Garcia Hansen et al., Perceptions of dayligtht quatlity from light transport systems

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PERCEPTIONS OF DAYLIGHT QUALITY DELIVERED BY LIGHT TRANSPORT SYSTEMS

Garcia Hansen, V.*; Isoardi, G.**; Miller, E.*

*School of Design, Queensland University of Technology

**School of Physics, Queensland University of Technology

ABSTRACT

Light Transport Systems (LTS) (e.g light pipes, fibre optics) can illuminate core areas within buildings with great potential for energy savings. However, they do not provide a clear connection to the outside like windows do, and their effects on people’s physiological and psychological health are not well understood. Furthermore, how people perceive LTS affects users’ acceptance of the device and its performance.

The purpose of this research is to understand how occupants perceive and experience spaces illuminated by LTS. Two case studies of commercial buildings with LTS, located in Brisbane, Australia are assessed by qualitative (focus group interviews) and quantitative (measurement of daylight illuminances and luminance) methods. The data from interviews with occupants provide useful insight into the aspects of LTS design that are most relevant to positive perception of the luminous environment. Luminance measurements of the occupied spaces support the perception of the LTS reported by occupants: designs that create high contrast luminous environments are more likely to be perceived negatively.

Keywords: light transport systems, light pipes, skylights, perception, performance

1. INTRODUCTION

The demand for energy efficient and healthy buildings has encouraged the development of innovative daylighting technologies that increase daylighting levels and illuminate much deeper areas within buildings than is usually achieved by simple windows. These new technologies, known as Light Transport Systems (LTS) (e.g mirrored light pipes, fibre optics, lenses, etc), can deliver daylight deep into building interiors. LTS usually comprise a light collection system (that captures daylight), a transport section (that transports the light over long distances into the building) and a system to distribute light and illuminate the space (Figure 1). LTS are designed to

integrate the benefits of natural light into buildings, which otherwise would rely totally on artificial lighting. Critically, the use of LTS in buildings reduces the need for artificial lighting and consequent cooling load of a building, thus reducing overall building energy consumption.

Figure 1: tubular skylight (solartube 750 DS) [1]

Regardless of their potential in energy savings [2]; the effects of LTS on health, how people perceive and experience these systems, and how they can affect occupant behaviour is not well understood. User acceptance and positive perception of these systems is significant in their successful application.

Whilst a growing body of research has experimentally tested the effect of different lighting conditions on employee satisfaction, mood and productivity (e.g.,[3]), significantly less is known about how office workers experience new lighting technologies, such LTS. Barrett [4] reported the perception of registered New Zealand architects and their experience in using core-daylight system, with Carter [5] first studying attitudes of office occupants towards daylight delivered by Tubular Daylight Guidance Systems (TDGS). From these studies, it is evident that open plan offices with LTS and windows provide better daylight penetration; however, users believe that windows are more efficient in delivery of both quantity and quality of

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daylight. The reasons for the failure of LTS to achieve user expectations in daylight performance were attributed to a lack of understanding the actual potency of the system.

Thus, this Australian pilot study explored how office building occupants perceived simple LTS. The aim was to relate the design of different LTS with their effectiveness, measured by (a) the daylight quality of the space and (b) occupant perception of the system.

2. METHODOLOGY

Participants & Case Study Buildings

Two commercial office buildings with LTS in south-east Queensland, Australia, were selected as case studies. The case studies represent two situations, 1- integration of LTS and windows, 2- windowless space only illuminated by LTS. Building 1 (Fig.2) is in the central business district of Brisbane. The two-storey building is obstructed by adjacent structures of similar height. It is an open plan architectural office with a visually pleasant interior and lighting design, which includes the use high ceilings, skylights (simple square light pipes with white transport section and lambertain diffusser), direct/indirect lighting and photosensor dimming controls. Building 2 (Fig.3) is in the regional community of Toowoomba, 100 km from Brisbane. The building is also double storey; however it is relatively free from sky obstruction by other buildings yet contains very few vertical windows. The windowless, open-plan office area of this building was retrofitted with Solatube 750 DS systems (Fig.1) used as skylight. The design of the office space is not particularly pleasing (low ceilings, recessed louvre-style electrical lighting, dark décor and no coordination between light pipes placement and office workstations). All workers residing in the buildings were invited to participate in four focus groups (2 per building), with 18 people participating; 11 from Building 1 in the City (5 male, 6 female and 9 from Building 2 in Toowoomba (5 male, 4 female).

Procedure, Measures & Analysis

Qualitative Study Methodology

Experiences of luminous conditions can be analysed in a similar way to experiences of the built environment. According to Bell et al. [6], factors involved in the experience of the built environment include: perception of

place, senses, needs, desire for satisfaction (choice and comfort), cognition and aesthetics. Applying this theoretical framework to lighting research, the interview schedule for the focus groups focused on occupants experience of luminous conditions in workspace, specifically:

– preferences for illuminance level, sources of light and overall luminous scenario

– needs and requirements for lighting quality and functional comfort in the workspace

– behavioural measures e.g., satisfaction with the lighting conditions, feelings about work environment, task performance and individual lighting control.

– Aesthetic appraisal of lighting design.

– Context, previous background and experience with lighting environments.

Qualitative Data Collection

After seeking consent from organisational managers, potential interviewees were contacted (via email and phone) and invited to participate in an in-depth face-to-face focus group to explore issues of the luminous environment. Within each building, two focus groups were conducted with occupants. Standard good practice research ethical protocols were followed; with the semi-structured focus groups lasting 60 minutes and covering major factors involved in experience of luminous conditions in the workplace. A movie voucher was offered as an incentive and thank you for participation. Transcripts and responses were analysed using a thematic approach, identifying categories, themes and patterns [7].

Quantitative Data Collection

Measurements of luminance and illuminance were made at each building location on the days of interview. Building 1 was monitored on one day in late November (late spring); Building 2 was monitored once in mid-June and again in late November (corresponding to winter and late spring). On each day, data was collected between 10:00am and 4:00pm, during normal operation of the offices. Staff used the spaces undergoing measurement. The measurement protocol in these circumstances had to be unobtrusive and minimal distraction to staff.

Illuminance measurements were recorded using a Konica Minolta multi-point illuminance measurement system with 9 photosensors logging data at 5-minute intervals. Sensors were placed within workstations to record the daily illuminance pattern experienced by occupants in their

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horizontal task areas. Sensor locations were selected to reflect a variety of workstation conditions (e.g. directly below light pipe, adjacent to light pipe and window, light pipe behind no window, etc). There was no attempt made to quantify the spatial uniformity of illuminance in the buildings due

to the large areas of measurement and the need to minimise disruption. Key metrics for appraising workstation illuminance were: average total illuminance and standard deviation of total illuminance (as a measure of the magnitude of temporal variation illuminance).

Figure 2: Case Study Building 1, in Brisbane (architecturally designed)

Figure 3: Case Study Building 2, in Toowoomba (no windows)

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Luminance measurements were recorded using High Dynamic Range (HDR) photography [8]. The uncertainty on luminance measurements made using this method is approximately 10 %. A HDR of the luminous environment observed by an occupant at a selection of workstations was recorded at 3 different times of day: 10:30am, 12:00pm and 3:30pm. The key metrics applied here for examining luminance surrounding the workstation were: maximum luminance in the field of view, average luminance in the task and immediate surrounding areas, and luminance ratio (of maximum value to task value).The quantitative photometric data reported in this paper is used to give general context to the experiences described by focus group participants. Results from individual workstations are not identified.

3. RESULTS

A preference for natural light

All participants identified numerous perceived advantages of daylighting, including health, psychological and environmental benefits, with these factors often intrinsically linked. There was a belief that electric light was ‘artificial’ and ‘not natural’ and therefore there was the perception that this light was not as ‘good for you’, with participants raising health and well-being concerns associated with electric light. This was particularly evident in the Toowoomba sample that did not have access to windows in their office: “I know it (electric light) doesn’t do me any good and it’s certainly disconnects me from the outside of the building”. Daylight was associated with ‘natural’ light and perceived to be good for health and the environment: Just feel like I’m getting a bit more of the sun’s energy or something [laugh] I don’t know how to explain it but I just like it better”

Evaluating Light Pipes

In Building 1, participants were much less aware of the light pipes – many people sat next to windows or had a view, which they enjoyed. It is significant to note that adjacent buildings and trees heavily obstructed the windows, so that they provided very little daylight to the interior. The ceiling height in this building was over 5 metres, placing the overhead LTS out of the field of view of occupants and permitting good spatial uniformity of skylight at the workplane. Occupants viewed the space as ‘wide and

luminous’, a beautiful space that they enjoyed working in. Critically, they did not automatically make the connections between the light pipes and the light working environment – after reflection and discussion, they acknowledged that without the light pipes (simple tube skylights) their office would have relied on artificial light (see Figure 1). As the quotes below illustrate, Building 1 was a good example of how well-designed luminous environment with good integration of windows and light pipes helps people enjoy the space and natural light.

"I think that it was one thing that attracted us before when we were looking for a new tenancy and when we came in here there were walls everywhere I did look up and notice the skylights and the natural light that was coming from them was one thing that would be a lovely feature for any office"

Measurement of the luminous environment at workstations in Building 1 generally showed that the maximum luminances in the field of view were at nearby luminaires (typically approximately 4,500 cdm

-2) or at

skylights (values up to 9,700 cdm-2

). Figure 4 shows the HDR view of a workstation with a view of a skylight. This photo is selected as the most extreme example of luminance contrast obtained in Building 1. In the task and immediate regions of the space the average luminance is 120 cdm

-2. This gives

a ratio of 80:1. To give this value context, from a visual comfort perspective, the suggested maximum for brightness ratios within the field of view is 50:1 [9].

Conversely, in Building 2, participants were generally negative about the light pipes. First, they were resentful because they had originally been promised a large window during the retrofitting. During construction, however, it proved to be impossible and they were given light pipes - many were still angry, as this ‘did not replace a window’. Second, half of the employees worked in an area with no windows and in the area with window there was limited access. Thus, light pipes were critical in this building – whilst they were sophisticated in design (e.g., collection device, dumper mechanism that reduce incoming light), the integration of the light pipes with the building and electrical design was poor. As the quotes below illustrate, Building 2 occupants experienced many problems with the design and implementation of the light pipes - especially glare (because the light pipes were badly positioned) and lack of control.

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"The negative is that you feel like you don’t much control over it and they haven’t really been placed in really thought out positions. So they have put a good thing in badly".

"Basically at a certain time of day on a really bright day just be way too glary and I have

manila folder stuck to the wall there and I have moved the plant a little bit some days to to block out that light otherwise it’s just really quite tiring.".

Figure 4. Luminance map at workstation 4, 10:30am, in Building 1.

Figure 5. Luminance map at workstation 7, 3:30pm, in Building 2.

The negative visual conditions reported by some Building 2 occupants were supported by luminance measurements

taken at workstations in this building. Figure 5 shows a workstation adjacent to a light pipe in a windowless area. The image

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shows the immediate working surfaces have luminances around 140 cdm

-2.The maximum

luminance in the field of view comes from the adjacent light pipe, approximately 19,000 cdm

-2. This gives a luminance contrast ratio

of 135:1, again exceeding the recommended value for visual comfort. All of the seven workstations surveyed in Building 2 displayed a similar high luminous contrast at some part of the day (either morning, midday or afternoon). Dark décor and low ceilings compounded the potential for visual discomfort in these conditions.

The Quality of Light

Monitoring of Building 1 revealed that illuminance levels were relatively constant throughout the day: the standard deviation of the average illuminance was less than 4 % of the average illuminance across all of the workstations recorded. The luminaires used in this building incorporated photosensors that maintained a combined daylight/electric light illuminance of 500 lux at the workplane. Accordingly, the average illuminance measured at workstations during the measurement period ranged from 490 lux to 540 lux. Conversely, Building 2 had no sensors and no means of dimming luminaires. The general illuminance levels were excessive where daylight and electric light combined with average illuminances at workstations ranging from 450 lux to 820 lux. Light from the skylight was described as fluctuating or variable light, rather than constant. Over the periods of measurement, the standard deviation of the average illuminance at each workstation ranged from 14 % to 23 % of the average values. A few isolated examples can be identified where in the period between measurement intervals (5 minutes), illuminances changed by more than 150 lux at every sensor. This type of variation in illuminance occurred with clouds temporarily obstructing sun and daylight levels dropping significantly – with no view of what is happening outside, participants felt this was a distraction and caused eye strain:

“but on a really bright day with a few clouds around you do really notice the change in light level. See what I don’t like about this is that your eyes are constantly adjusting backwards and forwards, backwards and forwards. I don’t understand how you can survive in that environment when you've got a bright screen and then you’ve got this bright thing that goes off and on, off and on behind you. I just think it will strain your eyes"

One Building 2 participant who described the light from the skylight as not ‘natural’ for an office setting due to its fluctuating nature, likening it to being outside in a rainforest.

"It’s not a natural sort of lighting arrangement. It might be natural if you are in a rainforest and you’ve got a thick canopy above you and you’ve got pools of light coming in like that, it’s sort of natural. But the way it is now with no other lights on it’s not that natural I don't think. Well to me."

Building 2 occupants also noted how the use of electric light could balance the variability of the skylight. Electric light was described as more ‘stable’ or ‘reliable’ lighting and less obtrusive. "The other thing I’ve found since we’ve moved here which is a little bit disappointing is that they really don’t seem to take away much of the need for the fluoro lighting as well… most of the office here people still require the lights to be on during the day simply to have enough reliable sort of lighting because the light coming through the skylight is just too variable…"

It is significant to note that there were limited controls for the electric lighting system in Building 2: single on/off switches controlled large groups of luminaires. There was no opportunity to dim the fluorescent lamps in response to increases in skylight, or to personalise luminous conditions at the workstation. This is in contrast with the integrated control system found in Building 1, where photosensors activated continuous dimming of the fluorescent lamps to maintain even illuminance throughout the day. Individual controls were available for workstations in Building 1 (although not frequently used during the day).

Connection to Outdoors

While participants believed skylights gave connection to the outdoors, by allowing variations in weather to be felt, participants from both buildings were unanimous in their belief that daylight from a skylight was no replacement for daylight from a window, with one participant describing the window as “a total experience”. Windows were perceived to have the additional benefit of being able to ‘see’ outdoors and gave context to the variation in lighting.

"You certainly feel connected to the outside still with skylight but not as much as you would if you had a window… I notice when cloud cover is moving across and even sometimes where there’s an aircraft and get a shadow flicker across the skylight"

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"In the absence of the windows these (skylights) are a much better option than the artificial lights. Natural is good.

The variability of the skylight was viewed positively by some , providing a connection to the outdoors and cues to the changes in weather and time of day. Skylights were important for allowing this connection in Building 2, which had limited access to windows and relied on the skylights.

"It gives an indication of what is going on outside so you can get up and find out for real. Yeah I am very much aware of the diurnal sort of movement during the day. I’d have to say it gives me some sort of feeling connection to the outside, nowhere near as good a connection with the outside I would like to have if we had a window"

Building 1, however, had windows that provided most of the cue to the outdoor variability. Most office workers in this office have direct access to a window, nevertheless, some mentioned they get cues of outdoor variability from the skylight depending on where they sit:

"depends where you sit. Definitely where I was sitting yesterday I only had the skylight for the reflection from outside".

"I have a skylight right behind me so I do notice when it goes dull and when you get full sun coming on it I get that shadow behind me, but it doesn’t impact on me it just gives a good sense of brightness and light "

The Importance of Skylight Placement

Participant’s perception of the lighting quality from the skylights was largely influenced by the placement of the skylights, which in many cases was not optimal. In Building 2, lights that were directly above or behind working spaces produced glare and participants reported negative associations with the light: “Anyone who has it directly over them, they don’t like it. They try and adjust their environment to suit, to make it more pleasant, less glary”. Building 1 participants also reported glare issues from skylights located above workstations, particularly at certain times of the day:

“When I was sitting behind it with the glare right behind me, it was about 11 til 1 which was really annoying because it was right above you, the sun was right above and it came right down and it just lit everything up right below it. And that’s when I sometimes wore sunglasses”

This comment is consistent with measurements of light pipe luminance in this office similar to that shown in Figure 5 exceeding 20,000 cdm

-2. In terms of optimal

placement, participants indicated that preferable placement of the skylight was central or behind the work area, with one describing how the use of a white table under the light helped disperse the light around the work space (Figure 6): “In the place where I’m at now, there is one sort of centrally located which is good and then there’s a white table so the light coming down actually reflects off the white table and disperses a bit more.”

Figure 6: Furniture (white table) that works as a diffusing surface for LTS in Toowomba building

DISCUSSION

The main issues that arouse from this research are:

Preference for natural light: all office workers had a generally positive perception of daylight in the workplace (even those who were unhappy with their current environment due to negative aspects of LTS).

Evaluating LTS: when LTS are integrated with windows (and problems of glare were not present) acceptance was higher. Lack of control of the device and poor integration with electrical lighting produced more negative views of the systems.

Quality of Light: where LTS provide major fluctuations in workplane illuminance, response was more likely to be negative

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Connection to outdoors: LTS provided cues to diurnal variation, especially significant and valued where there were no windows. For some, this was a higher priority than potential visual discomfort from LTS.

Importance of placement: Perception varied with position of workstation with respect to LTS. This was generally a glare issue, particularly where the skylight was within occupants field of view while working (i.e. dependant on ceiling height and aperture luminance)

The main implications people’ preferences may have on design for LTS are:

– Incorporating windows – even if they provide little light or are heavily obstructed (as they were in Building 1)

– Ceiling height is a major factor on the glare to occupants from skylights (lower ceilings places skylight apertures in the field of view at the workstation), glare avoidance is critical in positive perception of LTS – this issue is compounded by ill-considered space planning around skylights (the preferred position reported by occupants in Building 2 was for the skylight to be behind them – the least comfortable position was skylight directly above workstation)

– Control systems can increase occupant perception of lighting quality where LTS are applied (particularly with regards to temporal uniformity of light). Where photosensors maintained even workplace illuminance from the combination of electric and daylight in Building 1, there was no reporting of discomfort from variation in skylight. This is in contrast with Building 2, where there were no integrated lighting controls and skylight variation was reported by some as a negative quality of the LTS. Interestingly, the control system in Building 1 did not detract from the occupants’ ability to notice diurnal variation from the changing quality and quantity of light from the LTS (may be confounded by access to view windows - notably a participant with no window view commented positively).

– Workstation placement must be considerate of LTS layout – particularly in open plan spaces (Building 2), creating workstations with visual comfort issues.

– Design of output element - output elements that evenly diffuse light or redirect light upwards viewed as producing a more pleasing luminous environment

In conclusion, these results begin to

address the central aims of this research – understanding how people perceive and

experience spaces lit by LTS. However they are limited by the availability of only simple LTS systems and their confinement to a single geographical location (and cultural context). Further research is necessary to evaluate more complicated LTS.

ACKNOWLEDGEMENTS

The authors would like to thank Queensland University of Technology for the seed grant that supported this research the participants and project research assistants - Galyna McLellan Lorraine Bell and Michael Hirning Corresponding Author Veronica Garcia Hansen School of Design, QUT. 2 George St GPO

Box 2434. Brisbane Qld 4001 Australia Phone: +61 7 31381623 Fax:+ 61 7 3138 1528 e-mail: v.garciahansen@qut.edu.au

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