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Individual control over light source spectrum: effects on perception

and cognition

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I ndividua l c ont rol ove r light sourc e spe c t rum : e ffe c t s on pe rc e pt ion

a nd c ognit ion

N R C C - 5 4 0 1 1

V e i t c h , J . A . ; D i k e l , E . E . ; B u r n s , G . ; M a n c i n i ,

S .

A u g u s t 2 0 1 1

A version of this document is published in / Une version de ce document se trouve dans:

27th Session of the Commission Internationale de l'Éclairage, Sun City, South

Africa, July 10-15, 2011, pp. 213-218

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© National Research Council of Canada, Ottawa, ON, 2011.

Please cite as: Veitch, J. A., Dikel, E. E., Burns, G. J., & Mancini, S. (2011). Individual control over light source spectrum: Effects on perception and cognition. Proceedings of the 27th Session of the Commission Internationale de l’Eclairage, Sun City, South Africa

(CIE 197:2011, Vol. 1, Part 1, pp. 213-218). Vienna, Austria: CIE.

INDIVIDUAL CONTROL OVER LIGHT SOURCE SPECTRUM: EFFECTS ON PERCEPTION AND COGNITION

Veitch, J.A., Dikel, E.E., Burns, G.J., Mancini, S.

NRC Institute for Research in Construction, Ottawa, Canada

jennifer.veitch@nrc-cnrc.gc.ca

Abstract

For solid-state lighting to make a major dent in the lighting energy budget will require light sources and luminaires that are more efficacious than the existing technology, and a reason for building owners and tenants to change to an initially expensive, new and unfamiliar technology. New luminaire designs and operation modes using the unique characteristics of SSL will aid this transition. The ability to individually tune the light source colour is one such characteristic. Building on prior research concerning the benefits of individual control over illuminance, this experiment tests the effects of tunable light source colour and initial LED spectrum on office workers’ cognitive performance, mood, and satisfaction over the working day.

Keywords: solid-state lighting (SSL); light-emitting diodes (LEDs); tunable colour; light source spectrum; cognitive performance; mood; satisfaction; offices; work.

1 Introduction

Solid-state lighting (SSL) (light-emitting diodes [LEDs] and organic LEDs [OLEDs]) is a new technology undergoing rapid development. The efficacy of commercial white SSL sources is expected to double that of fluorescent in the long term, although commercial products are not yet as efficacious as linear fluorescents for general lighting. For SSL to make a major dent in the lighting energy budget requires addressing general white light applications, particularly in offices. However, as yet the industry is not conducting very much research and development on new luminaire designs and operation modes using the unique properties of SSL, but remains focussed on incremental source efficacy improvements and socket-for-socket replacements in existing luminaires. New luminaire designs and operation modes promise greater energy savings and value to the user; the latter will encourage earlier adoption of SSL systems. Put colloquially, few people will adopt an unfamiliar, complex, and (initially) more expensive technology unless it has the potential to offer features and functionality that is different from, and perceived as superior to, the existing technology.

Previous research at the NRC Institute for Research in Construction and elsewhere has demonstrated that personal control over light level in an office environment has benefits for occupant satisfaction, comfort, and mood, and also contributes to energy savings [e.g., Boyce et al. 2006; Newsham et al. 2004]. LEDs now offer the possibility of some degree of control over delivered light spectrum. This experiment is a test of the viability of individually tunable light source colour for general office lighting.

We hypothesized that individual preference for light spectrum does vary substantially across the population of office workers, and that having personal control over light source spectrum will enhance occupant well-being. Others have suggested that varying spectrum systematically by time of day might have direct physiological benefits, via linkages to circadian rhythms, or by other mechanisms [Bommel 2006]. Existing colour quality indicators (e.g., Ra) are poor predictors of colour quality for LED lighting,

and the search for better metrics is a major part of international LED research [Davis and Ohno 2009]. The results of this experiment will also address LED colour quality effects as a contribution to this international debate.

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Veitch et al. — CIE 2011 page 2

The experiment is a 2 x 2 (initial spectrum X afternoon tunability) factorial design. The initial spectra are ~3000 K and ~ 6500 K; half of the participants were given the option to change the light source spectrum after lunch in the full-day session. This experiment is designed to test five hypotheses:

H1: When given the opportunity, individuals will self-select different spectral power distributions as their preferred lighting.

H2: Being given control will stop or reverse the daily, fatigue-driven trend in reduced performance and mood (trends that have been observed previously).

H3: Those whose preference in the afternoon is most different from what they previously experienced (in the morning) will show the largest (favourable) changes in mood, satisfaction, and performance. H4: Overall, colour discrimination performance will be better for conditions with higher colour quality. (This hypothesis will be tested with the participants’ selected light source spectra.)

H5: Overall, performance and alertness immediately after lunch (post-prandial dip) will be higher for the 6500 K group than the 3000 K group.

The overall objective of this experiment was to demonstrate that providing individually tunable light source spectrum, as is possible with LED lighting, improves the office environment by improving the satisfaction of office workers. Such a demonstration would facilitate the uptake of this new technology, thereby increasing the potential for energy savings as system efficacy improves over the conventional fluorescent systems that LED lighting will replace.

2 Method 2.1 Participants

At the time of writing, data collection is not complete. We have preliminary data from 69 participants (31 men and 38 women) between ages 18 and 69 (median age ~ 35), hired from an office temporaries agency for a day’s work as a participant in this experiment. Half of the sample had taken university courses and 37% had completed an undergraduate or graduate degree. The final sample will consist of ~88 participants; complete data will be presented at the CIE Session.

2.2 Setting and Independent Variables

The experiment took place in a windowless mock-up office equipped with two cubicles separated by full-height furniture panels, but with no doors. Figure 1 shows one of the two identical cubicles. Note that a mirror at the top of the wall gives the appearance of a larger space extending beyond the individual cubicle boundary. Each cubicle had an L-shaped desk with a smooth grey surface (ρ = .55). The desk was equipped with a personal computer, an adjustable keyboard tray with mouse extension, and an LCD monitor. The office chair was fully adjustable. The painted walls were a very light grey (ρ = .79) and the fabric panels were beige (ρ = .48) with taupe supports (ρ = .23). The floor was carpeted in a tweed pattern of grey with coloured flecks (ρ = .12). The colour scheme was chosen to avoid dramatic chromatic shifts under the tunable colours. Accents were provided in the form of art on the walls, an artificial plant, and real wood desk accessories. Not shown in the photo are the two low-iron-glass mirrors on the walls, which participants used when making judgements of their own skin appearance.

The ambient lighting was provided from recessed prismatic-lensed 1 ft by 4 ft luminaires that were modified to use seven LED units as the light source, instead of fluorescent lamps (see Figure 2). The LED units are RGBW systems that provide variable white light by combining the output of four light-emitting diodes (red, green, blue, and white). Outwardly the luminaires look like conventional fluorescent office lights.

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Veitch et al. — CIE 2011 page 3

Figure 1. View of one cubicle.

Figure 2. Custom luminaire. Left photo: exterior of troffer (shown suspended from lab bench).

Right photo: LED sources are individually controllable and are aimed at the side of the troffer to provide a diffuse light through the prismatic lens.

Participants were randomly assigned to one of the two initial starting conditions, either 3000 K or 6500 K white light, both providing ~ 500 lx on the desk (see Figure 2) and CRI = 90. This lighting was fixed for the morning for all participants.

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Veitch et al. — CIE 2011 page 4

Half of the participants had the opportunity to individually tune the lighting spectrum (maintaining the same illuminance) for the afternoon using custom-designed colour selection software (“Colour Picker Pro“) presented on the computer screen (see Figure 4). The other half continued to work under the same lighting all day; they did not know that light source spectrum control was a possibility until the end of the day, at which time they used the colour selection software to indicate their personal spectral preference.

Changing the cursor position on the Colour Picker Pro image returned values for the RGB channels to the lighting system. The LED units added a variable input from the W channel (which was outside our control) to maintain the constant illuminance. Custom software logged the RGB values every minute while the system was on. Post-experimental spectroradiometric measurements of the re-created conditions will be needed to establish the correlated colour temperature, colour rendering index, chromaticity coordinates and other colour characteristics of the colours chosen by participants.

Figure 4. Colour Picker Pro interface. The RGB values changed dynamically as participants clicked and

moved moved the mouse (+); unclicking would fix the values. The ○ symbol marks the previous setting.

2.3 Procedure

Participants, who were recruited from an office temporaries firm, arrived at 8:30 a.m. and began the day with the signing of a consent form and a colour vision screening test based on the Ishihara colour plates. Those whose test results indicated a colour vision problem completed the day, but their data will be examined separately.

Participants then entered their assigned side of the cubicle space. For the rest of the day, participants completed a set of computer- and paper-based tasks and questionnaires delivered through custom software to keep them on schedule. The activities were designed to provide experience with the space and its appearance and to test the effects of the lighting conditions on cognitive performance, mood, and judgements of the space.

Participants reported their age, sex, education, years of work experience (both overall and experience as a temporary office worker), handedness (used for mouse), the state of their vision (corrected or not), eye colour, skin colour group, and hearing. They also completed the Early/Late Preferences Scale, a questionnaire that assesses the degree to which an individual prefers to be most active in the morning, or later in the day [Bohle et al. 2001]. This is a personality construct known as Morningness/Eveningness (or chronotype), which is thought to reflect underlying differences in circadian rhythm. Previous NRC-IRC work (in preparation) has found that chronotype predicts light level choices, and we expect that it will also influence light spectrum choices.

The cognitive and perception tasks and questionnaires for mood state, satisfaction, comfort, and appearance judgements were repeated three times: once after the morning coffee break (the first part of the morning consists of acclimation and training), and twice in the afternoon. Colour discrimination performance was assessed with the Farnsworth-Munsell 100 Hue Test [Kinnear and Sahraie 2002], and

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Veitch et al. — CIE 2011 page 5

colour appearance judgements were made of both the space and of the individual’s own appearance, as seen in a low-iron mirror.

The cognitive measures focused on vigilance and fatigue: They were a word judgement task using the task-switching paradigm [Allport et al. 1994; Wong and Leboe 2009] and the conveyor belt test used in previous NRC-IRC lighting quality research and known to be sensitive to the availability of individual control over lighting [Boyce, Veitch, Newsham, Jones, Heerwagen, Myer and Hunter 2006; Veitch and Newsham 1998]. The task-switching paradigm uses the increased response latency associated with a change in task as an indicator of task difficulty. In this case, participants performed a length sequence of word judgments based on the meaning of a word or the surface characteristics of its presentation (colour or size). The dependent measure is the difference in the time required to answer the second question in a pair; it is longer when the second question is of a different type than the first. Furthermore, this latency becomes longer as the participant becomes fatigued. To our knowledge this is the first time the task has been used in lighting research, but we expect its sensitivity to be appropriate to this context.

3 Results

The data are not complete at the time of writing; there remain a few experimental sessions, followed by extensive post-experimental photometric measurements, before analysis can begin. Data analysis will consist of between-groups multivariate analysis of variance of the performance, satisfaction, and mood data to test the five hypotheses described above. The analyses will include multiple regression analyses that examine the difference between participants’ tuned colours and the colours they experienced during the earlier part of the day and the effect of that discrepancy on their mood and satisfaction; this analysis will parallel the work of Newsham et al. [2004] using individual control over illuminance. We also plan extensive analyses of possible moderating effects of age, sex, and individual differences. These analyses will be presented at the CIE Session.

Preliminary examination of the use of the tunable colour program shows a wide variety of responses among the 33 people in that experimental condition. The range of use of the colour picker is from 0 (no changes) to 17 changes over the afternoon (we excluded from this count any change that was in place for less than 2 minutes on the basis that these were not stable settings).

4 Discussion

LED lighting represents a revolution in lighting and promises dramatic increases in lighting energy efficiency. Market penetration, however, will depend on user acceptance of a new, unfamiliar, and initially more expensive technology. The possibility of tunable colour is one feature that might intrigue potential users sufficiently that they would make the change from a conventional fluorescent system. This experiment will provide insight into how office employees might use this new feature, and also addresses outstanding hypotheses concerning the effects of light source spectrum on work performance and employees’ satisfaction and mood over the working day.

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Veitch et al. — CIE 2011 page 6

References

ALLPORT, D.A., STYLES, E.A. AND HSIEH, S. 1994. Shifting intentional set: Exploring the dynamic control of tasks. In Attention and performance XV, C. UMILTA AND M. MOSCOVITCH Eds. MIT Press, Cambridge, MA, 421-452.

BOHLE, P., TILLEY, A.J. AND BROWN, S. 2001. Psychometric evaluation of the Early/Late Preferences Scale. Ergonomics 44, 887-900.

BOMMEL, W.J.M.V. 2006. Dynamic lighting at work -- both in level and colour. In Proceedings of the 2nd

CIE Expert Symposium on Lighting and Health Commission Internationale de l'Eclairage, Vienna, Austria,

62-67.

BOYCE, P.R., VEITCH, J.A., NEWSHAM, G.R., JONES, C.C., HEERWAGEN, J., MYER, M. AND HUNTER, C.M. 2006. Lighting quality and office work: Two field simulation experiments. Lighting

Research and Technology 38, 191-223.

DAVIS, W. AND OHNO, Y. 2009. Approaches to color rendering measurement. Journal of Modern Optics

56, 1412 - 1419.

KINNEAR, P.R. AND SAHRAIE, A. 2002. New Farnsworth-Munsell 100 hue test norms of normal observers for each year of age 5-22 and for age decades 30-70. British Journal of Ophthalmology 86, 1408-1411.

NEWSHAM, G.R., VEITCH, J.A., ARSENAULT, C.D. AND DUVAL, C.L. 2004. Effect of dimming control on office worker satisfaction and performance. In Proceedings of the IESNA Annual Conference, Tampa,

FL, July 26-28, 2004 IESNA, New York, 19-41.

VEITCH, J.A. AND NEWSHAM, G.R. 1998. Lighting quality and energy-efficiency effects on task performance, mood, health, satisfaction and comfort. Journal of the Illuminating Engineering Society 27, 107-129.

WONG, J. AND LEBOE, J.P. 2009. Distinguishing between inhibitory and episodic processing accounts of switch-cost asymmetries. Canadian Journal of Experimental Psychology 63, 8-23.

Acknowledgements

This work is supported by Natural Resources Canada (Panel on Energy Research & Development; Office of Energy Efficiency; Clean Energy Fund; CANMET HBCS); BC Hydro; Philips Color Kinetics; Philips CANLYTE; GO Lighting, UBC Structured Surface Physics Laboratory; NRC Institute for Research in Construction; and volunteer advisors from industry and research labs. The authors acknowledge the additional contributions of students Derek Ludwig, Chris Hahn, Toby Pilditch, and Pavel Popov to the development of software and tasks and thank Morad Atif, Guy Newsham, and Trevor Nightingale for their ongoing support of this project.

Figure

Figure 1. View of one cubicle.
Figure 4. Colour Picker Pro interface. The RGB values changed dynamically as participants clicked and  moved moved the mouse (+); unclicking would fix the values

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