gold film windows

8/9/2019 Gold Film Windows

1/7

Gold Coated Glass in the Building IndustryTH E DESIGN OF INTERFERENCE SYSTEMS FO R HEAT REFLECTIONRolf G rothFlachglas AG Delog-Detag, Fiirth, West Germanyand Walter ReicheltWC Heraeus GmbH, Hanau, West GermanyT h in gold jilms on window panes reject infra-red radiation andtransmit a proportion of v isible light that is dependent on the thicknessof coa ting , so provid ing an eflective means of protection from solarradiation. Gold Jilms on double-glazed windows have the furtheradvantage of reducing heat losses i n win ter, so redw in g fuel costs.

In modern office buildings large areas of glass areused to give an even distribution of light and, in th ecase of large open-plan offices, to give workers anuninterrupted view and a sense of contact with theoutside world. Such large window areas have,however, created problems. Inten se sunlig ht causesoverheating in these offices, necessitating extensiveair-conditioning.Ordinary window glass is almost completelytransparent to radiation extending from the ultra-violet to the infra-red. Th is range represents the

spectrum of solar radiation received at the earth'ssurface.Figure 1 shows the distribution of solar energygiven by Moon's air mass, where m=2 (I) , corre-sponding to an angle of incidence of 30" to the hori-zontal. Of the total radiant energy about 3 per centis in the ultra-violet'rangc (A < 0.4 pm), 51 per centin the visible range (h=0.4 to 0.75 pm), an d 46per cent in the infra-red range. T h e distributionof solar spectral energy is dependent partly on theheight of the sun, and partly on the moisture con-

Fig. 1 The spectral distribution ofd a r energy. Of the total radiantenergy in this example about 3 percent ir in the ultra-vielet r a w ,Mow 0.4 pm, 51 per cent in thevisible range, from 0.4 to 0.75 pm,while 46 per cent is in the infra-red


2/7

A sultry summer heat prevails in the Rhineland, and to avoid overheating in the newly built GermanIndustry House in Cologne the windows have been double glaaed with solar insulating glass type Gold4 / 2 6 by Flachglas AG Delog-Detag, giving 40 per cent transmiseion of visible light rays with only 26per cent of the solar energy entering the building

centration and the amount of dust in the atmo-sph ere . Basically, half of all solar radiation isinfra-red, lying outside the visible range.Requirements for Sun Insulating GlassFor effective lighting through a sun-insulatingglass, a high degree of opacity will be required in thisarea of infra-red radiation. Attenuation of infra-red radiation must take place mainly by reflectionand not by absorption, as the absorption of solarradiation leads to heating of the glass, which at ahigh tempe rature will transfer a considerable amoun tof the absorbed solar radiation into the room byconvection and long-wave secondary radiation.Also, glass designed to give protection from sun-light must conform to various requirements of lighttransmission. For example, in large open-planoffices with floor-to-ceiling glazing, the use of normaldouble-glazing with 80 per cent light transmissionhas little point; the window area would be too brightby comparison with the artificially lit area at theback of the room. On the Continent, in such cases,glass windows with a 30 to 40 per cent transparencyare used; in the USA the transparency used ismuch lower, from 10 to 20 per cent. On the otherhand, for other applications such as hospitalsand schools, at least 50 per cent transmission isnecessary.

Figure 2 shows the design of a glass unit coated toprovide insulation from solar radiation. I t is com-posed of two glass sheets forming a hermeticallysealed unit with the metal-to-glass edges sealed by an,adhesive or soldering process. T he heat-reflectingfilm is applied to the inner surface of th e glass whichis on the outer wall of the building, and is thus pro-tected from mechanical damage.

3(21~2w ez YrW -A 32e z

INNER GLASS OUTER GL4SS


3/7

T he performance of this type of sun-insulatingglass is best shown by an energy diagram such asFigure 2. This shows how much of th e effective totalsolar radiation is directly reflected (solar reflection)and how much is transmitted (solar transmission).Then there is th e secondary heat emission of th eglass by convection and long-wave temperatureradiation outwards an d inwards, depending on th erise in temperature of th e glass produced by th eamount of solar radiation absorbed.T he total amount of heat reaching th e interiorof th e room (total energy transmission) is thereforeth e sum of th e solar transmission an d th e secondaryemission into the room. Obviously th e transparencyof th e glass (in relation to th e sensitivity to light ofth e human eye) is also important. I t is usual to giveboth figures in th e form of a coefficient, transparency1energy transmission. The figures shown in th e energydiagram of Figure 2 relate to double-glazing withsemi-transparent gold applied to a single glass pane.Simple Transparent Gold Films

Effective sun-insulating glass for medium lighttransmission can be produced by a single golddeposit. Figure 3 shows the spectra l traxismission andreflectivity curves of an insulating glass unit treatedwith a gold film. Lig ht transmission (as comparedwith the sensitivity to light of the human eye) is 40per cent and total energy transmission 26 per cent.

This transmission is greatest in th e blue-green areaof th e spectrum an d decreases rapidly towards th einfra-red area. T o th e same extent, reflectivity tolong wave lengths increases up to 70 pe r cent. Thismeans that the extinction of th e infra-red radiation ofth e sun is du e mainly to reflectivity an d no t toabsorption.Sun-insulating windows with such semi-transparentgold films have proved effective on account of theirgood optical properties. From inside, they appearlight green in colour, but th e tint is so slight as no t tobe unpleasant. Looked at from outside, buildingshaving these windows are impressive an d colourful,as may be appreciated from the photograph on page63.Sun-insulating glass treated with a single goldfilm is, however, no t effective if considerably morethan 40 pe r cent transmission is required; th e thick-ness of the gold film would have to be reduced. Butas this thickness is reduced, so th e film tends to loseits selective properties, that is its low infra-redtransmission in relation to transparency (2). Evenwith th e sun-insulating glass with very low lighttransmission (about 10 to 20 pe r cent) th e effectivenessof these films is doubtful because gold films of thisthickness would increase th e intensity of th e greencolouring.Then there are architectural considerations to betaken into account. From th e architect's point of


4/7

The new headquarters building ofBMW in Miinich. This is glazed with atype of solar insulating glass, also byFlachglas AG Delog-Detag, that givesa blue reflection. The optid effeet ofthe blue g l a ~nd the white buildingmaterial-blue and white are thehousecoloursof BMW-have a pleasing;architectural efleet. Solar insulatingglass with various shad- of blue, goldand bronze is available to provide scopefor the architect in deeigningthe fagade

view, the exterior of the building should show abroad band of colour. Th is will combine with thestructural lines of the building to give better pro-portions and give the architect greater scope in thedesign of the fasade than can be achieved with apaler shad e of gold.Gold Interference Layer Systems

So the problem is one of changing the opticalproperties of the gold film in such a way as to retainhigh heat-reflectivity while greatly reducing visiblereflectivity and increasing transmission, renderingit achromatic.The method used is closely related to the well-known process of reducing the reflectivity of glasssurfaces used for instance in photographic apparatusand the bloom ing of instrument lenses. By applyingthin dielectric films with a suitable refractive indexand thickness, the reflected light rays at the surfaceand the interface of this additional film can beobtained with almost similar amplitudes but witha shift in the phase angle of 180 so that they

almost cancel each other out. In th is case, sinceabsorption is minimal, the non-reflected lightintensity appears as enhanced transmission.The high reflectivity of gold at long wavelengthsis caused by the high electrical conductivity of themetal and this, in turn, by the numb er and mobility - .of its free s-electrons. A necessary condition is tha t "the gold film is not made up of isolated "islands"bu t that it is homogeneous and continuous.Methods of applying gold by firing on to the glassoften do not produce continuous films, whereasvacuum-evaporated or vacuum-sputtered coatingson suitable substrates produce very low surfaceresistances of a few ohms per square with a 30 to40 per cent transparency.At shorter wave lengths, reflection and absorptionbehaviour become more complicated, at first in anarrow energy band by plasma-resonance caused bythe interaction of free and bound electrons, followedby inter-band transference of d-electrons. I n thecase of gold, these phenomena lie in the middle ofthe visible spectrum; they are responsible for the


5/7

yellow colour of gold. One can therefo re say that theoptical properties at long wave lengths are governedby the behaviour of the free electrons and at shorrwave lengths by the behaviour of the bound elec-trons. The se complicated relationships are expressedmathematically by the optical constants n and kin the equation:Ii=n-ik

where n stands for the dispersion factor and k for theabsorption factor; for example, for a certain thicknessof a gold film n = l and k= 2 if the wave length is500nm. But it must be emphasised that these are nottrue constants but that both n and k are complexfunctions of wave length and film thickness. Thesefunctions have to b e known in order to m ake quanti-tative calculations; at one time, writers differed con-siderably on the values of n and k, but there has beena distinct improvement in recent years, due no doubt

--., o improved methods of film application and moresophisticated method s of measurement.But even if the optical constants are suffcientlyexactly known, their mathematical processing isdifficult and time-consuming. The solution to thisproblem consists in calculating the reflectivity valuesas functio ns of the wave length of layer systems, when,for instance, a transparent gold film of 200 i% isdeposited between two dielectric layers one of whichis bonded to the glass substrate. Film thickness andrefractive index of the dielectric layers have to becalculated so that th e whole reflectivity in the visiblespectrum is reduced to a minimum, w hile at the same

time the reflection of heat radiation is brought toa high value. T he necessary calculations ar e ele-mentary, but as both complex phase angles an d com-plex Frenel coefficients are involved, th e resultingmathematical equations are cumbersome. FortunatelyKard (3)has proposed a method of calculation whichprovides a graphical means of evaluating th e complexreflectivity factors.Test results can be conveniently verified byevaporating a wedge-shaped film composed of th efirst dielectric material on to a glass substrate,applying to this layer a transparent film of gold ofuniform thickness and evaporating on to this asecond dielectric material, similarly wedge-shapedbu t so that th e two wedges are at 90" to each other.By this method one obtains a good overall impressionof all possible thickness combinations of th e two di-electric films and th e required combination can beeasily selected.These layers do no t show th e normal colour ofgold. Fo r instance, in the blue-green spectral rangethey have a reflectivity value between 5 and 10 percent, so that 70 to 75 per cent transmission can beexpected. Compared with a single gold layer, th etransmission maximum is shifted towards longerwave lengths on account of th e interference layers.In transmitted light these films ar e slightly yellowishwhen looked through from th e inside, giving clearervision with greater contrast.Gold is a metal that does no t easily bond to oxideand silicate substrates. I t is therefore expedient to

The Deutsche Lloyd officebuilding in Stuttgart makesuse of a type of gold-coatedinsulating glass that givee ahighly reflective blue appear-ance from the outside. Thecost of cooling this air-conditioned building in sum-mer has been greatly reducedby the installation of thisform of solar insulation byFlachglas


6/7

. .. - . . .. .. . .- .-*

. . , I 3- .,-,. . - ,c .y ," 2* . ' a::.,: .> ?? !::A;. ,,...._....,,, ; , .,. . ..This imposing building is the headquarters of afirm of consultanrs in Wiesbaden. In this case the gold-coated insulating glass, also by Flachglas, has a soft bronze tone which contrasts effectively with thewhite facings of the constructional material

embed the gold film in dielectric layers to secure asatisfactory bond of the layers in the pack and goodadherence of the pack to the glass substrate. Th isproblem has now been solved and a process evolvedwhich satisfactorily produces coated m aterial for useas insulating glass. I t is also possible, by using smalladditives to gold and by means of modern techniques,for instance electron beam evaporation, to apply 2to 3 pm glass layers on to the gold surface that areabrasion-resistant, so that they can be used satis-factorily when exposed to the atmosphere.Applications of Gold InterferenceLayer Systems

By combining gold films with interference layersfor sun-insu lating glass, new applications have beendeveloped, particularly in cases where high lighttransmission is needed .Figure 3 shows the spectral transmission andreflectivity curves for a technical sun-insulating glasswith a gold interference layer system of this type,and for comparison the corresponding curves for aproduc t with a single gold film. Compared with thelatter, the interference layers increase transmissionin the visible range considerably. Transm ission isincreased from 40 to 66 per cent which is only 14per cent less than that of a clear glass double-glazingunit (80 per cent). I n the infra-red range, on the

other hand, the reflec tivity of the gold film is largelyretained. Tha t the reflectivity values differ some-what for wavelengths of about 2pm for which theeffect of the interference layering is almost nil isdue to the fact that the thickness of the gold filmsis somewhat different in these products.By its very nature, the total energy transmission,44 per cent of the type with the interference layer,is higher than in the type with the single gold film.This is to a large extent governed by the greaterheat caused by the higher light transmission en-gendered by solar radiation in the visible range.Looked at from outside, the typical gold effect is ..-lost because of the anti-reflection effect of the inter-ference layers; instead, they have a slightly blueappearance. The illustration on page 65 shows abuilding glazed with this type of glass.In recent years developments in sun-insulatingglass have tended to extend the colour palette soas to give architects more scope in designing thefa~ adesf buildings. Plate glass with varying shades ofblue, gold or bronze, when looked at from the outside,is available. The building illustrated opposite has ahighly reflective blue exterior in contrast with thatshown on page 63, while the building shown abovehas bronze-coloured glass. Light transmission liesbetween 66 and 22 per cent with the correspondingenergy transmission between 44 and 15 per cent (4).


7/7

I I I I 1 I II03 W 0:s 0.8 1 2 . 4 6 8 1 0WAVELENGTH IN p n

In all these cases the gold film is the actual selectivefilter element. Various technical values and differentcolours are obtained by a combination of the goldfilm with interference layers and alloy layers andthe selection of the appropriate film thicknesses.

from about 4 to 30 (zm with a maximum atabout 10 pm.In this range of wave lengths, transparentgold films possess very high reflectivity.Figure 4 shows as an example the spectralreflectivity of a gold interference coatingsystem on the side of the glass exposedto the atmosphere. In the visible range,reflectivity-including the effect of the inter -ference layer-is very low and transm issioncorrespondingly high (light transmissionof the corresponding insulating glass pane=66 per cent). In the near infra-red range,reflectivity increases steeply, reaching morethan 96 per cent in the 4 pm wave lengthregion. This corresponds to an emissionfigure of ~ =0 .0 4 compared with c=0.84(5) for non-layered window glass.This coating reduces the heat exchangebetween the two window panes to almost nil,with a considerable increase in the heatresistance of the air space and a correspond-ing improvement in the heat loss. In all,heat loss, 2.6 kcal/mahO C or norm al doub leglazing with an air space of 12 rnm isreduced to 1.5 kcal/m2h0C. The degree of heatinsulation resulting from the film is approximatelyequivalent to that given by a 30 cm thick wall ofpolished brick or tiles.

Reduction of Heat LossesDouble-glazing treated with a gold film has anotheradvantage, apart from that of protection from solarradiation. Its heat transfer value is much lowerthan that of norm al double-glazing. This meansthat in winter, w ith low outside tem peratures , heatingcosts can be reduced by about 40 per cent.A normal double glazing unit with the usual 12mm air space has a heat loss of 2.6 kcal/m2h0Ccompared with 5 kcal/mahOC for single glazing.This improvement is due to the additional heat in-sulation of the enclosed air space.Heat transfer between the colder outer pane andthe inner pane is due first to the therm al conductivityof the air space with additional convection andsecondly to the radiation exchange between the twopanes of glass. T he radiation exchange with an airgap of 12 mm is about double that of the figure ofthermal conductivity and convection.Th is heat exchange is considerably lessened by theinfra-red reflecting layer because the layered glasshas a very low emissivity. T he heat radiation of theglass panes corresponds quite well to the spectralenergy distribution of a black body at approximately300K (room temperature) and includes the infra-red

Methods of Applying the CoatingVacuum-film methods are used exclusively, asthey are the only ones which give continuous goldfilms without forming an "island" effect, a con-tinuous film being essential for high infra-redreflectivity. Th e coating is carried out by cathodesputtering or by evaporation deposition methods.Processing large glass sheets up to an area of 10 m2has been made possible by the development of largevacuum plants and by advances in coating technnques,thus ensuring a uniform thickness over such largeareas.Thus, in addition to its now well-established use

for coating glass for heat pro tection in plants operatingat very high temperatures, gold has found a newapplication in the building industry. Sun-insulatingglass is also being increasingly used for the windowsof air-conditioned trains on the Trans-EuropaExpresses and on the In ter-City lines.References

1 P. Moon, J. Franklin Znst., 1940,230,5832 H. Schroder, Glastechn. Ber., 1966,39, 1563 I?. G. Kard, Dokl. Akad. Nauk S.S.S.R., 1956,108, 604 "lnfrastop-~onnenschutz~1iiser", l a c h ~ l a sG ~ e l o ~ -Detag5 J. I. Yellot, ASHRAE Journal, 1964, (Jan.), 87

gold film windows

Documents