Temples of glass: Developing a relative transmissivity index for the in-situ analysis of medieval glazing Christopher Simmons Outline • Simmons and Mysak (2009). The Transmissive Properties of Medieval and Renaissance Stained Glass in European Churches, in preparation • Motivation • Methods • Results • Limitations • Future Work Motivation: Why should we care about stained glass transmissivity? • Quantify the light transmission characteristics of different colours of glass • My Thesis’s hypothesis: the increased adoption of lighter colours and high transmissivity glass in 14th century France/HRE may have been partly driven by increasing cloud cover in NW Europe • Determine level of glass corrosion and refractive properties • Compare the properties of stained glass from different eras • Evaluate the modification of the original lighting aesthetic provided by the inclusion of modern glass panes Background: Northern Continental Glass • Full Colour and Mixed Programs (1100-1260) • The Grisaille Revolution (1260-1350) • Slower uptake in HRE, ~1350 LeLe Mans • Persistence of white glass and transparent enamels through the Renaissance (1600) CologneCologne Absolute Glazing Transmission • Objective: to develop • Huge errors a method of using associated luminance values as with a proxy for glazing unevenness transmission of window plane • Difficult to achieve outside of the lab setting • Objective: to develop a method of using luminance values as a proxy for glazing transmission Motivation: Why do we need a remote, in-situ approach? • Ease of access • No need to remove glass • No need to elevate oneself to the level of the window panes • No possibility of experimental damage to glass • Low cost, simple methods • Large quantities of data can be collected in a short period of time Methods: HDR Luminance mapping • New method for evaluating luminance without a luminance meter • SLR camera, controlled remotely, placed on tripod • White balance (daylight), ISO (100), and aperture size (4.0) fixed • Exposure time doubled from 1/4000 s to 30 s • Photosphere® software developed by G. Ward (freely available) is used to assemble High Dynamic Range (HDR) images and create luminance maps based on the camera’s response function (RGB values) Assumptions • Basic Assumption # 1: the gradient in exterior luminance is negligible over a small window area • In other words, for a small window area the exterior luminance is considered constant • Any change in the interior luminance value obtained in the small window area should be a function of differential transmission characteristics of the glass • A relative measurement Sky Standards: An introduction • Empirical formulae based on observational data collected during luminance scan experiments • Based on the ratios of the luminance value obtained at a given point in the sky hemisphere to luminance at zenith or at the position of the sun Standard Overcast Sky Lz = luminance at zenith Lγ = luminance at sky element • No dependence on solar position • Maximum luminance at zenith __Lz 1 + 2 cos(Z) = _________ • Luminance at the Lγ 3 1 horizon is /3 of that at zenith Standard Clear Sky Lz = luminance at zenith Lγ = luminance at sky element • Strong dependence on position of sun -_____0.32 L (1 – e sin ( γ ) )(0.91 + 10 e- 3 χ + 0.45 cos2 (χ)) • Low gradient in __z = ________________________ -3Z luminance for large Lγ 0.274 (0.91 + 10 e s + 0.45 cos2 (Z )) s χ, near horizon Assumptions • Basic Assumption # 2: the variation in exterior luminance distribution is negligible on the order of a few minutes – It takes about 3 minutes to collect a set of photos to make an HDR image – Need clear skies or slowly-varying exterior cloud cover (i.e., slow variation in zenith luminance) – Low-dynamic lighting conditions (e.g., no cloud passes over the sun) Assumptions • Basic Assumption # 3: When we divide the luminance of one piece of glass by the luminance of another piece of glass in the same small window area, the following factors divide out: • Interior reflections off the glass (vast interior = slowly-varying interior illumination) • Effect of exterior protective grills and/or protective glazings • The effects of lens vignetting on the luminance value • What’s left in the measurement: • Differences in transmission, corrosion, and refraction between individual pieces Assumptions • Basic Assumption #4: the transmissivity of red glazing changes little between the 12th and 16th Centuries • Red glass was produced throughout the period by flashing a thin piece of copper-stained glass onto white glass • In the 15th century, flashing became popular for other colours, increasing their transparency • Any increase in red flashed glazing should be due to an increase in the transmissivity of white glass Belle Verrière Nave Window 23, Rouen Cathedral • 13th century red : average 8.4 cd/m2 • 15th century red : average 8.2 cd/m2 Two Simple Methods • For a window with N selected medallions (small window areas) N = average ________colour, i (1) luminance i = 1 red, i N ________total, i (2) red, i i = 1 • Only windows in good preservation and with as little replacement glass as feasible were selected (CVMA) Results White to Blue to Red Red Chartres Cathedral (Nave Window 39) 2.0 12.3 Le Mans Cathedral (North Triforium 1.2 8.5 Windows) Évreux Cathedral (Windows 10, 12, 3.7 21.4 14) Results: French coloured glazing (13th C.) Total Medallion Luminance/Red Luminance Chartres Cathedral (Window 39) 2.1 Chartres Cathedral (Window 41) 2.6 Chartres Cathedral (Window 121 lancets) 2.2 Le Mans Cathedral (North Triforium 1.8 Windows) Bourges Cathedral (Window 200) 1.8 The Belle Verrière of Chartres Well- preserved 13th C. 13th Century Medallions Medallions Medallion/Red 2.3 selected Blue/Red 2 for analysis 12th Century Medallions Light Blue/Red 8.9 Coloured Glazing in the Holy Roman Empire Total Medallion Luminance to Red Luminance Cologne Cathedral (Window SVI, 4.2 coloured bands only) Skt. Kunibert, Cologne (Central Apse 3.7 Window) Strasbourg Cathedral (Clerestory Windows, CVMA best preserved 6.0 panels only) Skt. Kunibert The Grisaille Revolution (14th C.) Total Medallion Luminance to Red Luminance Évreux Cathedral (Window 10) 10.3 Évreux Cathedral (Window 207) 9.7 St-Ouen, Rouen (Window 231) 7.6 Renaissance Glazing: Window 53 Rouen Cathedral Renaissance Glazing Total Medallion Luminance/Red Luminance 6.6 Troyes Cathedral (Window 232) Troyes Cathedral (Window 231) 4.7 St-Nizier, Troyes (Window 100) 7.7 Toledo Cathedral (Four Clerestory and Triforium Windows) 3.5 Modern Restoration Glass: Cologne Modern Restoration Glass: Cologne Hatch marks = modern glass Beauvais Cathedral – Lady Chapel, early 13th century Conclusions • A new methods for evaluating relative transmissivity is provided using red glass as a standard for comparison • Light transmittance into the cathedral could be controlled by colour palette selection from an early era • Belle Verrière in Chartres: Romanesque light blue is much more translucent than early 13th century blues • Stained glass in the HRE, with its broader colour palette, was generally more transmissive than red-and-blue dominated French glass • Less necessity to convert to brighter stained glass during cloudier times • Stained glass from the post-Grisaille Revolution period is generally 5-10 times more transmissive than 13th century full colour windows • Modern replacement glass often does not significantly alter glazing transmission Method Limitations • Corrosion • Reasonably high degree of variance (large sample set required) • Red glass required • Laboratory work needed to create other standards • More authenticity criticism charts needed • Inherent bias (small, overly corroded pieces eliminated), selected according to CVMA records • Program that selects pixels based on RGB ratios to identify specific colours would help reduce bias, but then each image would require extensive work to eliminate anachronous panes from the analysis, which isn’t an exact science Future Work • Refine the ‘red glass’ standard, develop standards for other colours • Lab tests, determine a standard variance of red with respect to other colours for a large data set, then test individual windows to determine the place of the red in those windows within the larger standard variance. • Corrosion index: relative to identified, original, well-preserved panes of glass • Before and after HDR images to document the effects of historical Corrosion index: The Belle conservation/cleaning efforts on Verrière of Chartres window transmission • Expand the database and make publicly available Thank You.