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 14 th 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
Cologne Background: Northern Continental Glass Full Colour and Mixed Programs (1100-1260) The Grisaille Revolution (1260-1350) Slower uptake in HRE, ~1350 Le Mans Le Mans Persistence of white glass and transparent enamels through the Renaissance (1600)
Absolute Glazing Transmission Objective: to develop a method of using luminance values as a proxy for glazing transmission Huge errors associated with unevenness 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
L z = luminance at zenith Standard Overcast Sky L γ = luminance at sky element L z = L γ 1 + 2 cos(z) 3 No dependence on solar position Maximum luminance at zenith Luminance at the horizon is 1 / 3 of that at zenith
L z = luminance at zenith Standard Clear Sky L γ = luminance at sky element -0.32 L sin (γ) -3χ z = L γ (1 e )(0.91 + 10 e + 0.45 cos 2 (χ)) -3Z 0.274 (0.91 + 10 e s + 0.45 cos 2 (Z s )) Strong dependence on position of sun Low gradient in luminance for large 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 12 th and 16 th Centuries Red glass was produced throughout the period by flashing a thin piece of copper-stained glass onto white glass In the 15 th 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 13 th century red : average 8.4 cd/m 2 15 th century red : average 8.2 cd/m 2
Two Simple Methods (1) For a window with N selected medallions (small window areas) N i = 1 colour, i red, i = average luminance (2) N i = 1 total, i red, i Only windows in good preservation and with as little replacement glass as feasible were selected (CVMA)
Results Blue to Red White to Red Chartres Cathedral (Nave Window 39) 2.0 12.3 Le Mans Cathedral (North Triforium Windows) Évreux Cathedral (Windows 10, 12, 14) 1.2 8.5 3.7 21.4
Results: French coloured glazing (13 th 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 Windows) 1.8 Bourges Cathedral (Window 200) 1.8
The Belle Verrière of Chartres Wellpreserved 13 th C. Medallions selected for analysis 13 th Century Medallions Medallion/Red 2.3 Blue/Red 2 12 th Century Medallions Light Blue/Red 8.9
Coloured Glazing in the Holy Roman Empire Total Medallion Luminance to Red Luminance Cologne Cathedral (Window SVI, coloured bands only) Skt. Kunibert, Cologne (Central Apse Window) Strasbourg Cathedral (Clerestory Windows, CVMA best preserved panels only) 4.2 3.7 6.0 Skt. Kunibert
The Grisaille Revolution (14 th 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 Troyes Cathedral (Window 232) Total Medallion Luminance/Red Luminance 6.6 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 13 th 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 13 th 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 13 th century full colour windows Modern replacement glass often does not significantly alter glazing transmission
Corrosion Method Limitations 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 conservation/cleaning efforts on window transmission Corrosion index: The Belle Verrière of Chartres Expand the database and make publicly available
Thank You