- Email: [email protected]
Sydney opera house glass walls: Colour measurement and control
Build. Sci. Vol. 10, pp. 65-72. Pergamort Press 1975. Printed in Great Britain
]
I
l
,
i
Sydney Opera House Glass Walls: Colour Measurement and Control J. A. HOOPER* M. P. W A S S A L L t
One o f the architectural requirements o f the tinted laminated glass used in the construction of the Sydney Opera House glass walls was that it should be uniform in colour relative to an initially accepted colour standard. As the glass was manufactured in small batches over a two year period, stringent controls were needed to maintain the necessary eolour quality of the laminate. The procedures adopted to check uniJormity of colour are described and results are given o f colorimetric analyses carried out on the colour standards and the production control specimens. The chromaticity data are interpreted in terms of subjective colour discrimination and are shown to be compatible with visual observations of the glass in situ.
a 0.8 m m thick clear plastic interlayer. In the view window areas located at podium level, nominal 15 m m thick clear glass was used, giving an overall laminate thickness of 21.8 mm. In the remaining areas the nominal thickness of clear glass is 12 mm, giving a standard laminate thickness of 18.8 mm. The thickness tolerances of the glass, particularly the tinted glass, are relevant to the problem of colour control. The specified tolerances were + 1.0 mm for the tinted glass and +0.3 m m for the clear glass. The specified tolerance for the two standard laminated sections was + 1.4 mm. The tinted glass may be described as pale bronze in colour, and is designated "demi-topaze" by the manufacturers, Boussois Souchon Neuvesel. It was specially formulated for the glass walls project and was manufactured using the traditional potcasting process [3]. In this process the basic constituent materials (e.g. sand, soda, lime), together with the colouring oxides, are contained in a refractory pot and heated in a furnace. On achieving the necessary mixing and fusion the molten glass is poured on to a roller mechanism from which it emerges as a red-hot continuous ribbon. After annealing, the glass is ground and polished. Each pot can hold little more than one tonne of glass and it took almost two years to produce the required 6000 m 2 of tinted glass. It was essential therefore that the most careful colour controls be exercised throughout this prolonged and intermittent manufacturing process. At the start of production, 75 and 300 mm square specimens of tinted, clear and laminated glass were produced by the manufacturers and were adopted as colour standards. Colorimetric data
INTRODUCTION T H E "glass walls" of the Sydney Opera House is the name given to the glass surfaces that enclose the openings between the roof shells and the podium structure. Each wall consists of a mosaic of glass sheets supported by a steel structure. The sheets vary considerably in shape and size, the maximum dimensions being approximately 4 m by 2.! m. Figure 1 shows the scale and geometrical form of the two north facing walls, the largest of these being 2 4 m high and 55 m wide. The general layout of the Opera House is shown in figure 2, in particular the numbering of the concert hall shells. The opera hall shells are numbered in the same way but prefixed with the letter B. The glass walls are referred to by the names of the shells they enclose. The glass walls were constructed during the period 1970-72. There were many technical problems that had to be solved before they could be built [1, 2], but one of the architectural requirements was that the colour of the glass should be uniform throughout the building.
T H E GLASS
The building is glazed in laminated glass consisting of a layer of clear plate or float glass, a nominal 6 mm thick layer of tinted plate glass, and *Ove Arup & Partners, 13 Fitzroy Street, London, W1P 6BQ.
tPye Unicam, Cambridge (Formerly of Imperial College, University of London). 65
66
J. A. Hooper and M . P. Wassall
Fig. I. View from harbour showing glass walls A4 (right) and B4 (left).
~
/Opera hall
where X, Y and Z are the tristimulus values that fix the proportions of the red, green and blue reference stimuli (X), (Y) and (Z) required to match the colour (C), and C is a measure of the amount of the colour and is given by C = X + Y + Z . The Xtristimulus value is given by x = y~ T ~ e ~ A , ~
(2)
),
Fig. 2. General layout of structure.
were obtained for the smaller specimens. Subsequently 7 5 m m square colour-quality control specimens of laminated glass were supplied by the manufacturers at a rate of one for every 200 m 2 produced. These specimens were compared both visually and colorimetrically with the laminated glass colour standard. DEFINITIONS AND T E R M I N O L O G Y Of the various systems of colour measurement currently available, the trichromatic system is undoubtedly the most widely used. In 1931 the Commission Internationale de l'Eclairage (C.I.E.) established a framework for colour specification in which a given colour, (C), may be expressed in the form C(C) - X ( X ) + Y ( Y ) + Z ( Z ) (1)
where T~ represents the spectral transmission curve for the glass, Pz denotes the spectral energy distribution of the illuminant, ff~ is the standard equal-energy distribution coefficient (see, for example, Wright [4]), and A2 is the wavelength interval, the summation normally being carried out over the range 380-770 nm. Similar expressions define the Y and Z tristimulus values. Furthermore, two of the reference stimuli are chosen in such a way that, upon suitable weighting of the distribution coefficients, the tristimulus value, Y, of the third reference stimulus also represents the percentage transmission of the glass. In order to separate colour quality from the amount of light being matched, the value of C in equation (1) is made equal to unity by normalizing the tristimulus values, i.e. x = X / ( X + Y + Z ) , and similarly for y and z. This gives the unit trichromatic equation ( c ) - x(X) + y(r') + z ( Z )
(3)
Since the sum of the coefficients x, y and z is unity, any colour may be represented graphically on a plane diagram with x and y co-ordinates.
Sydney Opera House Glass Walls: Colour Measurement and Control Such a diagram is the C.I.E. chromaticity chart (figure 3), which shows how all possible colours may be represented graphically in terms of these two coefficients. As the locus of the spectral colours is everywhere either straight or convex, and the resultant mixture of any two colours lies somewhere along a straight line between those two colours in chromaticity space, all other colours must lie within the area bounded by the spectrum locus and the straight line joining its red and violet extremities.
c'5 hd
0.7
s0
s{)~
~
Spectrum LoctJs (xyl
57o
O5
5~o
c
03
67
One of the advantages of colorimetric analyses performed to the C.I.E. system is that measurements can be obtained in an unambiguous form which, besides having an internationally agreed meaning, can be related to subjective descriptions of colour. Such analyses also avoid certain problems associated with the visual assessment of colour quality (e.g. different colour rendering characteristics of various illuminants; possible colour defective vision of the observer; differences in visual discrimination between observers with normal colour vision), particularly when these assessments have to be made over a long production period. An additional benefit to be derived from spectrophotometric analyses of laminated glass is that by extending measurements to the nearinfrared region of the spectrum the necessary data are obtained from which to estimate the moisture content of the plastic interlayer, a quantity which is directly related to the degree of adhesion at the glass/plastic interface and hence the structural integrity of the laminate. At the same time, however, it must be stressed that colorimetric analyses are designed to supplement rather than to replace the visual judgement of colour quality. INSTRUMENTATION
Oq
I 01
02
03
04
05
06
07
08
x
Fig. 3. The 1931 C.LE. chromaticity chart. Having plotted the chromaticity co-ordinates (x, y) for a given colour C (figure 3), a straight line may be drawn from the white point W (representing the illuminant) through C to intersect the spectrum locus at 2 a. The colour C may then be defined by 2a, known as the dominant wavelength, and the ratio of the distance CW to W2 a, known as the purity, p. From the subjective viewpoint this alternative definition of colour is sometimes preferable to that of chromaticity co-ordinates, especially when assessments of small colour differences have to be made. The colour of a Planckian (black-body) radiator at various absolute temperatures is also shown for reference. The chromaticity values for the C.I.E standard illuminants SA, SB, Sc and D65oo (corresponding to correlated colour temperatures of 2856, 4874, 6774 and 6504 °K respectively) are seen to lie close to the Planckian locus. The same applies to the equal-energy white, E, on which the units of the three reference stimuli (X), (Y) and (Z) are based.
The colour measurements were made using a modified version of the Wright spectrophotometer [5], illustrated in figure 4. The optical system of the present instrument is the same as the original, but the photocell detector system has been replaced by a photomultiplier and digital voltmeter. It is a non-recording instrument and measurements have to be taken at discrete wavelengths throughout the spectrum. In normal use the cross-sectional area of the beam incident on the sample is approximately 1 0 x 8 mm, and this configuration was used in the present investigation. The H.T. supply to the photomultiplier dynode resistor chain was chosen for stability of output and the chain itself constructed from high quality resistors. These were housed in a separate metal box to minimise heating effects in the photomultiplier tube and to complete the earth shield. The photomultiplier anode current was measured by monitoring the voltage developed across a 1 Mf~ load resistor. In order to reduce noise--which occurs mainly in the form of pick-up in the cables and Shot noise in the photomultiplier tube--a 10 #F capacitor was connected in parallel with the load resistor and could be switched in or out as required. The voltage displayed on the digital voltmeter was directed to a serializer unit which tripped the voltmeter at the required rate and
68
J. A. Hooper and M. P. Wassail
Fig. 4. Modified Wright spectrophotometer,
converted the signal into a form suitable for a tape punch unit. The paper tape readout could then be fed into a computer for the necessary calculation of chromaticity values. It is important in precise spectrophotometry that all possible sources of error are firstly isolated and then minimised. In the optical system, wavelength errors were minimised by regularly calibrating the instrument against spectral wavelength lines, and bandwidth errors were minimised by restricting the maximum bandwidth to 10 nm. Errors resulting from stray light, polarization and angular mis-alignment of the sample were considered negligible. Significant drift in the light source components could only be detected if measurements were made over periods longer than about fifteen minutes, and as readings at any given wavelength were taken in less than one minute, drift errors were also considered negligible. In the detector system, careful checks were carried out to establish the linearity of response of the photomultiplier over a wide range of illumination. Errors common to automatic recording instruments, e.g. pen inertia and slit width variation, were clearly absent in the present case. Although it is normal practice to quote chromaticity co-ordinates to four places of decimals, the fourth figure cannot be regarded as exact, Even using a high precision instrument such as the one referred to, there are some random fluctuations which are unavoidable and which give rise to small errors. The effect of such fluctuations on chromaticity values has been considered by Nimeroff [6] on the assumption that the chromaticity co-ordinates x i, y;, obtained from the ith set of individual measurements of Ta have a joint normal distribution about their mean values x,,, .v,,. On the basis of
wilh corers removed.
this analysis it is possible to construct an ellipse around any point in the chromaticity diagram within which a given proportion of the values are likely to fall. Wassail [7] has assessed the stability of the Wright spectrophotometer by taking repeated measurements on the same sample (x,, = 0.3644, Ym = 0"3284). In this case the computed size of the ellipse containing 95 per cent of all chromaticity points was defined by a major axis of length 0.0003 in the chromaticity chart, aligned in the yellow-blue direction, and a minor axis of length 0.0002. The size of the error ellipse may depend to some extent on its position in the chromaticity diagram, but the size quoted can be taken as being directly applicable to the glass sample measurements. RESULTS OF M E A S U R E M E N T S The spectral transmission curves, T~, for the standard glass laminate and the standard tinted glass are shown in figure 5, and the transmission and chromaticity values are given in Table 1. These values relate to the standard illuminant D6500, which is generally considered the one most representative of average daylight conditions. The spectral transmission values, Y, for the laminated and tinted glass specimens were 64.1 per cent and 71.3 per cent respectively. The corresponding C.I.E. coefficients x and y are plotted on the chromaticity chart (figure 6), but the values of dominant wavelength, 2~, give a clearer indication of the colour difference between the two specimens. Data for the clear plate and float glass are also included in ]'able I and figure 6. The chromaticity coefficients are similar to those for the illuminant, but there is a marked difference between the
Sydney Opera House Glass Walls: Colour Measurement and Control dominant wavelength of the plate and float glass specimens. The "green" of the plate glass and the "blue-green" of the float glass are plainly visible to the eye when the specimens are viewed "endon", i.e. through the 75 mm thickness, but the degree of colour saturation in both types of glass is so low that these differences will have a very 100
SO
6O
40 .7:
69
small effect on the overall colour quality of the laminated glass. Transmission and chromaticity data for the 26 laminated glass colour-quality control samples are given in Table 2. The thickness of these samples varied from 17.7 to 21.8 ram, well within the specified permissible range of 17.4-23.2 ram. The transmission values varied from 63.5 to 68.9 per cent, with an average value of 65.9 per cent. Values of x ranged from 0.3244-0.3279, with an average of 0.3264; for y the range was 0.34930.3539, with an average of 0.3517. Based on these average values of x and y, the dominant wavelength is 571 nm and the colour purity is 0.11. Evidently the average chromaticity values are almost identical to those pertaining to the standard laminate (Table 1), but such close agreement must be considered as fortuitous.
20
0
30(]
400
i
500 600 Wavelength-rim
100
DISCUSSION
800
Fig. 5. Spectral transmission curves for glass standards. 09
O~
Laminated Glass
07 ', P..... Glass ~ ' X ~
0 6
Ob
,\
/
,, O4
03
/ Z T
e~ Glass
sue
"2
(
,
\ '\ 01
0
j-
\ 01
02
03
o'4
o5
o!6
o7
Fig. 6. Chromaticity data for glass standards.
If a variable colour is visually matched with a colour of fixed chromaticity there will be a point at which it is just possible to distinguish one colour from the other. This difference in colour between them is termed the "just noticeable difference" (j.n.d.) and may be represented by a short line or bar on the C,I.E. chromaticity chart. By changing the colour mixture of the variable colour and again matching with the fixed colour a twodimensional representation of the j.n.d.'s is obtained. The results of a large number of colour matching trials reported by MacAdam [8] indicated that the locus ofj.n.d, points around a given reference colour is approximately elliptical. MacAdam's ellipses are shown in figure 7 and represent the standard deviation of colour matching multiplied by a factor of ten. In the strictest sense the ellipses shown in figure 7 should be regarded as cross-sections of the j.n.d. ellipsoids in the colour solid corresponding to constant spectral transmittance, but the method of plotting the j.n.d.'s on a plane chromaticity chart remains a simple and effective way of representing small colour differences. In particular, the
Table 1. Transmission and chromaticity data for glass standards (D 65 o o illuminant)
Specimen
Thickness (mm)
Y (per cent)
x
y
24 (nm)
p
Tinted glass Float glass Float glass Plate glass Laminated glass
5-7 11-8 15.0 15.2 18"8
71.3 87"4 86.0 86'8 64.1
0-3284 0-3090 0.3079 0-3107 0'3264
0.3467 0.3310 0.3316 0-3332 0.3521
575 498 498 519 570
0-09 0-02 0.02 0'01 0.11
70
J. A. Hooper and M. P. Wassall
large variations in the magnitude of the j.n.d.'s in different regions of the chart are immediately apparent. Thus in the colour specification for railway signal glasses [9], for example, the permissible chromaticity zone for green glasses is much larger than for white, yellow and red glasses.
09 -
osi o]
'\
s;s "<'
,
•
/
The chromaticity of the standard laminate (Table I) is shown in figure 7, and its position in the chart indicates that j.n.d, values approximately half-way between the highest a n d lowest of M a c A d a m ' s values may be expected for this glass. The numerical differences between the ( x , y ) chromaticity values of the standard laminate (Table l) a n d those relating to the colour-quality control specimens (Table 2) are shown in figure 8: once again the differences have been multiplied by a factor of ten. A b o u n d i n g ellipse, symmetrical a b o u t the co-ordinate origin, is also shown. By
iI
Standard Glass
I
06
i
5oo
'!
(103
....
I
I
,
i
05
0"02 -x
@"~
t\
//
~
! / .d U
:
/
~j
\
--.
/
/
~
i
~
4!
",<'-
O O1
!
4
03
J
<
U
/7
. . . .
(J
..>;oo4
~-
.--I
_ cjP
o
I
o
/,
-001 •
/J O
O1
(32
/J03
/~"
_oo
O-4
0'5
O6
0 /
O8
_oo
-003
Fig. 7. Standard deviations of chromaticityfrom indicated standards, represented ten times actual scale on C.LE. chromaticity chart (after MacAdam [8]) with control specimen results superposed.
/
,/ ,,";/
Spec:rmerm
-002
-OO1
O 001 1Otx-×s ',
002
03
Fig. 8. Differences between chromaticity values of standard laminate and colour-quality control specimens (subscript s denotes standard).
Table 2. Spectral transmission values and chromaticity co-ordinates for laminated glass control specimens (D65o0 illuminant)
Location
Sample No.
Thickness (mm)
Y (per cent)
x
y
A4 A4 A4 A4 A4 A4 A4 A5 A5 A6 A6 A7 A7 A8 A8 B4 B4 B4 B4 B4 B5 B5 B5 B6 B7 B8
2 3 4 5 6 7 8 1 2 1 2 1 2 1 2 1 2 3 4 5 1 2 3 1 1 1
18"9 18'9 18"9 19'0 19'0 18"3 21 '8 18'3 18'5 18"8 18"3 18"8 21"6 21 '6 21'6 17.9 21 "5 21 '5 17"7 21.6 21-6 18'5 21"3 18"6 21 "5 18"2
64"2 65'0 64"5 63"5 64"1 68"8 65"4 67-5 65"7 66"6 65'9 66"5 65"5 66"0 65"3 66.9 65"7 64-5 67'8 64"9 66"6 65'6 68"9 66'8 65"2 66"7
0'3277 0"3270 0'3269 0'3269 0"3268 0"3253 0'3271 0'3255 0"3262 0-3273 0'3257 0"3270 0"3279 0"3263 0"3275 0-3246 0"3272 0"3277 0"3244 0"3261 0"3258 0"3264 0"3256 0"3260 0"3272 0"3261
0-3532 0"3523 0"3514 0'3517 0'3515 0'3504 0-3530 0'3502 0"3511 0"3519 0"3506 0'3517 0"3539 0"3522 0"3533 0"3495 0"3531 0'3538 0'3493 0'3520 0"3520 0"3513 0"3514 0"3506 0"3532 0"3508
Sydney Opera House Glass Walls: Colour Measurement and Control
71
Fig. 9. Viewthroughglass wall A8 (west) towardsharbour bridge.
transposing this ellipse onto figure 7 and comparing its size with that of others in nearby regions of the chart, it is reasonable to postulate that there should be no noticeable difference in colour between the standard laminate and any of the control specimens. In support of this assertion, a visual comparison of the specimens carried out under room lighting conditions (fluorescent tubes) did not reveal any perceptible colour differences. The foregoing results for the small laboratory specimens are most satisfactory, but inevitably the real test of the effectiveness of colour control is whether or not colour variations can be seen in the glass walls themselves. In this connexion the comments of Wright [4] are relevant: " O u r powers of discrimination are at their best when we are looking at two adjacent areas of colour with a sharp boundary line between them, when the areas are large, when the illumination is good and when we are using unrestricted binocular vision." These conditions are very similar to those prevailing in the glass wall areas, particularly for an observer inside the building looking out. In the view window areas (figure 9), for example, some of the largest of the glass sheets are separated only by a comparatively thin bead of silicone rubber. Despite these exacting conditions, however, there
were no visible colour variations in any part of the glass walls. CONCLUSIONS Chromaticity measurements carried out on small quality control samples during the production period indicated the presence of small colour variations in the laminated glass relative to the colour standard. These measured variations were slightly less than the j.n.d, values for the glass estimated on the basis of MacAdam's perceptibility ellipses. This interpretation was upheld by a visual inspection of the samples, which showed that the measured differences were imperceptible to the eye under room lighting conditions. In subsequent observations of the glass in situ there were no visible variations in colour over any region of the glass walls.
Acknowledgements--This work forms part of a wider study of laminated glass undertaken by Ore Arup & Partners on behalf of the New South Wales Government and the architectural consortium of Hall, Todd and Littlemore. The chromaticity measurements were carried out at the Department of Applied Optics, Imperial College, University of London, and thanks are due to Professor W. D. Wright for providing the necessary laboratory facilities and for his interest in the work.
REFERENCES 1. J. A. HOOPER,On the bending of architectural laminated glass. Int. J. mech. Sci. 15, 4, 309 (1973). 2. D. D. CROFT and J. A. HOOPER,The Sydney Opera House glass walls. Struct. Engr. 51, 9, 311 (1973). 3. H. SOWDEN,(Editor) Sydney Opera House Glass Walls, J. Sands, Sydney (1972). 4. W. D. WRIGHT, The Measurement of Colour. Adam Hilger, London, 4th Edition (1969). 5. W. D. WRIGHT,A photoelectric spectrophotometer and tristimulus colorimeter designed for teaching and research. Optica Aeta 1, 102 (1954).
72
J. A. Hooper and M. P. Wassail
6. 1. NIMEROFF,Propagation of errors in spectrophotometric colorimetry. J. Opt. Soc. Am. 43, 6, 531 (1953). 7. M. P. WASSALL,The analysis (~[su~/ace texture by goniospectrophotometry. D.I.C. Thesis. University of London (1972). 8. D. L. MACADAM,Visual sensitivities to colour differences in daylight. J. opt. Soc. Am. 32, 5, 247 (1942). 9. British Standard Specification No. 623; 1940. Colours for Signal Glasses./br Railway Purposes.
Une des n6cessit6s architecturales de la construction de la Maison de l'Op6ra Sydney est que la couleur du verre teint6 lamin6 employ6 sur les tours doit ~tre exactement conforme aux normes de couleur d6j/L accept6es. Comme le verre a 6t6 fabriqu6 par petits lots au cours d'une p6riode de deux ans, des contr61es s6v~res furent n~cessaires pour maintenir la qualit6 de la teinte requise. Les proc6d6s employ6s pour v6rifier l'uniformit6 de la couleur sont d6crits, et les r6sultats des analyses colorim6triques effectu6es sur les normes des teintes et le contr61e de la production sont donn~s. Les informations chromatiques sont 6tudi6es el, regard des teintes voulues et sont trouv6es conformes ~ l'aspect visuel du verre in situ.
Eine der architektonischen Bedingungen ftir buntes Schichtglas, das beim Bau der Glasw/inde des Opernhauses in Sydney benutzt wurde, war die Bedingung, dab es in Bezug auf die zu Beginn angenommene Farbnorm einheitlich in der Farbe sein mtisse. Da das Glas tiber einen Zeitraum von zwei Jahren in kleinen Posten hergestellt wurde, waren strenge KontrollmaBnahmen n6tig, um die erforderliche Farbeigenschaft des Schichtglases einzuhalten. Die Verfahren zur Prtifung der Gleichm/iBigkeit der Farbe werden beschrieben, u n d e s werden Ergebnisse kolorimetrischer Analysen dargestellt, welche an den Farbnormen und den Produktionkontrollproben durchgeftihrt wurden. Die Farbtondaten werden in Form subjektiver Farbunterscheidung gedeutet, und es wird gezeigt, dab sie mit visuellen Beobachtungen des Glases auf dem Bau vereinbar sind.
]
I
l
,
i
Sydney Opera House Glass Walls: Colour Measurement and Control J. A. HOOPER* M. P. W A S S A L L t
One o f the architectural requirements o f the tinted laminated glass used in the construction of the Sydney Opera House glass walls was that it should be uniform in colour relative to an initially accepted colour standard. As the glass was manufactured in small batches over a two year period, stringent controls were needed to maintain the necessary eolour quality of the laminate. The procedures adopted to check uniJormity of colour are described and results are given o f colorimetric analyses carried out on the colour standards and the production control specimens. The chromaticity data are interpreted in terms of subjective colour discrimination and are shown to be compatible with visual observations of the glass in situ.
a 0.8 m m thick clear plastic interlayer. In the view window areas located at podium level, nominal 15 m m thick clear glass was used, giving an overall laminate thickness of 21.8 mm. In the remaining areas the nominal thickness of clear glass is 12 mm, giving a standard laminate thickness of 18.8 mm. The thickness tolerances of the glass, particularly the tinted glass, are relevant to the problem of colour control. The specified tolerances were + 1.0 mm for the tinted glass and +0.3 m m for the clear glass. The specified tolerance for the two standard laminated sections was + 1.4 mm. The tinted glass may be described as pale bronze in colour, and is designated "demi-topaze" by the manufacturers, Boussois Souchon Neuvesel. It was specially formulated for the glass walls project and was manufactured using the traditional potcasting process [3]. In this process the basic constituent materials (e.g. sand, soda, lime), together with the colouring oxides, are contained in a refractory pot and heated in a furnace. On achieving the necessary mixing and fusion the molten glass is poured on to a roller mechanism from which it emerges as a red-hot continuous ribbon. After annealing, the glass is ground and polished. Each pot can hold little more than one tonne of glass and it took almost two years to produce the required 6000 m 2 of tinted glass. It was essential therefore that the most careful colour controls be exercised throughout this prolonged and intermittent manufacturing process. At the start of production, 75 and 300 mm square specimens of tinted, clear and laminated glass were produced by the manufacturers and were adopted as colour standards. Colorimetric data
INTRODUCTION T H E "glass walls" of the Sydney Opera House is the name given to the glass surfaces that enclose the openings between the roof shells and the podium structure. Each wall consists of a mosaic of glass sheets supported by a steel structure. The sheets vary considerably in shape and size, the maximum dimensions being approximately 4 m by 2.! m. Figure 1 shows the scale and geometrical form of the two north facing walls, the largest of these being 2 4 m high and 55 m wide. The general layout of the Opera House is shown in figure 2, in particular the numbering of the concert hall shells. The opera hall shells are numbered in the same way but prefixed with the letter B. The glass walls are referred to by the names of the shells they enclose. The glass walls were constructed during the period 1970-72. There were many technical problems that had to be solved before they could be built [1, 2], but one of the architectural requirements was that the colour of the glass should be uniform throughout the building.
T H E GLASS
The building is glazed in laminated glass consisting of a layer of clear plate or float glass, a nominal 6 mm thick layer of tinted plate glass, and *Ove Arup & Partners, 13 Fitzroy Street, London, W1P 6BQ.
tPye Unicam, Cambridge (Formerly of Imperial College, University of London). 65
66
J. A. Hooper and M . P. Wassall
Fig. I. View from harbour showing glass walls A4 (right) and B4 (left).
~
/Opera hall
where X, Y and Z are the tristimulus values that fix the proportions of the red, green and blue reference stimuli (X), (Y) and (Z) required to match the colour (C), and C is a measure of the amount of the colour and is given by C = X + Y + Z . The Xtristimulus value is given by x = y~ T ~ e ~ A , ~
(2)
),
Fig. 2. General layout of structure.
were obtained for the smaller specimens. Subsequently 7 5 m m square colour-quality control specimens of laminated glass were supplied by the manufacturers at a rate of one for every 200 m 2 produced. These specimens were compared both visually and colorimetrically with the laminated glass colour standard. DEFINITIONS AND T E R M I N O L O G Y Of the various systems of colour measurement currently available, the trichromatic system is undoubtedly the most widely used. In 1931 the Commission Internationale de l'Eclairage (C.I.E.) established a framework for colour specification in which a given colour, (C), may be expressed in the form C(C) - X ( X ) + Y ( Y ) + Z ( Z ) (1)
where T~ represents the spectral transmission curve for the glass, Pz denotes the spectral energy distribution of the illuminant, ff~ is the standard equal-energy distribution coefficient (see, for example, Wright [4]), and A2 is the wavelength interval, the summation normally being carried out over the range 380-770 nm. Similar expressions define the Y and Z tristimulus values. Furthermore, two of the reference stimuli are chosen in such a way that, upon suitable weighting of the distribution coefficients, the tristimulus value, Y, of the third reference stimulus also represents the percentage transmission of the glass. In order to separate colour quality from the amount of light being matched, the value of C in equation (1) is made equal to unity by normalizing the tristimulus values, i.e. x = X / ( X + Y + Z ) , and similarly for y and z. This gives the unit trichromatic equation ( c ) - x(X) + y(r') + z ( Z )
(3)
Since the sum of the coefficients x, y and z is unity, any colour may be represented graphically on a plane diagram with x and y co-ordinates.
Sydney Opera House Glass Walls: Colour Measurement and Control Such a diagram is the C.I.E. chromaticity chart (figure 3), which shows how all possible colours may be represented graphically in terms of these two coefficients. As the locus of the spectral colours is everywhere either straight or convex, and the resultant mixture of any two colours lies somewhere along a straight line between those two colours in chromaticity space, all other colours must lie within the area bounded by the spectrum locus and the straight line joining its red and violet extremities.
c'5 hd
0.7
s0
s{)~
~
Spectrum LoctJs (xyl
57o
O5
5~o
c
03
67
One of the advantages of colorimetric analyses performed to the C.I.E. system is that measurements can be obtained in an unambiguous form which, besides having an internationally agreed meaning, can be related to subjective descriptions of colour. Such analyses also avoid certain problems associated with the visual assessment of colour quality (e.g. different colour rendering characteristics of various illuminants; possible colour defective vision of the observer; differences in visual discrimination between observers with normal colour vision), particularly when these assessments have to be made over a long production period. An additional benefit to be derived from spectrophotometric analyses of laminated glass is that by extending measurements to the nearinfrared region of the spectrum the necessary data are obtained from which to estimate the moisture content of the plastic interlayer, a quantity which is directly related to the degree of adhesion at the glass/plastic interface and hence the structural integrity of the laminate. At the same time, however, it must be stressed that colorimetric analyses are designed to supplement rather than to replace the visual judgement of colour quality. INSTRUMENTATION
Oq
I 01
02
03
04
05
06
07
08
x
Fig. 3. The 1931 C.LE. chromaticity chart. Having plotted the chromaticity co-ordinates (x, y) for a given colour C (figure 3), a straight line may be drawn from the white point W (representing the illuminant) through C to intersect the spectrum locus at 2 a. The colour C may then be defined by 2a, known as the dominant wavelength, and the ratio of the distance CW to W2 a, known as the purity, p. From the subjective viewpoint this alternative definition of colour is sometimes preferable to that of chromaticity co-ordinates, especially when assessments of small colour differences have to be made. The colour of a Planckian (black-body) radiator at various absolute temperatures is also shown for reference. The chromaticity values for the C.I.E standard illuminants SA, SB, Sc and D65oo (corresponding to correlated colour temperatures of 2856, 4874, 6774 and 6504 °K respectively) are seen to lie close to the Planckian locus. The same applies to the equal-energy white, E, on which the units of the three reference stimuli (X), (Y) and (Z) are based.
The colour measurements were made using a modified version of the Wright spectrophotometer [5], illustrated in figure 4. The optical system of the present instrument is the same as the original, but the photocell detector system has been replaced by a photomultiplier and digital voltmeter. It is a non-recording instrument and measurements have to be taken at discrete wavelengths throughout the spectrum. In normal use the cross-sectional area of the beam incident on the sample is approximately 1 0 x 8 mm, and this configuration was used in the present investigation. The H.T. supply to the photomultiplier dynode resistor chain was chosen for stability of output and the chain itself constructed from high quality resistors. These were housed in a separate metal box to minimise heating effects in the photomultiplier tube and to complete the earth shield. The photomultiplier anode current was measured by monitoring the voltage developed across a 1 Mf~ load resistor. In order to reduce noise--which occurs mainly in the form of pick-up in the cables and Shot noise in the photomultiplier tube--a 10 #F capacitor was connected in parallel with the load resistor and could be switched in or out as required. The voltage displayed on the digital voltmeter was directed to a serializer unit which tripped the voltmeter at the required rate and
68
J. A. Hooper and M. P. Wassail
Fig. 4. Modified Wright spectrophotometer,
converted the signal into a form suitable for a tape punch unit. The paper tape readout could then be fed into a computer for the necessary calculation of chromaticity values. It is important in precise spectrophotometry that all possible sources of error are firstly isolated and then minimised. In the optical system, wavelength errors were minimised by regularly calibrating the instrument against spectral wavelength lines, and bandwidth errors were minimised by restricting the maximum bandwidth to 10 nm. Errors resulting from stray light, polarization and angular mis-alignment of the sample were considered negligible. Significant drift in the light source components could only be detected if measurements were made over periods longer than about fifteen minutes, and as readings at any given wavelength were taken in less than one minute, drift errors were also considered negligible. In the detector system, careful checks were carried out to establish the linearity of response of the photomultiplier over a wide range of illumination. Errors common to automatic recording instruments, e.g. pen inertia and slit width variation, were clearly absent in the present case. Although it is normal practice to quote chromaticity co-ordinates to four places of decimals, the fourth figure cannot be regarded as exact, Even using a high precision instrument such as the one referred to, there are some random fluctuations which are unavoidable and which give rise to small errors. The effect of such fluctuations on chromaticity values has been considered by Nimeroff [6] on the assumption that the chromaticity co-ordinates x i, y;, obtained from the ith set of individual measurements of Ta have a joint normal distribution about their mean values x,,, .v,,. On the basis of
wilh corers removed.
this analysis it is possible to construct an ellipse around any point in the chromaticity diagram within which a given proportion of the values are likely to fall. Wassail [7] has assessed the stability of the Wright spectrophotometer by taking repeated measurements on the same sample (x,, = 0.3644, Ym = 0"3284). In this case the computed size of the ellipse containing 95 per cent of all chromaticity points was defined by a major axis of length 0.0003 in the chromaticity chart, aligned in the yellow-blue direction, and a minor axis of length 0.0002. The size of the error ellipse may depend to some extent on its position in the chromaticity diagram, but the size quoted can be taken as being directly applicable to the glass sample measurements. RESULTS OF M E A S U R E M E N T S The spectral transmission curves, T~, for the standard glass laminate and the standard tinted glass are shown in figure 5, and the transmission and chromaticity values are given in Table 1. These values relate to the standard illuminant D6500, which is generally considered the one most representative of average daylight conditions. The spectral transmission values, Y, for the laminated and tinted glass specimens were 64.1 per cent and 71.3 per cent respectively. The corresponding C.I.E. coefficients x and y are plotted on the chromaticity chart (figure 6), but the values of dominant wavelength, 2~, give a clearer indication of the colour difference between the two specimens. Data for the clear plate and float glass are also included in ]'able I and figure 6. The chromaticity coefficients are similar to those for the illuminant, but there is a marked difference between the
Sydney Opera House Glass Walls: Colour Measurement and Control dominant wavelength of the plate and float glass specimens. The "green" of the plate glass and the "blue-green" of the float glass are plainly visible to the eye when the specimens are viewed "endon", i.e. through the 75 mm thickness, but the degree of colour saturation in both types of glass is so low that these differences will have a very 100
SO
6O
40 .7:
69
small effect on the overall colour quality of the laminated glass. Transmission and chromaticity data for the 26 laminated glass colour-quality control samples are given in Table 2. The thickness of these samples varied from 17.7 to 21.8 ram, well within the specified permissible range of 17.4-23.2 ram. The transmission values varied from 63.5 to 68.9 per cent, with an average value of 65.9 per cent. Values of x ranged from 0.3244-0.3279, with an average of 0.3264; for y the range was 0.34930.3539, with an average of 0.3517. Based on these average values of x and y, the dominant wavelength is 571 nm and the colour purity is 0.11. Evidently the average chromaticity values are almost identical to those pertaining to the standard laminate (Table 1), but such close agreement must be considered as fortuitous.
20
0
30(]
400
i
500 600 Wavelength-rim
100
DISCUSSION
800
Fig. 5. Spectral transmission curves for glass standards. 09
O~
Laminated Glass
07 ', P..... Glass ~ ' X ~
0 6
Ob
,\
/
,, O4
03
/ Z T
e~ Glass
sue
"2
(
,
\ '\ 01
0
j-
\ 01
02
03
o'4
o5
o!6
o7
Fig. 6. Chromaticity data for glass standards.
If a variable colour is visually matched with a colour of fixed chromaticity there will be a point at which it is just possible to distinguish one colour from the other. This difference in colour between them is termed the "just noticeable difference" (j.n.d.) and may be represented by a short line or bar on the C,I.E. chromaticity chart. By changing the colour mixture of the variable colour and again matching with the fixed colour a twodimensional representation of the j.n.d.'s is obtained. The results of a large number of colour matching trials reported by MacAdam [8] indicated that the locus ofj.n.d, points around a given reference colour is approximately elliptical. MacAdam's ellipses are shown in figure 7 and represent the standard deviation of colour matching multiplied by a factor of ten. In the strictest sense the ellipses shown in figure 7 should be regarded as cross-sections of the j.n.d. ellipsoids in the colour solid corresponding to constant spectral transmittance, but the method of plotting the j.n.d.'s on a plane chromaticity chart remains a simple and effective way of representing small colour differences. In particular, the
Table 1. Transmission and chromaticity data for glass standards (D 65 o o illuminant)
Specimen
Thickness (mm)
Y (per cent)
x
y
24 (nm)
p
Tinted glass Float glass Float glass Plate glass Laminated glass
5-7 11-8 15.0 15.2 18"8
71.3 87"4 86.0 86'8 64.1
0-3284 0-3090 0.3079 0-3107 0'3264
0.3467 0.3310 0.3316 0-3332 0.3521
575 498 498 519 570
0-09 0-02 0.02 0'01 0.11
70
J. A. Hooper and M. P. Wassall
large variations in the magnitude of the j.n.d.'s in different regions of the chart are immediately apparent. Thus in the colour specification for railway signal glasses [9], for example, the permissible chromaticity zone for green glasses is much larger than for white, yellow and red glasses.
09 -
osi o]
'\
s;s "<'
,
•
/
The chromaticity of the standard laminate (Table I) is shown in figure 7, and its position in the chart indicates that j.n.d, values approximately half-way between the highest a n d lowest of M a c A d a m ' s values may be expected for this glass. The numerical differences between the ( x , y ) chromaticity values of the standard laminate (Table l) a n d those relating to the colour-quality control specimens (Table 2) are shown in figure 8: once again the differences have been multiplied by a factor of ten. A b o u n d i n g ellipse, symmetrical a b o u t the co-ordinate origin, is also shown. By
iI
Standard Glass
I
06
i
5oo
'!
(103
....
I
I
,
i
05
0"02 -x
@"~
t\
//
~
! / .d U
:
/
~j
\
--.
/
/
~
i
~
4!
",<'-
O O1
!
4
03
J
<
U
/7
. . . .
(J
..>;oo4
~-
.--I
_ cjP
o
I
o
/,
-001 •
/J O
O1
(32
/J03
/~"
_oo
O-4
0'5
O6
0 /
O8
_oo
-003
Fig. 7. Standard deviations of chromaticityfrom indicated standards, represented ten times actual scale on C.LE. chromaticity chart (after MacAdam [8]) with control specimen results superposed.
/
,/ ,,";/
Spec:rmerm
-002
-OO1
O 001 1Otx-×s ',
002
03
Fig. 8. Differences between chromaticity values of standard laminate and colour-quality control specimens (subscript s denotes standard).
Table 2. Spectral transmission values and chromaticity co-ordinates for laminated glass control specimens (D65o0 illuminant)
Location
Sample No.
Thickness (mm)
Y (per cent)
x
y
A4 A4 A4 A4 A4 A4 A4 A5 A5 A6 A6 A7 A7 A8 A8 B4 B4 B4 B4 B4 B5 B5 B5 B6 B7 B8
2 3 4 5 6 7 8 1 2 1 2 1 2 1 2 1 2 3 4 5 1 2 3 1 1 1
18"9 18'9 18"9 19'0 19'0 18"3 21 '8 18'3 18'5 18"8 18"3 18"8 21"6 21 '6 21'6 17.9 21 "5 21 '5 17"7 21.6 21-6 18'5 21"3 18"6 21 "5 18"2
64"2 65'0 64"5 63"5 64"1 68"8 65"4 67-5 65"7 66"6 65'9 66"5 65"5 66"0 65"3 66.9 65"7 64-5 67'8 64"9 66"6 65'6 68"9 66'8 65"2 66"7
0'3277 0"3270 0'3269 0'3269 0"3268 0"3253 0'3271 0'3255 0"3262 0-3273 0'3257 0"3270 0"3279 0"3263 0"3275 0-3246 0"3272 0"3277 0"3244 0"3261 0"3258 0"3264 0"3256 0"3260 0"3272 0"3261
0-3532 0"3523 0"3514 0'3517 0'3515 0'3504 0-3530 0'3502 0"3511 0"3519 0"3506 0'3517 0"3539 0"3522 0"3533 0"3495 0"3531 0'3538 0'3493 0'3520 0"3520 0"3513 0"3514 0"3506 0"3532 0"3508
Sydney Opera House Glass Walls: Colour Measurement and Control
71
Fig. 9. Viewthroughglass wall A8 (west) towardsharbour bridge.
transposing this ellipse onto figure 7 and comparing its size with that of others in nearby regions of the chart, it is reasonable to postulate that there should be no noticeable difference in colour between the standard laminate and any of the control specimens. In support of this assertion, a visual comparison of the specimens carried out under room lighting conditions (fluorescent tubes) did not reveal any perceptible colour differences. The foregoing results for the small laboratory specimens are most satisfactory, but inevitably the real test of the effectiveness of colour control is whether or not colour variations can be seen in the glass walls themselves. In this connexion the comments of Wright [4] are relevant: " O u r powers of discrimination are at their best when we are looking at two adjacent areas of colour with a sharp boundary line between them, when the areas are large, when the illumination is good and when we are using unrestricted binocular vision." These conditions are very similar to those prevailing in the glass wall areas, particularly for an observer inside the building looking out. In the view window areas (figure 9), for example, some of the largest of the glass sheets are separated only by a comparatively thin bead of silicone rubber. Despite these exacting conditions, however, there
were no visible colour variations in any part of the glass walls. CONCLUSIONS Chromaticity measurements carried out on small quality control samples during the production period indicated the presence of small colour variations in the laminated glass relative to the colour standard. These measured variations were slightly less than the j.n.d, values for the glass estimated on the basis of MacAdam's perceptibility ellipses. This interpretation was upheld by a visual inspection of the samples, which showed that the measured differences were imperceptible to the eye under room lighting conditions. In subsequent observations of the glass in situ there were no visible variations in colour over any region of the glass walls.
Acknowledgements--This work forms part of a wider study of laminated glass undertaken by Ore Arup & Partners on behalf of the New South Wales Government and the architectural consortium of Hall, Todd and Littlemore. The chromaticity measurements were carried out at the Department of Applied Optics, Imperial College, University of London, and thanks are due to Professor W. D. Wright for providing the necessary laboratory facilities and for his interest in the work.
REFERENCES 1. J. A. HOOPER,On the bending of architectural laminated glass. Int. J. mech. Sci. 15, 4, 309 (1973). 2. D. D. CROFT and J. A. HOOPER,The Sydney Opera House glass walls. Struct. Engr. 51, 9, 311 (1973). 3. H. SOWDEN,(Editor) Sydney Opera House Glass Walls, J. Sands, Sydney (1972). 4. W. D. WRIGHT, The Measurement of Colour. Adam Hilger, London, 4th Edition (1969). 5. W. D. WRIGHT,A photoelectric spectrophotometer and tristimulus colorimeter designed for teaching and research. Optica Aeta 1, 102 (1954).
72
J. A. Hooper and M. P. Wassail
6. 1. NIMEROFF,Propagation of errors in spectrophotometric colorimetry. J. Opt. Soc. Am. 43, 6, 531 (1953). 7. M. P. WASSALL,The analysis (~[su~/ace texture by goniospectrophotometry. D.I.C. Thesis. University of London (1972). 8. D. L. MACADAM,Visual sensitivities to colour differences in daylight. J. opt. Soc. Am. 32, 5, 247 (1942). 9. British Standard Specification No. 623; 1940. Colours for Signal Glasses./br Railway Purposes.
Une des n6cessit6s architecturales de la construction de la Maison de l'Op6ra Sydney est que la couleur du verre teint6 lamin6 employ6 sur les tours doit ~tre exactement conforme aux normes de couleur d6j/L accept6es. Comme le verre a 6t6 fabriqu6 par petits lots au cours d'une p6riode de deux ans, des contr61es s6v~res furent n~cessaires pour maintenir la qualit6 de la teinte requise. Les proc6d6s employ6s pour v6rifier l'uniformit6 de la couleur sont d6crits, et les r6sultats des analyses colorim6triques effectu6es sur les normes des teintes et le contr61e de la production sont donn~s. Les informations chromatiques sont 6tudi6es el, regard des teintes voulues et sont trouv6es conformes ~ l'aspect visuel du verre in situ.
Eine der architektonischen Bedingungen ftir buntes Schichtglas, das beim Bau der Glasw/inde des Opernhauses in Sydney benutzt wurde, war die Bedingung, dab es in Bezug auf die zu Beginn angenommene Farbnorm einheitlich in der Farbe sein mtisse. Da das Glas tiber einen Zeitraum von zwei Jahren in kleinen Posten hergestellt wurde, waren strenge KontrollmaBnahmen n6tig, um die erforderliche Farbeigenschaft des Schichtglases einzuhalten. Die Verfahren zur Prtifung der Gleichm/iBigkeit der Farbe werden beschrieben, u n d e s werden Ergebnisse kolorimetrischer Analysen dargestellt, welche an den Farbnormen und den Produktionkontrollproben durchgeftihrt wurden. Die Farbtondaten werden in Form subjektiver Farbunterscheidung gedeutet, und es wird gezeigt, dab sie mit visuellen Beobachtungen des Glases auf dem Bau vereinbar sind.