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Page 1: Mirror lightpipes : Daylighting performance in real buildings

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Lighting Research and

http://lrt.sagepub.com/content/30/1/37The online version of this article can be found at:

 DOI: 10.1177/096032719803000106

1998 30: 37Lighting Research and TechnologyL. Shao, A.A. Elmualim and I. Yohannes

Mirror lightpipes : Daylighting performance in real buildings  

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Mirror lightpipes : Daylighting performance in real buildings

L Shao BSc PhD CEng MCIBSE, A A Elmualim MSc and I Yohannes MScInstitute of Building Technology, University of Nottingham, Nottingham NG7 2RD, UK

Abstract Mirror lightpipes are useful for providing healthy and energy-efficient daylight into build-ings where windows and skylights are unsuitable, insufficient or generate too much heat gain. Thelightpipes have been installed in dozens of buildings in the UK. Field monitoring has been carried outto assess their performance in four different buildings: the headquaters of a major insurance company, ahealth clinic, a residential building and a college dining hall In those cases where lighipipes with mod-erate aspect ratios were installed, good illuminance of up to 450 lux has been obtained withinternal/external illuminance ratios around 1%. When long and narrow lightpipes with many bends areused, however, the ratio reduced to around 0.1%. These results showed that lightpipes can be effectivedaylighting devices provided that excessive aspect ratios and numbers of bends are avoided. Lightpipeswith larger diameters should be used whenever possible. The lightpipes often improved signiScantlythe visual quality af the interior environment, and high user satisfaction was found even in buildingswhere a relatively low level of daylight was admitted through the lightpipes

List of symbols .

d Diameter of lightpipe (mm)defí Effective diameter; od/4 (mm)int Integer junction (units?)I Length of lightpipe (mm) )p Aspect ratio of lightpipe; i.e. ratio of lightpipe length to

diameter.R e ce of interior surface oflightpipe~’ Transmittance of lightpipe,8 Angle between incident light and lightpipe axis (0)

1 Introduction

Increased global warming and deterioration of the ozone layerhave stimulated interest in the use of renewable energy sys-tems. Daylighting is increasingly employed in modem build-ings~ underground spaces and tunnels to minimise energyconsumption and the release of harmful emissions to theenvironment. Innovative daylighting techniques includinglightshelves, prismatic glazing, holographic f~Im~ and light-pipes have facilitated the effective use of daylighting in a widerange of spaces(I-8~ In addition to bringing energy savings,these daylighting technologies also help to create healthierinteriors for occupants. Bouchet<S) suggested that as little as 50Ix of daylight may provide ‘ c~t relief of the feeling ofisolation for people working in underground spaces. Naturaldaylight has also been found to relieve seasonal affective dis-order, cholesterol problems, chronic fatigue and ’jet laf aswell as benefitting people in shift work and computer t1work<’.1Q,11). Improved psychological well-being would lead toincreasing morale, work performance and productivity(9). Itwas reported that students attending a daylit school achievedbetter examination results (14% higher marks) and higherattendance rates than those in non-daylit schools, includingnewly built non-daylit schools.<12)

The lightpipe is a simple and effective daylighting deviceused widely in North America and Australia, but its applica-tion in Europe is relatively new and smaller in scale. SinceI lightpipes have been installed in over 50 buildings inthe UK. Field monitoring has been carried out to assess theirperformance in four different buildings: the headquaters of a

I

major insurance company, a health c3.inic, a residential build-ing and a college dining hall. Internal illuminance levels anddistribution., external illuminance and the ratio between thetwo were measured. Occupiers of the buildings were alsoquestioned regarding their satisfaction with the lightpipe sys-tems.

2 The lightpipe

The lightpipe, lined with highly reflective material, is used toguide sunlight and daylight into occupied spaces. Highlyreflective materials include anodised aluminium and coatedplastic film such as Silver1ux9, which have reflectancesgreater than 95%. Commercial lightpipes are available from anumber of manufacturers, in straight and bend sections foron-site assembly and installation. They allow the lightpipe totraverse complex roof spaces to reach rooms that are not easi-ly accessible to skylights. A lightpipe is normally fitted with adear top dome which prevents the ingress of rainwater anddust, and removes harmful ultraviolet radiation. A diffuserfitted to the bottom of the lightpipe improves the ‘ c~rm~tyof light distribution more uniformly in the room it illumi-nates. Compared with skylights or windows, the lightpipetransmits less solar heat onto the illuminated surfaes. is

particularly valuable in summer for preventing hotspots in abuilding. In winter, a light collector ~e.~. a sun scoop or lasercut panel(4) could be mounted above the top opening to allowsignificantly more sunlight frum low angles to be collected.

The light transmission performance of the lightpipes is givenby Swift and Smith{8):

T = (4/~j~ ~s~l~~ -~~~~’~~~~~X ~1-~~-~~~’ ~~-ant~t~~~~~~~

(1)

where T is the transmitance of the lightpipe. i.e. the ratio ofthe amount of transmitted light to that of incident light, R isthe reflectance of the interior surface of the lightpipe~ is theaspect ratio of the lightpipe, i.e. the ratio of the lightpipelength to its diameter, 0 is the angle between the incidentlight and the lightpipe axis, int is the integer function, that isint(a) is the the integer less than or equal to a.

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Equation 1 is based on the assumption that the surfacereflectance is independent of light incident angle and wave-length. Transmission calculations for surfaces which do nothave this characteristics can be found in Reference 8.Equation 1 is not integrable, and the transmittance of light-pipes must be obtained using numerical techniques. Theresults obtained by Edmonds et a~ indicated that the trans-mittance of a lightpipe is critically dependent on the incidentlight angle, the reflectance of its interior surface and its aspectratio. Applying the calculation procedure to the lightpipeused at the Gurney Surgery (95% reflectance, 600 mm longand 330 mm in diameter; see section 3) yields a transmittancevalue of 87%.

A simpler equation(13) is also available which may be used toestimate the transmittance of a lightpipe: .

T = RltJmfltl df (2)

where 1 is the length of the lightpipe, ~ = m~4, ~ is thediameter of the lightpipe.Measurement of lightpipe performance carried out inAustralia has shown that an illuminance of 300 Ix can beachieved in a room of 3 X 3 X 2 m fitted with a light-pipe of 255 mm diameter without using any device toenhance light collection(4). The aspect ratio of the lightpipe,the reflectivity of the interior surface and the solar altitudewere 7,95% and 60° respectively. The corresponding externalilluminance value was not reported. Information on lightpipeperformance in Europe is limited. Bouchet andFontoynon65>, in France, analysed performance of light wellslined with a specular and highly reflective material using aMonte-Carlo based ray-tracing technique. Their computersimulation predicted a minimum illuminance of 100 Ix forover 70% of the period between 09.00 and 18.00 under over-cast conditions. The corresponding external illuminance hada minimum value of 10 000 Ix. The simulation was based on alight well of 1 X 1 X 6 m, which was sealed at the top usingMocks, with a transmission of 0.6 and a reductioncover. The separation between the light wells was 4.5 m.

The attractiveness of the lightpipe, compared with skylights,lies in its flexibility in traversing roof spaces, lower heat lossduring the winter season, more uniform light distributionand the potential for application in multi-storey buildings(using pipes of diameter). Interiors with only side win-dows are often perceived as gloomy even on sunny days whenthere is sufficient indoor daylight. A lightpipe would reducethe luminance contrast between the room and the bright exte- ’rior, thus reducing the probability of occupants resorting toelectric lighting to correct the perceived gloom. /

3 MomtoredMMN~amdmeasttrementconSgum~ns

3.1 T~P&B-FC~~ :

The Pearl Centre (Headquarters of Pearl Assurance plc) is ~located on the outskirts of Peterborough, UK and houses ~

2200 The building makes use of natural day- ’

light. All main Mocks, each three storeys high, have a ’

central atrium and window overlooking the land- ’

scaped grounds. To prevent blocking out natural light and <

views of the surrounding countryside, only the most senior <

executives have cellular offices and the height of screens and. I

storage is restricted to 1.5 m. The waiting area next to the <

executive offices has limited access to daylight compared with ’

that of other parts of the building; The area was some dis- :

away from the central atrium and light from side win- 1

dows was blocked by the walls of the executive ~~~e~. Thearchitect chose lightpipes from Monodraught Ltd to rectifythis problem by bringing daylight from above the angledroof

The waiting area (Figure 1) was a square space approximately9 X 9 X 2.75 m. It had walls to all four sides and an openingin each of the four comers for circulation. The walls and ceil-ings were white, except Wall 3 which was lined with full-height wooden storage furniture with a polished light yellowsurface. A painting approximately 2 X 2 m was placed in thecentre of Wall 1, and on each of the two other walls there wasa painting of 1.5 X 1.5 m. Two coffee tables and several chairswere placed close to the centre of the dark carpeted floor.

Figure I Schematic of the waiting area, Pearl Centre

A 5 X 5 grid, with a uniform spacing of 1.5 m between neigh-bouring grid lines, was used in the measurement of lightinglevels. The dffusers of the lightpipes coincided with the cen-tres of some of the grid squares as indicated in Figure 1. Alleight lightpipes were of 330 mm diameter, and each pipe wassealed at the top with a dear acrylic dome. Pearl white dif-msers were fitted to the lower openings of the lightpipes foreven light distribution. The interior surfaée of the lightpipehad a reflectance value of 95%. The lightpipes passed throughme ceiling, the attic space and the roof The attic had an over-all height (roof ridge over attic floor) of 8 m and housed vari-ous building services plant and ductwork. The lightpipeswere between 8 and 12 m in length and contained up to fourbends, so as ? pass beams and ductwork for other ser-within the attic. The upper of the lightpipes termi-nated above the just below the roof ridge heightHorizontal illuminance at a height of 0.85 m were mea-sured at the 25 grid points indicated in Figure 1. The photo-cell was mounted on a tripod to facilitate movement betweenthe points, and the tripod was adjusted to ensure that thecell height was correct and its detecting surface horizonal. Allelectric light sources within the waiting area were switchedoff during the measurement. The luxmeter used was aHagner Digital E2X, which is on a silicon diode photo-cell and capable of measuring illuminances in the range 0.01to 200000 lux. The meter has an accuracy of ±3% and a virtu-

ally perfect cosine correction curve. The meter connects to aremote photocell via a flexible lead, making it easier to read

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illuminance levels without blocking incident light reachingthe photocell. The meter also had analogue output for a data-logger. The same equipment were used for other measure-ments reported in this paper.

3.2 The Gurney Surgery

The Gurney Surgery is a general medical practice housed in asingle-storey building located close to the city centre ofNorwich. It comprised spaces for various functions includingconsulting rooms, health visitor rooms, treatment room,management office, staff room, reception office, record stor-age room, a pharmacy and reception areas, covering a totalfloor area of over 400 m~. Due to its inner-city location andthe nature of its business, the surgery required a high degreeof security and privacy. Most of the rooms in the buildingwere without side windows and the lightpipe provided anideal solution to the problem of balancing privacy, securityand access to daylight. A total of 11 Lightpi were installedin five rooms of the surgery, including the health visitorrooms, the staff room, the management offices and a patientwaiting area. All lightpipes were straight, 600 mm long and330 mm in diameter. A clear top dome was attached to theopening above the roof and a pearl white diffuser attached todie lower opening against the ceiling. The material and con-struction of the lightpipes were identical to those used in thePearl Centre.

Measurements concentrated on the health care room, thelargest room in which lightpipes were installed. The room, 75 3.6 X 2.3 m, had four lightpipes at the positions shown inFigure 2. The walls of the room were finished with light yel-low wallpaper and the suspended ceiling tiles were white.Five square, recessed mirror-louvre luminaires were htted tothe ceiling. The iu the offices included a table, anexamination couch with white covering, a washbasin, waist-height storage cupboards and a number of c. The door inWall 3 had a light wood a~a.d the floor was covered withdark heavy-duty carpets. A number of brightly colouredposters, generally A3 or A2 in size, were displayed on thewalls. A 3 X 5 grid at the working plane height of 0.85 m wasused for surveying the illuminance distribution in the space(illuminances were measured at intersections). The spacingsbetween the grid lines are also shown in Figure 2. The posi-tion of the lightpipes in relation to the grid is alsomarked.

Figure 2 Schematic of the health care room in the Gurney Surgery

_ _ _ . _ _ _ .. _ _ ._~ _ _ _ _ _ .... _~ ... r .z. _ .

3.3 Rivermead Court

25 Rivermead Court is a flat in a luxury residential develop-ment on the bank of River Thames in Marlow. The two-bed-room flat was on the upper floor of a two-storey building andhad its entrance on the ground floor. All five rooms in the flathad windows providing natural lighting during the day.However, the stairs and landing were without any access todaylight and appeared very gloomy. Two lightpipes of diame-ter 330 mm were installed in this area, as indicated in Figure3 by two circles. The ’L’ shaped landing had an overalldimension of 2.9 X 2 m. The walls were finished with light-coloured wall paper and the floor was covered with beige-coloured carpets. The five doors opening to the landing andthe ceiling in the area were white.

The lightpipe over the stairs was 6 m long and contained twobends to allow the pipe to pass around the obstructions in theattic space. The other pipe was straight and 1.8 m in length.Both lightpipes were otherwise identical to those used in thePearl Centre and there were no obstructions in the vicinity oftheir external openings. The positions of the lightpipe dif-msers are marked in Figure 3. A grid of five points at theworking plane height of 0.85 m was used for surveying theilluminance distribution in the space and their locations,marked by numbers 1-5, are also shown in Figure 3.

Figure 3 Schematic of the landing area at 25 Watermead Court

3.4 (~t~w~Co&~Guildford College is a further education institution locatedof London, providing GCE A-level and vocational train-ing. Lightpipes were installed in the large dining hall on theupper floor of a modem two-storey building. The dining hall,approximately 20 X 40 X 2.75 m, was daylit from continuous,full-height windows in all four walls. The centre of the hallwas occupied by food counters and structures housing build-ing services. An area close to the centre of the hall appearedgloomy despite the extensive glazing in the perimeter walls.Nine lightpipes of diameter 530 mm were installed to bringin additional daylight.The locations of the lightpipes are shown in Figure 4. Thenine pipes were installed in three rows and three columns,and the spacing between neighbouring rows and columns was

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Figure 4 Schematic of hghtpipe arrangement in the dmmg b4 GuddfordCollege

3m Seven of the lightpipes were 1.65 m long, with diffusersflush with the ceiling. The remaining two were integratedinto a natural ventilation tower and had a length of 1.2 m.The diffuser was mounted to the top of the tower which was1.65 m above the ceiling This extra height was neces-sary both to allow the of the tower to wind and tothe stack effect for natural The two tow-ers had square openings of 2.4 X 2.4 m in the ceiling. Theirpositions are marked in Figure 4 by two circles withfine lines. Schematics of both types of installation are alsoshown in Figure 4. The material and construction of thelightpipe systems were identical to those used at Pearl Centre,but the diameter of the lightpipes used at Guildford Collegewas much larger at 0.53 m as compared with 0.33 m at PearlCentre. The hall contained a large number of light-coloureddining tables and chairs, and had a white ceiling and darkuncarpeted floor. The wall next to the lightpipe-lit areaextended the full height between the floor and ceiling, andwas white in colour.

A uniform 5 X 5 grid at the working plane height of 0.85 mwas used for surveying the illuminance distribution in thespace. The spacing between neighbouring grid lines was 1.5m, and nine of the 25 grid points coincided with the positionsof the lightpipe diffusers, as shown in Figure 4.

4 Results and discussion

4.1 Pearl Centre

Three sets of measurements were ’ed out, at 11.30, 13.00and 16.30, respectively., on 5 June 1997. The weather wassunny around noon and y overcast towards the evening.The results obtained at 11.30 are shown in Figure 5. Eachvalue in the Figure 5 corresponds to a grid point in Figure 1,and the correlation is identified by aligning the correspond-ing walls. For example five values in the first row of Figure 5are the illuminance values for the five grid points next to wall1 in Figure 1.

.

wall 1

48 54 62 58 .~

47

57 _

90 .,.__

84 90 66

Wall 2 76 ~ ~ ~ 5 87 84 68 Wall 4

70 133 123 100 69

37 1 73 ~

80 71 ’~ 49

Wall 3

Figure 5 Illuminances in the waiting at 11.3~

The maximum illuminance in the waiting area was 133 luxobtained clc~se to the er of the shortest lightpipe amongthe eight. This may be compared with the illuminance valueof 250 lux measured on the same floor 1 m away from the atri-um, which has a large roof In the middle of the atriumfloor, receiving direct sunlight, the illuminance increaseddramatically to 49 000. .

The average illuminance of the 25 grid points was 75.6 lux.The unobtructed external illuminance on a horizontal plane,measured immediately after the completion of indoor tests,was 84000 lux. The concept of daylight factor is not appropri-ate here as the sky was clear and does not meet the overcastcondition upon which the daylight factor is based.Nevertheless a ratio of the internal illuminance to externalilluminance was calculated for each of the grid points and theaverage value was 0.

The results obtained at 13.00 are shown in Figure 6. Similarto the illuminance distribution pattern obtained at 11.30,lower light levels were found around the perimeter of the areaand higher levels at the tre. The maximum illuminancewas found at the same location, but its value increased to 152lux. The average illuminance also increased, to 89.2 lux andthe external illuminance, under a clear sunny sky, was 94 000I The average ratio of internal to external illu-minance increased to 0.095%.

Hgtne 6 Dhiminances in the waiting at 13=t?U

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The results obtained at 16.30 are shown in Figure 7. Both themaximum illuminance and the average illuminance were

greatly reduced, to 42 lux and 27.2 lux respectively. Thisreduction was due to the sharp drop of external daylight lev-els to 23 500 lux as the sky became overcast. However, theaverage ratio of internal illuminance to external illuminanceincreased to 0.12%. This is consistent with the previous expe-rience of the authors and is probably due to most of lightfrom an overcast sky originating from around the zenith, andundergoing fewer reflectionslabsorpfons as it travels downthe lightpipe.

aFiguce 7 illuminances in the waiting at 16:30

The lightpipes used at the Pearl Centre were of the narrowerof the two types available from Monodraught Ltd. This, com-bined with the exceptional pipe length, resulted in aspectratios up to 36: 1, far beyond the conventional range. Thelarge number of bends adopted in some of the pipes also con-tributed to the modest illuminance levels. An additional fac-tor is that none of regular array of measurement points laydirectly under a lightpipe ~r, causing an underestima-tion of the average illuminance. A much greater light levelcould be achieved by using the larger 530 mm diameter light-pipes, which will not only more than double the cross-sec-tional area but also result in fewer reflections due to thereduced aspect ratio. Consequently the light transmittancewould increase by 3()()OIb to produce a maximum illuminanceof 500 lux and average illuminance of 250 lux at 1300. Theselevels would be commensurate with those found in the neigh-bouring open-plan offices spaces. Measurement showed that inopen-plan areas, the maximum illuminance at working planeheight was 500 lux (close to the uphghter posts) and the leveldropped to 250 lux I m away from the posts. Reducing thenumber of bends and the length of the lightpipe by caretdesign would result in er increases in light levels. Thesesimple changes would create a harmonious space consistentwith the generally high standard of daylighti~ag found in thisbuilding. Even at the present modest daylight levels, thelightpipe systems were well received by occupants of the sur-rounding spaces. This is probably due to the intrinsicallyhealthier e of the natural daylight.

4..2 ~’u~~ .

Performance of the straight 600 mm lightpipes installed atthe Gurney Surgery was measured at approximately 1700 on12 June 1997. The sky was heavily overcast with moderaterain. The unobstructed external illuminance on a horizontalplane was 16 000 lux. The results are shown in Figure 8.Similar to the presentation of Pearl Centre results, each valuein the figure corresponds to a grid point in Figure 2. Forexample the five values in the first row of in Figure 8 are theilluminance values for the five grid points next to wall 1 inFigure 2.The maximum illuminances of about 230 lux occured under-neath the sers. The spatial variation of illuminance ismuch greater in the direction of the longer wall. This is prob-ably due to the larger grid spacing in this direction (1.2 m ver-

Ftgore 8 Illuminance (lux) distribution in the health care room, GurneySurgery

sus 0.9 m in the other direction). The sharp reduction of illu-minance over a short distance indicated that the light distrib-ution of the lightpipe is relatively concentrated in the regionunderneath the diffuser. The data also showed that a light-pipe separation of 3 m would in practice result in a mini-mum-to-maximum illuminance ratio of approximately 0.6.The lowest light levels were found at the grid points next towalls 2 and 4. Comparison between these and similarly lowvalues in the central column shows that increasing wallreflectances would not give rise to a significant increase in illu-minances in these regions. The average illuminances for the15 grid points is 177 lux and the average ratio of internal illu-minance to external illuminance is 1.1%.

The average illuminance level achieved at the end of normal

working hours under very dark sky conditions is very encour-aging, and demonstrates the energy-saving potential offeredby lightpipes. Similar to the situation at the Pearl Centre, thelight level would be doubled by the use of lightpipes of 530mm diameter. Both the Gurney measurements and the PearlCentre measurements were taken under overcast conditionsin rooms with similar lightpipes. However, the average ratioof internal illuminance to external illuminance at GurneySurgery was almost ten times higher. This substantialincrease is due to the much shorter lightpipes used and to theabsence of any bends. The Surgery manager reported satisfac-lion with the lightpipe daylighting system and interest in thesystems from other local health centres. One user neverthe-less expressed a preference for windows for the benefitof a view to the outside.

4.3 Rh*nrad G~rt

The daylighting performance of the Lightpipes installed at 25Rivermead Court was measured at approximately 1330 on 20June 1997. The sky was overcast with very light rain. Theexternal illuminance on a horizontal plan was 38 500 lux. Theresults obtained are shown in Table 1 The point numbers inthe table correspond to those for the grid points in Figure 3.

Table 1 Illuminance (lux) distribution in the stair/landing area atItiverinead Court

The maximum illuminance of 450 lux occured underneaththe diffuse of the short/straight lightpipe (point 1 in Figure3). The illuminance value directly underneath the di~s~r ofthe long(bent lightpipe was much lower, indicating the dra-matic effect of pipe length and bends on light transmission.The a~ ~ illu~m’ c~ ~t~r ~a.e ~~~ grid goi~t~ ~~ ~24 I~,The average illuminance for the five grid points was 224 lux,and the average ratio of internal illuminance to external illu-minance was 0.580/o. However, these values could be mislead-ing because the two lightpipes were very different and theaverage value of the two is not inf~r~~t~v~. Separate averageilluminance ratios for the two points would give a better indi-cation of their respective performances, and they are estimat-

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ed to be approximately 1% for the short/straight pipe and 0.3for the lon one.

The illuminance value of 450 Ix was the highest obtained dur-ing the monitoring exercise at the four buildings, and it wasachieved under overcast, rainy conditions. A maximum valuein excess of 1000 ~ would be expected under sunny condi-tions. The results obtained at Rivermead Court again pointedto the superior performance of shorter lightpipes with no orfew bends. Both the occupier and neighbours visiting die flatlike the lightpipe installation. The lightpipe drasticallyimproves the visual environment of the stairs and landingarea which were previously lit with a number of low-voltagehalogen lamps housed in ’hi-tech’ style ceiling-recessed lumi-naires.

4.4 Guildford CoflegeThe daylighting performance of the lightpipe installations atGuildford College was measured at approximately 1630 on 20June 1997. The sky was heavily overcast with rain. The exter-nal illuminance on a horizontal plane was steady but verylow, at 8 260 lux. To determine the effects of the lightpipes onthe improvement of lighting levels, two sets of measurementwere carried out. First, the inlets of the lightpipes above theroof were covered up with cardboard boxes and the illnance distribution due to light from the side windows alonewas measured. The covers were then removed to allow themeasurement of illuminnance due to light from both the sidewindows and the lightpipes. The two sets of results are pre-sented in Figures 9 and 10. Each value in these figures ~rxe-sponds to a grid point in Figure 4. For example the five val-ues in me first row of in Figure 9 are the illuminance valuesfor the five grid points next to the wall in Figure 4.

Figum 9 illuminance (lux) distribution in the dining hall at GoiM&rdCollege, inlets of bgh4upes covered up

Fagum 10 illuminance (lux) distribution in the dining hall at GuildfordCDIICGC; inlets of lightpipes not covered up.

When the lightpipes were covered up, the illuminance aver-aged over the 25 points in the area was 65.7 lux. This is a verylow value, and the visual appearance of the area was gloomy,particularly against large bright windows. The averagevalue increased to 128 lux when the lightpipe covers wereremoved, doubling the illuminance level in the area. Thevisual was than the illuminance value suggests,probably because the presence of the bright diffusers and the

brightened ceiling/wall, which balanced the brightness ofthese areas with that of the side windows. The ratio of inter-nal illuminance due to lightpipes to external illuminance was0.8%. This is approximately eight times greater than thatfound in the Pearl Centre, where longer lightpipes with asmaller diameter and bends were used.

The highest illuminance of 217 lux was achieved at the centreof the area and the greatest increase (3.6 times) with the cov-ers was found underneath the middle difuser next to the. The lowest percentage increase was found in the middleof the row furthest from the wall. Although the lightpipesincreased the illuminance s~ tiy, They reduced the illu-minance uniformity in the area. The ratio of average illumi-nance to maximum illuminance decreased from 77% to 5~°’~.Nevertheless both values are satisfactory. The two lightpipesmounted at the ventilation towers gave performance similarto that of the others despite their higher mounting. This isprobably because of their shorter length and the reducedreflections loss.

5 Conclusion

Lightpipe performance has been monitored in the field atfour buildings in the UK, chosen as representative of fourbuilding types: co residential, education and health.The lightpipes used in the buildings varied in diameter (330mm and 530 ~,1 length (0.6 to 12 m), aspect ratio (2 to over30), and in the number of bends in the pipe (0 to 4). In theGurney Surgery, the Guildford College and a part ofRivermead Court where lightpipes with moderate aspectratios (up to 6 in these cases) were installed, good illuminanceof up to 450 lux was obtained with inter illumi-nance ratios around 1%. However, in where long andnarrow lightpipes with some bends were used, for instance inthe Pearl Centre and in part of Rivermead Court, the ratioreduced to around 0.1% with internal illuminance reaching aslow as 27 lux at 16:30. These results showed that lightpipescan be effective devices for introducing natural daylight intobuildings, provided they are designed with care to avoidexcessive aspect ratios and numbers of bends. Lightpipeswith larger diameters should be used whenever possible. Thebenefit of employing lightpipes is not confined to energy sav-ings ; the quality and health-giving nature of the daylight arealso important factors in the widespread user satisfactionround in this investigation. In addition, the improvement tothe subjective, visual quality of the indoor environment isoften greater than the illuminance from the lightpipes wouldsu

n

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of lightpipes under cloudy and sunny conditions in the UK Proc. 4thEuropean Conf. on Energy-Efficient Lighting, Copenhagen, Denmark (1997)

11 Liberman J Light - Medicine of the future (Santa Fe, USA: Bear) (1991)12 Smiley F Students delight in daylight International Association for

Energy-Efficient Lighting Newsletter 5(2) 11-12 (1996)13 Zastrow A and Wittwer V Daylighting with mirror lightpipes and

with fluorescent planar concentrators Proc. SPIE (1987)

Discussion

D J Burton (Urbis Interior Lighting (UK»

It is exciting to read work by new researchers in this area, andmost importantly this paper concentrates on practical aggli~-tions.

Much has been written about the ~ff beneficial and other-wise, of daylight on human performance. The authors areright to consider the qualitative effects of this light as well asits potential to reduce the energy requirements of a building.However, the extent to which daylight transmitted along apipe retains the qualities of traditional methods of daylight-ing should be confirmed. Is it the ’active* nature of daylightthat gives an improved perception over ’passive’ artificiallighting? Work relating to seasonal effective disorder suggeststhat vastly higher levels of illuminance than those recordedby the authors would be needed to influence melatonin sup-pression. Further work in this area would be helpful

That light pipes have not been used more widely in north-west Europe is undoubtedly due to the moderate availabilityof sunlight in these latitudes. It would be interesting to hearthe views of the authors on this, and of the possibilities oflinking light pipes to a heliostat.

The results of the site measurements given in this paper sug-gest that light pipes may well prove to be one of the compo-nents needed to achieve the step-change often talked about toproduce much more en ~i~t buildings than we havetoday. The values of illuminance found represent a usefulcontribution to the overall lighting. Have the authors investi-gated the potential of adding daylight-linking controls to theelectric lighting used in these areas? Section 4 rightly ques-tions the value of ’average illuminance in such cases. Thisd not give any indication of the appearance of a space, norof the luminance patterns within it - do the light-pipe ’t_ters look like luminaires? What range of luminance is pro-duced at the emitters themselves? I would be eager to hearwhether the authors intend to pursue these aspects as theirresearch progresses. .

Dr D J Carter (University of Liverpool, UK)

The problems of utilisation of skylight as a source for light-pipe systems have constituted a major barrier to their use intemperate latitudes. The work reported in this paper suggeststhat passive lightpipe systems can deliver useful amounts ofilluminance to building interiors in UK latitudes by redirect-ing either skylight or direct solar radiation. These results givegreat encouragement to enthusiasts for such systems!

The results give measured working-plane illuminance valuesfor pipe systems of a variety of confi tions and lengths.These are of great interest and could serve as the basis of ‘rule

of thumb’-type design guidance. A further problem is the pre-diction of the performance of proposed applications of vary-ing complexity which do not resemble those measured. Havethe authors attempted any simulation of the systems, mea-sured or otherwise, which would go some way towardaddressing this problem?

,

The authors allude several times in the paper to improve-ments in interior visual quality produced by use of the sys-tems, but no evidence in the form of photometric measure-ment is presented by way of ~us~;~tion. Were any values ofdi or room surface luminance taken during the surveysso that the brightness patterns created in the installationcould be determined? Similarly, luminance values for thetpipe differs and their immediate surrounds could givean indication of the possibly of visual discomfort caused byIt would also be interesting to known how the apparentuser satisfaction with the installations was determined. Was astructured survey of user subjective opinion undertaken? Iseek clarification on this because my own, strictly anecdotal,survey of user satisfaction gathered during a visit to theGuildford installation indicated that a large proportion of theusers did not realise that it was a lightpipe system! It is per-haps also worth noting that the visual quality, or otherwise, ofthe exterior of the building can be influenced by the natureand positioning of the upper ends of the pipes.It is evident that the use of the lightpipes enables some elec-tricity substitution to be made in the installations. Any eco-nomic analysis of such systems should also include capitalcost and embodied energy. This is a very difficult area, butcould the authors make some estimation of the likely savingsin a temperate climate?

Authors’ response to discussion

As pointed out by Mr David Burton, although people gener-ally prefer daylight to artificial light, the exact mechanism isnot fully understood. The suppression of melatonin is onlyone aspect of the daylight effect and cannot be used to explainthe above general preference for daylight. Many years ofresearch may be required to understand the psychological andphysiological effects of daylight. Experiences in the UnitedKingdom have shown that the light pipes are valuable forimproving indoor visual environment and reducing the needfor electric lighting even under cloudy sky conditions.Linking light pipes to a heliostat would undoubtedly increaselighting levels, but the cost of the system would increase bymany times. A more cost-effective approach would be to uselaser-cut panels or fixed reflectors. Daylight linking in elec-tric lighting systems is critical for realising the po ener-gy benefit of daylighting, and the authors are investigatingcost-effective ways of implementing this technology.Research will also be carried out by the authors on emitterdesign to ensure that the luminance distribution of a givenemitter is suitable for the activity in the space it illuminates.

Simulation of light pipes mentioned by Dr David Carter isimportant for accurate design of complex systems and assess-ment of performance of new light pipe components. Theauthors are currently working on a ray-tracing modellingmethod to determine the light transmission perforanance oflight pipes using computers. In addition, a twelve-month

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monitoring excercise is being cari-ied out on the performanceof light pipes under UK climatic conditions, and the resultwill be used to estimate the likely energy and cost savings.User satisfaction with the li~ht pipe installations was assessed

using unstructured interviews with users and building f~citi-ties managers. Although users were often not interested in the

lighting equipment, they were sensitive to the change of visu-al environment brought about by the light pipes.

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