of white marble (S32), grey Carrara marble (S55), white Carrara marble (S56) and grey diorite (S861) 4 Results Comparison between the directly measured and calculated x and y chromaticity coordinates shows that the theoretically calculated data is accurate enough even for complex simulations. Fig. 11 and Fig. 12 show the two different dataset alongside each other for samples S29, S30 and S31 in the xy chromaticity diagram. It is clearly visible that the calculated and measured points are perfectly lapping each other for illumination D65, and the points are relatively close for Illuminant A as well. However, for F11 the separation between the measured and calculated data requires some explanations. Fig. 11 xy chromaticity data for S29, S30 and S31 If we compare the theoretical spectra of the F11 standard illumination, and the measured data from the same source, the distributions are noticeably different (Fig. 13). The height of the two main peaks of the spectra shows an opposite trend, therefore the total energy distribution of the measured spectra shifts towards the higher wavelength region compared to the theoretical one. 72 Period. Polytech. Civil Eng. Fig. 12 Magniied xy chromaticity data for S29, S30 and S31 under the applied illuminations. Filled symbols: measured data; Open symbols: theoretical data Á. Antal, P. Görög, Á. L. Veres, P. Balla, Á. Török Fig. 13 Measured spectra of the applied sources (F11s theoretical, F11 measured spectra) Tardos red limestone samples - S29, S30 and S31 - relect more red lights thus the colorization effect of the applied illumination is more sensitive to differences in the higher spectral ranges. This difference explains the shift of the xy chromaticity data. According to the colour difference calculations the white coloured stone slabs (S32, S55 and S56) the marble specimens, have different results than the other rock materials. The DeltaChroma and the DeltaE values of these white coloured specimens are higher than the other stones under all the three different lighting conditions. Only the DeltaChroma value of the white marble (S32) is smaller than that of the diorite (S861) under D65 illumination. The results of the three marble specimens, one white and two grey shows, that the little differences can be also perceived with this method. Mainly the grey ones have smaller DeltaChroma and DelteE values. The red coloured specimens have lower DeltaChroma and DeltaE values - the lowest values related to the dark grey diorite. The red and grey limestones gave almost the same results. It is interesting that the values of the dark greenish grey Zuhánya limestone, is a little bit higher than the one of Tardos red limestone. Between the three Tardos red limestone specimens it is also possible to see slight differences – mainly under Illumiant A, and Illuminant F11 (Fig. 14, Fig. 15 and Fig. 16). Fig. 14 ΔE and |ΔC| under Illuminant D65 compared to Illuminant E (S32-S861 represent no. of tested stone slabs – see Fig. 8) Colour of Stone Slabs under Different Illuminations Fig. 15 ΔE and |ΔC| under Illuminant A compared to Illuminant E (S32-S861 represent no. of tested stone slabs – see Fig. 8) Fig. 16 ΔE and |ΔC| under Illuminant F11 compared to Illuminant E (S32-S861 represent no. of tested stone slabs – see Fig. 8) 5 Conclusions Eight polished rock slabs representing ive main lithologies were studied under different artiicial lighting conditions to quantify colour differences and visual appearance of stone slabs. According to the results of the calculations and the spectral measurements it is possible to divide the different stone materials by using colour spectra. In addition the applied method is capable to recognize changes in the colour of the stone slabs under different lighting conditions. The ive different dimension stones (two types of limestones, two types of marbles and a diorite) can be divided into two main colour classes: the light and the darker stones. The classiication depicts signiicant differences between the spectral properties of very light marbles and the other darker stones. Within the group of darker stones marked differences were measured: Tardos red limestone and Siklós green limestone form one subgroup, while diorite represents another one. Consequently, this method can be used to classify not only the stones with high differences in colour, but it is also applicable to identify the slight differences in colour. With this colour measurement it was possible to recognize the slight difference between the three red Tardos limestone specimens. 2017 61 1 73 The application of colour spectra measurements provides an additional tool for synchronizing the colour of the stone slabs with the applied lighting conditions. It is possible to use it as a design the colour effect of a stone slab with knowing the spectral properties of it. Acknowledgement The inancial support of National Research, Development and Innovation (NKFI) Fund (K 116532) is appreciated. References [1] [2] [3] [4] [5] [6] [7] 8] [9] 74 Zamora-Mestre, J.-L., Mesalles-Ruiz, J., Soriano-Gabarró, X. “Proposal for optical method of quality control of the surface of the slabs of natural stone for cladding facades.” In: 37IAHS World Congress on Housing Science: “Design, Technology, Refurbishment and Management of Buildings”, Santander, Spain, Oct. 26-29, 2010. 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