Evaluating Visual Comfort and Performance of Three Natural Lighting

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Building and Environment 41 (2006) 1128–1135

Evaluating visual comfort and performance of three natural lightingsystems for deep ofce buildings in highly luminous climates

Carlos Ernesto Ochoa, Isaac Guedi CapelutoClimate and Energy Laboratory in Architecture, Faculty of Architecture and Town Planning, Technion—Israel Institute of Technology,

Technion City, Haifa 32000, Israel

Received 2 August 2004; received in revised form 18 October 2004; accepted 3 May 2005

Abstract

This work, part of a wider study, presents a qualitative and quantitative approach to evaluate daylighting systems for use in ofcebuildings located in latitudes where natural luminous conditions throughout the year are of high solar radiation, as in Israel. Theirwidespread application in this kind of climate, where the excessive penetration of direct radiation can be a problem, is possible. Theycan produce a consequent improvement of working conditions and energy savings, yet this is not the case now.

Three different systems that affect penetration of daylight in a sidelit ofce space were analysed: a single window without anyexternal protection, a horizontal lightshelf and a basic anidolic concentrator, mounted on the view window, together with improvedreectances of the surface’s nishes making ofce space. These were simulated through Radiance in a prototype that responds to adeep ofce space typology for different seasons of the year and hours of the day. The systems are compared for illuminance andglare performance.r 2005 Elsevier Ltd. All rights reserved.

Keywords: Daylighting; Ofces; Lightshelf; Anidolic concentrator; Glare; High solar radiation

1. Introduction

Countries located in climates with high solar radiationcan take advantage of favourable natural lightingconditions for energy savings and visual comfort inplaces like ofces. One such case is Israel. From datapublished by its National Electric Company [1], ofcebuildings might use for articial lighting, on a typicalsummer day, up to a third of the electricity they

consume. This is a contrasting fact, given the favourablenatural lighting conditions present during workinghours and throughout the year [2].

However, this can be a potential source of problems,due to the excessive contrast between the zone close to

the window and that in the opposite end of the ofceroom. Furthermore, uncontrolled penetration of solarradiation can increase the thermal loads during summer,producing an extra load to air-conditioning systems.Considering integral glazing/shading systems helps toachieve improved overall energy performance as well asenhanced lighting levels with visually comfortableuniformity.

Such systems are not extensively included in current

buildings given that designers usually neither considerpotential contributions of material properties liketexture and colour to achieve visual requirements orcomfort, nor contemplate using devices to increase theeffectiveness and use of daylighting. Qualitative aspectslike human visual comfort are solved in empirical waysor not at all. Another reason is little research on anddiffusion of daylighting devices for use in climateshaving high solar radiation throughout the year. Thislimits the options of designers, who usually apply

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Corresponding author. Tel.: +9724 829 4013;fax: +972 4 8294617.

E-mail addresses: [email protected] (C.E. Ochoa),[email protected] (I.G. Capeluto).

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shading devices or highly reective glazing systems butrely almost exclusively on electrical lighting for perform-ing visual tasks.

This paper is a rst step in comparing, under a climateof high solar radiation, three systems for improvingquality of ofce spaces through daylighting; it is a

selection from a wider study by the authors that tries tocombine both qualitative and quantitative factors andincludes more systems, times of the year and orienta-tions [3]. It must be pointed out that most conventionalresearch lines tend to focus either on the quality or theperformance of a device, but few on both.

The rst studied condition consists of a basic sidelitofce with improved properties of materials used incommon building practice, through increased surface’sreectances. The second one is a horizontal lightshelf,and the third an anidolic light concentrating system, aswill be detailed in Section 3.1. The systems presented inthis paper have extensive research for high latitudes. Yetresearch on such are scarce for environments with highsolar radiation where issues of radiation, overheating,light admission and glare are crucial.

The comparison is made through the computerprogramme Radiance and its Windows’ interface Desk-top Radiance [4]. For the most accurate results, anadequate representation of the sky conditions had to bechosen based on the available standards, which had notbeen done for Israel, the location of our simulations.

2. Sky-type selection for the luminous conditions of Israel

2.1. Brief description of the experiment

The computer programme Radiance has been widelyvalidated by different researchers [5,6]. The objective of this experiment is to verify the sky model that betteradapts to our reality and adjusts to the present skyconditions, in order to compare the systems rather thandetermine their precise behaviour. The space for the

experiment is located in the Technion, Israel Institute of Technology (Haifa, 32.5N 35E, standard meridian 30E).It is a meeting room with a large oor to roof stripwindow opening to the southwest, facing large reectivesurfaces just outside it (see Fig. 1 ). The materials used inthe room are easily found in the Israeli construction

market.A 100 100 cm grid was used to subdivide the space,except for a ‘‘centre line’’ (column C in Fig. 2 ) at every50 cm. A Minolta digital illuminance meter (modelT-1H, Japan) was used for taking readings; thereectances of the surfaces were also measured. Asimulation of the room was input to Radiance andDesktop Radiance, using CIE clear sky (with andwithout sun), CIE Intermediate with sun [7] and Perezall weather model [8,9]. Even though it is very difcult toreproduce the exact weather conditions, at least a goodapproximation is given through different models thatare close enough to replicate them. The basic data onglobal solar radiation come from tables compiled byNe’eman [10].

2.2. Results of the sky selection experiment

The results are partially presented in Fig. 2 . The CIEclear sky showed extreme divergence with the measureddata and was not plotted. The CIE intermediate sky

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Fig. 1. Test room used for the sky selection. Left, simulation, and right, photograph.

Haifa, December 13, 9.30 am Column C (center line)

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Fig. 2. Radiance-simulated comparison of CIE Intermediate andPerez sky models for Haifa, Israel, December 13, 9.30 AM

C.E. Ochoa, I.G. Capeluto / Building and Environment 41 (2006) 1128–1135 1129



with sun model showed an acceptable degree of divergence. The Perez sky model showed some peculiardeviations, especially towards the profound part of theroom.

Some possible reasons for the discrepancies were thatin the Perez model average data were used and not the

precise data for the day. For the CIE intermediate,slight variations in the orientation of the modelversus exact real-life orientation might cause somedifferences. Neither the exact degree of turbidity inthose hours was known, nor its type (for light scatter-ing pattern) [11].

3. Evaluating the performance of three daylightingsystems for ofce use

3.1. Systems

Three systems are presented here for use in highly

luminous climates. They were selected and sized in orderto achieve the important goals of reecting light to thedepth of the room while providing shade and improvingvisual comfort to the occupants through a morebalanced light distribution. The devices included in thisstudy may help also to reduce glare and energyconsumption through control of solar radiation. Thisis shown in Fig. 3 , adapted from a study on optimisationof lighting/shading devices performed by Shaviv et al.[12]. In this gure it can be seen that for a south-facingofce in the city of Tel Aviv with depth 6.7 meters, thecorrect application of a lightshelf decreases energy loadsper square meter per year in terms of cooling and electriclighting.

Determining the geometry of the daylighting systemsincluded using the angle of incidence of solar rays forthe month of March (55 1 ), considered as an intermediateposition for shading during summer and winter time. Animportant factor many times overlooked in evaluationsis the user himself; to mimic human behaviour the use of dynamic shading devices like internal venetian blinds isalso considered as described in Section 3.2.

The rst system consists of a sidelit ofce congura-tion with a single view window of clear double glazingand height 170 cm (see Fig. 4(a) ). The materials of the

walls, ceiling and oor have improved optical qualities(reectances ¼ walls 0.65, carpet 0.20, and acoustic slabceiling 0.80) [13]. These properties represent use of

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Fig. 4. Proles of the daylighting systems used (dimensions in cm): (a) basecase, (b) lightshelf, (c) anidolic concentrator.

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improved building materials with visual comfort inmind. This basic conguration will be considered thebasecase .

The second system has the same characteristics as therst one, except that a lightshelf of highly reectivematerial ( r ¼ 0:80) has been placed according to the

dimensions of Fig. 4(b) in the view window area. Thethird system is presented in Fig. 4(c) ; for this casethe device is an anidolic concentrator [14].

3.2. Testing conditions

The systems were modelled using Desktop Radiance,and added to a prototype ofce of dimensions 800(width) 1200 (depth) 270 (height) cm with the refer-ence working plane at 80 cm. The prototype is supposedto be in the third oor of an ofce building placed at aheight of about 10m over the ground which has areectance value r ¼ 0:20. The location studied is TelAviv (32N 35E, standard meridian 30E) due to the highconcentration of ofce buildings found there.

Simulation dates presented here are 21 June and 21December even though the experiment also included 21March and September. Testing hours are 10, 12, 14 and16 h. The four main orientations are tested, with sky-type CIE intermediate with sun, as mentioned in theprevious section.

This study aims at integrating evaluations for lightquality and quantity, since presenting only illuminancedata can be misleading if human factors are notincorporated. It is a known fact that users will react to

correct the amount of light entering a space in order toachieve visual comfort [15]. A rst standard approach inthe simulations to potential human behaviour is the useof automatic horizontal venetian blinds. They areassumed to be lowered when the Radiance simulationpicture shows direct solar penetration (sunspot) over theofce’s oor beyond 1 m from the view window. Thisblinds system does not cover the clerestory part of thedevices.

A second approach to the qualitative issue of humanvisual comfort is made through glare analysis. Glarefrom large natural sources needs more research tounderstand it; but a reasonable assessment can be givenwith what is known today. For this study, the daylightglare index (DGI) is used. This index was derived fromresearch for glare produced by articial sources, buttries to reconcile the fact that users can stand betterglare from natural sources [16].

Radiance provides routines to evaluate the luminanceof a given scene and assigns to it different glare indexesas requested by the user. The evaluation of electronicpictures to determine the presence of glare, throughmethods unrelated to Radiance, has also been proposedby Schiler to analyse through digitized video images,real-life conditions in test ofces and is based on

statistical analysis of pixel luminances [17]. For thesimulations of this study, the view point is 100 cm fromthe window and faces it directly. It is an unfavourableposition, yet this proved to be a limiting condition frompresently accepted glare formulas.

3.3. Results and analysis

The results of the simulations are presented in Figs.5–7 . Even though hundreds of parametric simulationswere done in the original study, here the mostrepresentative data of south orientation are shown.The minimum acceptable horizontal illuminance for theworkplane is 300 lx, as required by Israeli standards [18],while the upper limit illuminance is 4000lx [19].

Fig. 5 presents year-round histograms for thecompared systems (basecase with and without blinds,lightshelf and anidolic concentrator) taken at 200, 600and 1200cm from the view window. The histogram at200cm has its range of data from 0 to +4501 lx, whilethose at 600 and 1200cm range from 0 to +901 lx. Fromthem, it can be seen that the effectiveness of the systemsdecreases signicantly after 6 m from the view window,yet only the anidolic concentrator could keep illumi-nance levels above the minimum limit at 1200 cm.

Fig. 6 presents illuminance line graphs for 21December and 21 June for south orientation. Dashedlines indicate the use of blinds; the window is presentedon the left-hand side of every graph. In them it can beseen that the proposed geometry of the devicesaccomplishes the goals for highly luminous climates of

providing shading at the front part of the room toreduce the contrast between front and back of the room(therefore, lowering the chances for glare). They alsoconrm that the effect of the devices decreases after600 cm. The sudden peaks observed are ‘‘sunspots’’ thatin reality would be less apparent.

Qualitative analysis is given through glare graphs inFig. 7 , which also shows Radiance-generated picturesfor the basecase without blinds. Parametric simulationswere done for glare, but only few produced conclusiveresults. In the graphs the view angle is on the x-axis, andthe DGI index on the y-axis. The grey area indicates thevalues where DGI is acceptable. At noon the glarepattern is nearly ‘‘symmetric’’ and at 16 h ‘‘asymmetric’’,due to the asymmetric luminance distribution of the sky.This could also be seen on the view inside the room,depending on solar penetration (striking the oor or oneof the walls, respectively).

3.4. Discussion

This study focuses not on proposing novel systemsbut on evaluating their potential application to highlyluminous climates. The quantitative evaluation in termsof illuminance shows that the anidolic concentrator

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is clear; yet using them might not always be thebest solution to achieve it. The anidolic system reducedglare levels in most cases throughout the room. Basedon these results, it can be stated that the daylightingdevices (which were also thought as shade providers)reduce glare when the sun shines at a high angle. But thepicture changes when the sun shines at a lower angle,and the anidolic concentrator, which gives thehigher lighting levels, also becomes a possible sourceof glare.

4. Conclusions

Few evaluations with a double focus to evaluate theperformance of daylighting devices and their inuenceon visual comfort have been made for highly luminousclimates. Such scope is necessary to evaluate potentialdevices that can assure energy savings for cooling andlighting, and user satisfaction. This can be seen in thecase of the anidolic system, where errors in the sizing ororientation of the concentrator can give undesired glareresults, even though system performance might be asplanned.

Under the conditions described in this study, theanidolic concentrator provides the highest illuminancelevels in quantitative terms. However, in a qualitativesense, care must be taken with solar angles where thereections of the concentrator can cause glare. Fromthis point of view, the lightshelf provides a ‘‘safer’’approach, by reducing the contrast between levels at theview window and those at the back of the room, yetsacricing on illuminance levels.

This gives the guideline that devices must be consideredfor year-round behaviour of both illuminance perfor-mance and glare. Even though, the subject of glare fromlarge natural sources remains an open eld of researchboth to explain, measure and evaluate the phenomenon inorder to nd formulas that can be satisfactory for everysituation and climate (such as the response to highamounts of sunshine for prolonged times).

The study also can help as a guideline for ofcebuilding designers in climates with high solar radiation,in that spaces with one view window should not havedepths beyond 7 m, even when using daylighting devices.If such needs to be the case, opening a second viewwindow should be considered (for example, as in a patioor an atrium).

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Fig. 7. Daylight glare index (DGI) graphs and corresponding Radiance-generated sh-eye views for 21 December at 12 and 16 h, south orientation.Location, Tel Aviv, Israel.

C.E. Ochoa, I.G. Capeluto / Building and Environment 41 (2006) 1128–11351134



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Evaluating Visual Comfort and Performance of Three Natural Lighting