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DAYLIGHT AND ITS APPLICATIONS

Prof. Christian BartenbachBartenbach LichtLabor, Rinnerstraße 14, A-6071 A-Aldrans

Tel. 0043-512-3338-0Fax. 0043-512-3338-88

E-mail: info@bartenbach.com

Abstract - According to evolution, man´s visual perception is adapted on daylight. Thus, daylit offices are ofhigher acceptance than rooms which are predominately illuminated by artificial light.Beside this subjectiveaspect, economical considerations plead for an optimized use of daylight.To fulfill both, economical andpsychological requirements, innovative daylighting systems are needed which avoid thermal loads but createwell ballanced luminances between task zone, immediate and general surrounding zone to guarantee a stabilevisual environment.Comparing conventional daylighting systems with new innovative systems by means ofscientific-statistical methods redirecting devices are objectively and subjectively advantageous regardingdistribution of light in depth of rooms, glare protection and visual link to the outside.This means, besides anincreased efficient usage of poorly daylit areas (windowless zones), a demonstrable increase of psycho-physiological efficiency and less fatigue can be achieved by generating stressless reading-conditions for workon VDUs.By seperating openings in upper regions for daylight penetration and distribution and lower regionsfor glare free visual link to the outside, both, individual user defined demands and requirements concerningthermal protection (room climate) can be fulfilled with new integrated components.By means of innovativedaylighting techniques the psychological well-being and the productivity are significantly increased.Thepossibility of integrating artificial light within the daylighting concept investment costs and running costs canbe decreased, too.

DAYLIGHT AS A MEDIUM

Daylight has determined the development of the opticalorgan. The eye reaches its highest capacity in daylight;the subjective judgement of a lighting situation is made inconscious or unconscious comparison with daylightexperiences.Due to the great distance of the earth from the sun, sunrays reaching the surface of the earth are almost parallel.However, the angle of incidence is subject to ongoingchanges that differ with time and place and follow thelaws of nature; these changes are characteristic of daylightand thus distinguish it from artificial light.Sunbeams are refracted when they enter the atmosphere,but they are primarily diffused. The degree of lightdiffusion depends on the local composition of thepenetrated medium. With regard to time and place, thechanges in the atmosphere are largely manifested in theclimate; regionally, they are evident in the form ofdifferent weather conditions which influence intensity,composition of spectrum and directedness of sunlight.Many periodic cycles in nature and man (seasons;alternation of day and night; etc.) have their cause in thefluctuations of daylight according to the laws of nature.Under clear skies, daylight possesses an illuminance of upto 100,000 lux and a strict directeness which is visible inthe „geometrically defined“ sharpness of hard shadows.Under cloudy or foggy conditions, the level ofilluminance reaches only a fraction of this (one tenth to

one twentieth); the spreading light is diffused so stronglyand is reflected so often that a state of shadowlessdiffusion occurs. The sky changes its colours from thecrystal blue of midday to the flaming red of sunset and tothe colourless dusk and the darkness of night.The primary spectrum of the sun is changed or reducedwhen radiation passes through the atmosphere; theremaining „part“ represents the daylight with which weare familiar. The orginial structure of the sunlightspectrum exhibits a maximum of energy in the blue-greenarea, whereas the energy peak of the „terrestrial“ sunlightis shifted to the yellow-green wavelenghts.With its luminance of about 109 cd/m2, the sun outshinesall artificial light sources. Its energy on earth measuresbetween 600 to 900 W/ m2. Another characteristic ofdaylight is the ratio between horizontal and verticalilluminance.When the sun is in a medium or high position, the verticalcomponent amounts to about 70 to 80 % of the horizontalcomponent. If the sun is relatively low, the verticalcomponent predominates, thereby giving objects anappearance different from that caused by a high sun.Whenthe sky is overcast, shadows are by and large absentbecause horizontal and vertical primary light incidence(luminous intensity) and the increased component ofdiffused light are about equal.More typical for the daylight situation, however, is thechanging distribution of primary light caused by themovement of the sun, clouds, objects and the observer.

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Even a clear or overcast sky does not have the sameluminance everywhere; this depends on the height of theangle and on the direction of the sky. Under a cloudlesssky, the luminance (on the side opposite the sun) is higherat the horizon than at the zenith, whereas the luminosity atthe zenith is highest when the sky is perfectly overcast.For human beings and other life forms, light is the mostimportant medium that transmits information. Theinformation capacity of light consists in its intensity,time-space distribution, direction or straightness, creationof shadows and the composition of its spectrum. Primarylight does not bear information and is actually invisible.In the way that a series of letters turns into informationthrough systematic structuring and combinations, light isendowed with its information structure only when itcomes into contact with reflecting, absorbing andtransmitting material. Primary light is the „cause“ ofillumination, not its effect. The „effective light“, whichmakes objetcs visible, is the secondary or environmentallight.For the most part, secondary light is reflected light whichenters the eye as a modulated form of primary light andmakes an object, a scene or a fact identifiable. Thequalitative variety of secondary light can only developaccording to the inherent possibilites of primary light. Inthis regard, daylight possesses the highest potential. Thecontinuous changes in primary light therefore influencesecondary light.The almost endless variety of appearances in which light;especially dayhlight, can cast an object, becomes evenmore powerful when movements of the object or of theobserver take place.

DAYLIT ROOMS

If a room is illuminated by daylight, it requires windowswith an appropriate form and size. Through them,daylight enters the interior of the room. A window is amaterial, light-transmittance system which light mustpenetrate before it can become, through multiplereflection the secondary light that is characteristic of eachroom.The relation between light reflection and light absorptionat the window and inside the room determines (subject tothe incoming primary light) the luminous intensity in theform of a luminous flux balance. If no light wereabsorbed in the room (given a theoretical degree ofreflection of 1) and no light were reflected to the outside,this fictitious room would become in short time (that is,with the speed of light) infinitely bright since endlesslyincoming light would not be »disposed of«. But sincereflection never reaches a degree of 1, each reflectionabsorbs part of the light so that, without incomingprimary light from outside , there would occur, with equalspeed, the absence of a secondary »effective« light.Through the supplied primary light, the luminous fluxbalance guarantees a constant surplus of inflowing over

reflected light (the secondary effective light), but ensuresthat neither absorption nor reflection becomes toodominant. The more light enters the room, the more it willbe absorbed and reflected to maintain this balance. At thesame time, the repeatedly reflected surplus of light alsoincreases so that the luminous intensity of the room varieswith the incoming daylight. The structure of thesecondary light in the room, the effective light that can beused, changes very little because of a higher or lowerincidence of light. The radiation energy can fluctuatewithin certain limits without changing the visual objects(i.e., the available information).

CRITERIA OF THE WINDOW OPENING

These include:

- quantity of daylight- distribution of daylight- visual perseption and distribution of luminance in the room, and relation to the outside environment- sun protection- energy

With the aid of these elements, we will analyse belowactual daylight technology in conjunction with visualperception processes.

QUANTITY OF DAYLIGHT

All components contained in primary daylight are subjectto a time rhythm and are changeable in this rhythm. Theexisting quantity of light outdoors for a specificgeographic location is always set. The quantitativechanges in the course of the day and the year are shown inPictures 1a-1c for the geographic location of Munich.

Pic. 1a: Illuminace levels in the open air; Overcast sky

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Pic. 1b: Illuminace levels in the open air; Medium bright sky

Pic. 1c: Illuminace levels in the open air; Clear sky

This statistical data was gathered over a period of timeand is prerequisite to examine the quantity of availablelight that is subject to changes in the course of time.The data forms the basis for calculating the amount ofdaylight and energy. Actual values are juxtaposed withthe necessary nominal values for the interior of the rooms.The nominal values were chosen according to the sameconsiderations that guided the selection of values forinterior illuminance documented under DIN 5035. Theconcept of nominal illuminance (i.e., the necessaryprimary luminosity as it relates to immediate use) is thesame as that used in DIN 5035.

THE DISTRIBUTION OF DAYLIGHT

It was necessary to analyze the amount of daylight as anabsolute quantity in order to establish the dimension oflight quantity for the various work areas and to point outthe accompanying components. From a logical point of

view, one cannot separate the amount of daylight from itsdistribution in a room. The only question is how muchlight is necessary in which areas. Such demands arisefrom our knowledge of visual perception which presetsthe distribution of luminance. In an ordinary room withside daylighting, a window of a certain size (whichdetermines the quantity) results in a typical distribution ofdaylight because the position of the opening in the roomeffects a certain distribution of light.Pictures 2a and 2b show that, given the same windowsurface and the identical amount of light, the distributionof light changes toward the interior of the room. Quantityand distribution act in combination and are co-dependent.

Pic. 2a: Daylight distribution, normal situation

Pic. 2b: Daylight distribution by means of staggered levels

Structural aspects of the room, such as

-size of window-height of room-depth of room and other geometrical apsects-reflective surfaces and their location-compass direction-the technology of the materials, glass, sun-shading, redirecting systems-outside factors-outside building structures-outside reflection

have a relevant influence on the quantity and distributionof daylight.

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REDIRECTING OF THE SKY’S LIGHT

The classical methods used to direct light includereflecting surfaces and prisms.Pictures 3a-3c show systems of light redistribution.Irradiation can be effected via simple redirecting systemsor via the ceiling. Pictures 4 shows these possibilities.In rooms with more depth, redirecting can be carried outvia the ceiling. However, a reflecting surface (e.g. onemade of aluminium) mounted on the ceiling directs anddistributes the light into the depth of the room. The moredirected reflection there is on the ceiling surface, thebetter the control of direction.There are, of course, various possibilities to achievedaylight transparency through redirection of daylight. Thechoice of technology depends on the task at hand and onadditional demands, such as glare restriction surfacefacing outwards, integration with sun-shading, etc.

Pic. 3a: Prisms, no transparency

Pic. 3b: Re-direction lamellae (partial transparency)

Pic. 3c: Voucher building (exaple)

Pic. 4: Targeted re-direction of daylight with al reflecting skylight system

REDIRECTING OF DAYLIGHT IN THE CASE OFSKYLIGHTS

Some installations carried out by the BartenbachLichtLabor show the influence of the reflector effect ofskylight systems. It should be made clear that daylight,despite the dimensions of geometry, can be directed.Such results indicate the possibilities of directing daylightin a new dimension because the precision of the applied,directed reflection was used in no small way with theskylight lighting direction.

The technology can- be carried out with large mirror reflectors, including those that are not round (Picture 5a),- throug miniaturisation, including a mirror grid system (Picture 5b),- or through prism systems (Haus der Geschichte, Picture 5c).

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Pic. 5a: Airport Zürich-Kloten; reflecting skylight system

Pic. 5b: Art Museum Berne

Pic. 5c: German Hisory Museum, Bonn

VISUAL PERCEPTION

Distribution of luminance in roomsPerception mechanism are dependent on certain laws ofnature in order to work without interference. In this sense,it becomes necessary that the attentiveness zones whichare identical with the object areas supplied with themaximum level of luminance as that of the infield.The ambient field, i.e. visual environment, receives aluminance (environment luminance) that is incoordination with it. These regular connections can bederived, for example, from stable conditions forperception (Picture 6) which can be observed at VDUstations.

Pic. 6: Constant of perception coordinationg lighting intensity of infield and surrounding area

They were determined on the basis of the constant outputof luminance whose essential parameter is the adaptationconstancy (area of invariance of luminance).Such connections (i.e., objective research results,experience data with DIN-recommendations, but alsoideas for particular appearances) can help develop theappropriate distribution of luminance for a specific task oruse.The constantly changing luminance of daylight is coupleddirectly with the colour temperatures. It is safe to assumethat these range from 500 to 7,000° Kelvin.If one uses only levels of luminance which are calculatedon the basis of a TQ = 2-5 %, this indicates a range ofilluminance of 200-500 lux (on average), (Ea = 10.000lux).The areas of luminance in the room thus measure L = 25-150 cd/m². In combination with a colour temperature of4,500-7,000° Kelvin, a room ambience is created that hasthe appearance of dusk in relation to the outsideenvironment.

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EXAMPLE OF A ROOM WITH SIDEDAYLIGHTING

If a room whose window surfaces are dirctly in the fieldof vision and have a luminance of at least 1,000 cd/m²(given the aforementioned outside illuminance of 10,000lux), the appearance of the room follows this adaptationluminance .But for the right functioning of visual perception duringvisually-oriented activities , the adaptation luminanceshould be adjusted to the demanded infield luminance;the environmental luminance should therefore not surpassL = 25-150 cd/m².The window whose luminance is about ten times higherrepresents a substantial source of glare and mental stress.Pictures 7a-7c show 30 persons who were tested at VDU-stations in the Bartenbach LichtLabor with eight differentmethods that correspond to different brain activities.

Pic. 7a: Test room at Bartenbach LichtLabor with daylight redirection louvres

Pic. 7b: Results of fatigue tests

Pic. 7c: Results of performance tests

Three completely different window positions wereinstalled for the test persons to execute their work:

-clear window-clear window with glare protection device and a reduction of luminous intensity to c. 300 cd/m²-daylight re-distribution system with a reduction of luminous intensity to c. 300 cd/m² providing the same brightness in the user’s field of vision as a clear window does.

Picture 7c demonstrates that the light distributionachieves significantly improved values with regard tomental stress.The curves of fatigue clearly indicate that, given the sameilluminance, the situation with a simple window (glare),compared to one with redirecting lamellae, represents afar higher level of stress.The result in Picture 7b confirm this. The problem ofaccepting this situation is that such disturbances areunconsciously corrected in our perception, but arementally very stressful. We have gotten used to theappearance of such rooms. They appear bright to us, whenin fact they are not. The optical illusion is created by thebright outside landscape which we visually transpose intothe inner environment. This also prevents the appearanceof twilight; the rooms appear to be radiantly (glaringly)bright.Picture 7a shows the impression of a room with VDU-stations in which daylight is redirected via the ceiling andthe »anti-glare curtain« and window’s luminance wereadjusted to the luminance required by the room. Theimpression emitted by the room is tyPical for its moderatelevels of luminance in the different areas.

LI = 100-150 cd/m² LU = 30-100 cd/m².

This impression is also pleasant for this type of work. Atsuch rather low intensities of daylight, the room’sambience is sensitive, reacts to changes and suits opticalperception processes. The twilight impression disappears

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due to the mirror surfaces, which transform part of theoutside world, and to the colouring used.

ANTI-GLARE SYSTEMS

The criteria for anti-glare systems are

-the required reduction of luminance ,-luminous transmittance,-the view to the outside environment,-the light distribution.

This reduction of luminance with current state of the artanti-glare systems entails an almost proportionate loss ofthe light. The loss of light can be somewhat alleviatedonly through light diffusion. With an increase intransparency, a direct cancellation of luminance should beexpected.

ANTI-GLARE BLINDS

Picture 8 shows a test room with installed textile anti-glare blinds for VU work. For the mind, the anti-glareblind is advantageous compared to a clear window, but issubstantially less so than a redirecting device with noglare.This anti-glare system works by installing a limited light-transmissive foil in front of or behind the glaring windowsurface. The reduction of glare luminance is controlled bylimiting the passage of light and by changing the lightdistribution which is dependent on the outside lightdistribution of the sky.

Pic. 8: Test room with glare shield roulau

Picture 8 shows the effect of the resulting appearance aswell as the light-engineering changes such as luminance

values and distribution of illuminance. With respect tomental stress and fatigue, the results are shown incomparison with a clear window in Picture 7b and 7c.This test is symptomatic for systems of this kind. It leadsto the conclusion that the reduction of the amount of lightis offset by the reduction of luminance, with minimaleffectiveness.

VERTISO

The Vertiso anti-glare system has similar characteristics.However, the flexibility of the system makes it possible tohave a better effect of light of the anti-glare device interms of light reduction. It is necessary, however, to find acarefully adjusted level of glare luminance by taking intoaccount a reduced degree of reflection and to establish theprecise location for seating. This solution is usually notpractical for rooms with greater spatial depth.

ABSORPTION GLASS AND REFLECTION GLASS

Their use as anti-glare devices for glare luminance andlight transparency is limited. The reduction of glareluminance ist practically proportional to lighttransparency. The resulting effect is similar to the anti-glare blinds. In terms of appearance, the transparency ofcolour-neutral glass is unchanged. The result is similar tothat of anti-glare blind systems.The outside luminance, reduced to the glare luminance, isadjusted to the adaptation process and results in aperfectly sufficient outdoor view.

ANTI-GLARE VENETIAN BLINDS

When appropriately installed, anti-glare venetian blinds(picture 9) can have properties similar to the Vertiso. Thehorizontal positioning and the flexibility can for a certainglare luminance improve the light values, and the positionof the lamellae can be adjusted better to the specificlocation of the users.

Pic. 9: Perforated louvres

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As we can see in picture 9, the transperancy can beimproved with the appropriate perforation.

ANTI-GLARE SYSTEMS WITH INTEGRATEDDAYLIGHT DISTRIBUTION

The study of state of the art anti-glare systems shows thatthe various demands, e.g. reduction of glare luminance,maximizing transperency, luminous transmittance, outsideview, are difficult to reconcile.Classical anti-glare blinds, in particular, clearly show thatthe reduction of glare luminace also drasticallydiminishes luminous transmittance and obstructs theoutside view. If one changes the principle by directing theincoming daylight to the ceiling, the amount of incominglight remains almost constant and the glare luminance canbe reduced to the desired degree.These systems are devised for maximum redirection ofdaylight and must be expanded for the function ofluminance reduction.The following systems are available:

1. Adjustable redirecting mirror-lamellae

These systems exhibit a luminous transmittance of about60% throughout the room. The system is geared towards arigorous reduction of luminance even for deeper rooms.To make it possible to look outside (similar to a curtain orfabric), the lamellae were perforated.Glare luminance ist adjusted to the VDU work stations.The E-distribution is even, sufficient and redistributed;and pictures 7b, 7c show the „mental comparisons“ andthe fatigue data. These are far superior for redirection andanti-glare systems. Interestingly enough, the perforatedsystems (i.e. with an outside view) result in less fatigueand mental stress than the closed systems.The profile („fish“) redirection system in picture 10shows a specific reduction of glare luminance andluminous transmittance for deep rooms.

2. Anti-glare prisms and daylight redirecting

3. As an optical medium, a prism redirects light, andreflects it in the area of total reflection. Prisms aretherefore very practical if functions such as light-redirections and reduction of luminance throughredirection are appropriately optically applied.

APPLICATIONS AREAS FOR INTEGRATEDDAYLIGTH REDIRECTION WITH ANTI-GLAREFUNCTION

As shown in picture 11, the light distribution is effectedby means of mirror reflectors, redirecting the diffusedaylight evenly onto walls and into the wider area around

Pic. 10: Switch-over divice for light deflection, called "fish"

Pic. 11: Art Museum Wolfsburg

the workplace. The daylight medium floods the room in away that permits the exhibition of any kind of objects.The redirecting elements are glare-free, and in thebrightness they produce they integrate into the ambientluminous intensity.Reflecting venetian blinds can become „shining daylightwalls“ with reduced luminance by installing the anti-glareblinds separately. The view to the outside (given distancesbetween the lamellae that are geared towards certainlocations) is limited but still sufficient. The effect ofperforated systems is reminiscent of transparent curtains.It is therefore a technological system that can „correct“the usual window combination.Permanent systems, like the „fish“ reflector profile or theprism profile plates, are suitable as units by themselves,installed beteween the window panes or as sliding

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partitions, and also as prefabricated glass elements(similar to glass blocks). Depending on the opticaltechnological treatment as a so-called "„Japanese wall“ orin combination with Refloglas, the appearance of suchwalls can express an individual character.

RELATIONSHIP TO THE OUTSIDE

The use of daylight today is mostly limited to sidedaylighting through windows. Punctuated facades serveas a solution to demands for smaller openings.Because buildings heights are limited for economicalreasons, the resulting rooms are at most 5 meter deep. ThetyPical structure of office buildings is one room on eachside separated by a hallway. Since lighting conditions areinsufficient, daylight is very often supplemented byartificial light.How much opening do human beings need for a normalrelationship to the outside world?This issue was ivestigated in a number of tests. To statethe result upfront: the necessary opening to the outsideshould be at least 20 % of the facade`s surface.

Pic. 12a/b: Glass test results by the firm Flachglas

Increasing this to 30 % does not give better illuminance.(Picture 12a, 12b)The “view to the outside” represents a substantial anddifficult criterion for all installations of light redirecting,anti-glare and sun-shading devices. Blocking light,redirecting, and sun-protection obstruct the view to theoutside. An easy solution to the problem is to separate thedayligth openings according to the necessities of theirfunction. Some openings serve the incidence of light andothers maintain the relationship to the outside. At the rightplace and de-glared, the openings for the incidence oflight can take in the necessary amount of light andredistribute it appropriately without glaring. The openingsto the outside can be designed to fullfill this task. Thismight prevent potential distractions and interference inmental concentration.

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THE PRINCIPLE OF SUN-PROTECTION

Two principles are to be differentiated here:fixed and flexible sun-protection.

Flexible sun-protection.The course of the sun during the day and year can bedetermined with regard to a specific daylight openingwhich is characterized by the geographic location of thebuilding and the geometric position of the window. Thismakes it possible to construct a sun-protection device thatallows for a precise separation between the path of the sunand the rest of the sky, that possesses a high degree ofreflections to the outside for the areas and angles thatcomprise the sun`s daily and yearly course, and that has ahigh degree of transmittance to the inside for theremaining areas. Such systems can be realised as prismstructures in glass-like building materials or as structuresmade of mirror material.

Flexible sun-protection:The disatvantage of a fixed sun-protection is that a staticsystem is used to shut out all possible paths of the sun.This results in a faulty blocking of the sun, therebylimiting the values of possible luminous flux.Theoretically it is only necessary to shut out the 0,5° ofthe sun`s opening. However, this would necessitate anongoing tracking of the sunprotection system along twoaxes. If one settles for a compromise and limits themovement in one direction (e.g., the altitude angle), oneattains a sun-protection with only a very narrow area ofhigh reflection, thereby cutting out a small segment fromthe section of the sky that is available.

Pic. 13: Prinzipal function of a flexible sun-protection

The rest can be transmitted through the sun-protectionwithout substantial loss.This system technology makes it possible to increase theselection number substantially especially with themoveable prisms based on the principle of retro-reflection. One can achieve a luminous transmittance of τ0.7-0.85 and g-value for outside mounted prisms of g=0.06-01.

INTEGRATED SUN-PROTECTION AND LIGHT-REDIRECTING ELEMENTS

The seperation of the functions of sun-protection andlight-redirecting is actually only possible in areas withoutdirect infiltration of sunlight: in our altitudes, on parts ofthe north side, in areas of inner courtyards, or in shadedareas, depending on the surroundings. Since, in general,light redirecting possesses funcitons other sun-protection,an integration into a specific construction project is onlypossible in an individual fashion.

Pic. 14: Airport Zürich-Kloten, roof

It is possible in the case of prismatic optimized systems(especially flexible ones) to let in portions of the sun (indoses related to quantity and time). The optimum g-valuesare, for example, not necessary in winter, and part of thesun may pass through the light-redirecting device. Sincethe sun produces 8 to 10 times more than the diffused skyradiation, light can be increased during the time of lesslight and the accompanying heat can be used. Withsystems optically constructed in a suitable way withreflector lamellae, the optimum g-values are not necessaryfor specific tasks. However, if a compromise can beaccepted, such integrations can be implemented.