A Critical Investigation of Common Lighting Design Metrics For

This article was downloaded by: [University of Washington Libraries] On: 20 February 2014, At: 08:59 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK LEUKOS: The Journal of the Illuminating Engineering Society of North America Publication details, including instructions for authors and subscription information: http://ies.tandfonline.com/loi/ulks20 A Critical Investigation of Common Lighting Design Metrics for Predicting Human Visual Comfort in Offices with Daylight Kevin Van Den Wymelenberga & Mehlika Inanicib a University of Idaho Integrated Design Lab, Boise, Idaho, USA b University of Washington, Architecture, Seattle, Washington, USA Published online: 20 Feb 2014. To cite this article: Kevin Van Den Wymelenberg & Mehlika Inanici (2014) A Critical Investigation of Common Lighting Design Metrics for Predicting Human Visual Comfort in Offices with Daylight, LEUKOS: The Journal of the Illuminating Engineering Society of North America, 10:3, 145-164 To link to this article: http://dx.doi.org/10.1080/15502724.2014.881720 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. 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Terms & Conditions of access and use can be found at http:// ies.tandfonline.com/page/terms-and-conditions LEUKOS, 10:145–164, 2014 Copyright © Illuminating Engineering Society ISSN: 1550-2724 print / 1550-2716 online DOI: 10.1080/15502724.2014.881720 A Critical Investigation of Common Lighting Design Metrics for Predicting Human Visual Comfort in Offices with Daylight Kevin Van Den Wymelenberg1 and ABSTRACT Existing visual comfort metrics are reviewed and critiqued based 2 Mehlika Inanici upon their ability to explain the variability in human subjective responses in a daylit 1 University of Idaho Integrated private office laboratory environment. Participants (n = 48) evaluated visual comfort Design Lab, Boise, Idaho, USA 2University of Washington, and preference factors, totaling 1488 discreet appraisals, and luminance-based met- Architecture, Seattle, rics were captured with high dynamic range images and illuminance-based metrics Washington, USA were recorded. Vertical illuminance outperformed all commonly referenced visual comfort metrics including horizontal illuminance, IES luminance ratios, daylight glare probability (DGP), and daylight glare index (DGI). The bounded border- line between comfort and discomfort is introduced, and preliminary visual comfort design criteria are proposed for several existing metrics. Fundamental limitations of glare indices are documented, and the implications of inconsistent application of luminance ratio calculation methods are quantified. Future research is detailed. KEYWORDS daylight glare, daylight metrics, luminance ratio, vertical illuminance, visual comfort 1. INTRODUCTION It is generally accepted that daylight and views help to create healthy, comfortable, and productive work environments for users and therefore should be included in Downloaded by [University of Washington Libraries] at 08:59 20 February 2014 contemporary office spaces. Equally understood is the need to minimize discomfort glare, disability glare, and veiling glare for occupants in spaces with daylight. In cur- Received 23 September 2013, revised rent practice, design guidance to support visual comfort in daylit spaces has revolved 6 January 2014, accepted 7 January primarily around horizontal illumination, simple luminance ratios, and, in advanced 2014. Address correspondence to Kevin Van applications, absolute luminance thresholds or glare indices. The application of Den Wymelenberg, University of luminance-based techniques remains primarily within the research community and Idaho Integrated Design Lab, they have gained little traction among design practitioners. Both illuminance- and Architecture, 306 S. 6th Street, Boise, ID 83702, USA. E-mail: luminance-based methods suffer from an established lack of confidence or con- [email protected] sensus by the research and design communities regarding what metrics should be Color versions of one or more of implemented and what criteria are recommended. This is primarily because there the figures in this article can be found online at www.tandfonline. is presently inadequate human visual comfort research to support consensus-based com/ulks. design recommendations. 145 This article presents original research from a 6-month bounds of human acceptance and preference in spaces repeated-measures experimental design in which 48 par- with daylight, it stands to reason that luminance-based ticipants assessed their visual comfort in a private daylit metrics will more closely correlate with subjective accep- office. It begins with a brief review of common visual tance and preference measures than illuminance because comfort metrics (both illuminance and luminance based) luminance more closely relates to human perception of and recommended criteria. It identifies the strengths and brightness (see discussion in Cuttle [2004]). This section limitations of these metrics and recommends revised mea- reviews luminance ratios, absolute luminance values, and surement techniques and design criteria as relevant. Finally, glare indices that have been used to characterize visual it establishes the need for future research to develop a new preference and acceptance of the luminous environment. suite of luminance-based analysis metrics. 1.2.1. Luminance ratios 1.1. Illuminance-Based Metrics Current recommendations by the IES [DiLaura and others 2011, p. 12.20] list the maximum luminance ratios in Due to its ease of use and low cost to measure, horizon- daylight settings as “20:1 between daylight-media and tal illumination is the most widely applied architectural daylight-media-adjacent-surfaces.” No specific references lighting design metric. However, even under electric light are offered for the IES’s 20:1 recommendation, and other sources only, illuminance preference varies greatly, from ratios cite the previous handbook [Rea 2000], which also 100 to 800 lux [Boyce and others 2006; Newsham and lacks substantial reference to original research. Few previ- Veitch 2001; Veitch and Newsham 2000], and has a mean ous studies describing preferred luminance ratios in set- value between 400 and 500 lux. It has been reported that tings with daylight are available. Halonen and Lehtovaara the choice of any fixed horizontal illumination value will [1995] reported that under a wide range of daylight con- only be preferred by at most 55% of the users [Boyce et al. ditions, participants (n = 20) preferred average luminance 2006]. Few studies have reported user preference for illu- ratios of a white paper-based task and a light-colored back mination under daylight conditions alone. One found that wall opposite the window ranging from approximately 300 lux of daylight was preferred (n = 20) [Laurentin 1:2.25–10 with an average of approximately 1:5. Note that and others 2000] and another study, conducted primarily ratios using window luminance values were not reported. during sunny winter days, found a wide range of pre- Sutter and others [2006] found that a space with daylight ferred desktop daylight illuminance and a preferred mean was comfortable for users with luminance ratios of 1:6:20 of 3623 lux (n = 18) [Van Den Wymelenberg and others (task: adjacent: remote), twice as extreme as those tradi- 2010]. tionally recommend by the IES (1:3:10) but in line with the new recommendation for daylight media (1:20). The 1.2. Luminance-Based Metrics authors also found that users tolerated up to 1:50 as long as it was restricted to relatively small areas, comprising less As noted previously, human acceptance and preference than 5% of the field of view. vary widely under primarily electrically illuminated spaces. Due to the complexities related to daylight in buildings (for example, variability with time of day, time of year, 1.2.2. Glare indices Downloaded by [University of Washington Libraries] at 08:59 20 February 2014 sky condition, view quantity, view quality, extremes of Glare indices have been used to evaluate visual com- brightness values, discomfort glare, et cetera) the bounds fort in the luminous environment. Two recent literature of human preference are wider in spaces with daylight. reviews provide historical overviews of the multitude of Several attributes provide moderating effects to subjective glare indices [Eble-Hankins

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