Analysis of Occupants’ Visual Perception to Refine Indoor Lighting Environment for Office Tasks

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  The combined effects of color temperature and illuminance in a small office on visual response and mood under various lighting conditions were examined in this study. Visual annoyance tests were conducted using a sample of 20 subjects in a full-scale
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   Energies   2014 , 7  , 4116-4139; doi:10.3390/en7074116 energies ISSN 1996-1073 www.mdpi.com/journal/energies  Article Analysis of Occupants’ Visual Perception to Refine Indoor Lighting Environment for Office Tasks Ji-Hyun Lee 1 , Jin Woo Moon 2  and Sooyoung Kim 3, * 1  Graduate School of Culture Technology, Korea Advanced Institute of Science & Technology, Daejeon 305-701, Korea; E-Mail: jihyunlee@kaist.ac.kr 2  Department of Building & Plant Engineering, Hanbat National University, Daejeon 305-719, Korea; E-Mail: jwmoon@hanbat.ac.kr 3  Department of Interior Architecture & Built Environment, Yonsei University, Seoul 120-749, Korea *  Author to whom correspondence should be addressed; E-Mail: sooyoung@yonsei.ac.kr; Tel.: +82-2-2123-3142; Fax: +82-2-313-3139.   Received: 27 April 2014; in revised form: 17 June 2014 / Accepted: 18 June 2014 /  Published: 27 June 2014 Abstract:  The combined effects of color temperature and illuminance in a small office on visual response and mood under various lighting conditions were examined in this study. Visual annoyance tests were conducted using a sample of 20 subjects in a full-scale mock-up test space. Computer and paper-based reading tasks were conducted for 500 lx and 750 lx illuminance levels under 3,000 K, 4,000 K and 6,500 K conditions. Two hypotheses were considered for the test in this study. The primary hypothesis was that visual perception is affected by the color temperatures of light sources. The secondary hypothesis was that  better moods, such as relaxed and cozy feelings, are associated with low color temperatures given equal illuminance levels. The visual environment under the 3,000 K condition was characterized by glare and brightness, resulting in visual discomfort when target illuminance was higher than 500 lx. Occupants preferred 500 lx under the 6,500 K condition, and 500 lx and 750 lx under the 4,000 K condition, reporting better visual satisfaction when performing office tasks. Prediction models for visual comfort suggest that the less that subjects are visually bothered by light during tasks, the more visual comfort they feel. User satisfaction with light source color is critical for the prediction of visual comfort under different lighting conditions. Visual comfort was the most influential factor on mood. Lower color temperature was associated with better mood at lower illuminance levels, while higher color temperature was preferred at higher illuminance levels. OPEN ACCESS    Energies   2014 , 7   4117 Keywords:  color temperature; illuminance; visual response; reading task; office; visual comfort; mood 1. Introduction Fluorescent light sources with well-designed shielding devices are effectively utilized for lighting in office environments, since wide areas can be covered uniformly and the resulting visual comfort is acceptable. Despite these advantages, careful attention should be paid to the application of fluorescent light sources for office lighting, since their spectral composition in the visible light region is not sufficient for good color rendering and visual comfort. In some cases, the spectra are not uniformly distributed in the visible light region and obscure the perceptions of colors and objects. These limitations influence visual performance relevant to office tasks. The uneven composition of spectra in the visible light region, as summarized by the color temperatures of light sources, influences visual sensation [1,2]. The color temperatures of light sources and illuminance levels influence human visual responses and perceptions that are critically linked to visual satisfaction and comfort [3,4].   Illuminance and color temperature are quantitative indexes used to evaluate office lighting. A variety of studies have been conducted to examine the effects of color temperature on visual  perception. Visual perception and mood are affected by color temperature even if the illuminance level is constant [5  –  10]. Positive visual perception and mood increase efficiency when making some types of decisions and promote creative work performances [11]. Negative visual effects impaired task  performances and productivity when office tasks were performed [12,13]. People showed a strong tendency to believe that visual perception and mood are influenced by lighting conditions [14]. A majority of office workers in enclosed spaces prefer better lighting conditions [15,16]. The magnitude of illuminance changes is a critical factor that influences visual comfort [17]. When illuminance is constant in an office space, visual perception may still be affected by the color temperature of light. This phenomenon directly affects visual comfort and mood in office workers. Various studies have examined the influence of lighting conditions on office workers [3,4,6  –  13]. Higher illuminance conditions were evaluated as more intense than lower illuminance conditions. General interpretations regarding glare perception under different illuminance conditions were  provided by these studies; however, the visual environments and conditions considered in the studies did not include general office tasks such as paper- and computer-based visual tasks. These studies were also confined to analyses of mood under different lighting conditions, and lacked analyses regarding the effects of stimulating factors on visual discomfort. Accordingly, meaningful implications of visual  perception in terms of visual comfort were not obtained. Therefore, this study examine visual responses to a variety of lighting conditions including different target illuminance levels and color temperature conditions, since these two factors function as visual stimulation elements that affect visual perception. The combined effects of target illuminance level and color temperature on visual responses and mood in office spaces are analyzed in this study in order to design comfortable lighting environments for office tasks. Visual perception and annoyance tests were   Energies   2014 , 7   4118 conducted in a full-scale mock-up space, where six office lighting conditions were set up for computer and paper-based reading tasks. 2. Research Hypothesis Two hypotheses were considered in this study to examine visual perception and mood responses under different color temperature and illuminance conditions that are common in office environments. The primary hypothesis was that visual perception is affected by the color temperatures of light sources. It was predicted that low color temperatures that primarily consist of long wavelengths in the visible spectrum are associated with stronger visual stimuli, such as glare, brightness, and distraction, and are more likely to lead to visual discomfort, compared to the short wavelengths that represent the high color temperatures. The secondary hypothesis was that better moods, such as relaxed and cozy feelings, are associated with low color temperatures given equal illuminance levels. It was predicted that mood would become worse as the target illuminance level increased under the same equal color temperature conditions. The perceptions of visual comfort that are affected by visual stimulation factors were also expected to significantly influence mood under different lighting conditions. 3. Research Method 3.1. Space and Lighting Conditions Visual annoyance tests were performed in a full-scale mock-up office space, which was constructed for the evaluation of visual responses in a variety of control settings. Figure 1 illustrates the detailed layout of the mock-up space. The dimensions of the space were 4.9 m wide, 2.8 m deep and 2.65 m high. The windows were 1.5 m in height, and were glazed with double pane glass with 62% light transmittance. Venetian blinds were installed on the windows to control daylight. The space was furnished like a small private office. The floor was covered with beige linoleum, and the walls were painted a light green color that generated no specular reflection. An array of 0.6 m by 0.6 m suspended grids covered with white acoustic panel boards was installed on the ceiling. Two wooden desks with dimensions of 1.2 m (L) × 0.45 m (W) × 0.75 m (H) were placed in the room. The desktop surfaces were light brown and generated no specular reflections, and two chairs were  provided. A notebook computer with a 39.6 cm thin-film transistor (TFT) screen was installed on each desk for the office tasks. The resolution of the notebook screen was 1024 × 768 pixels. Fluorescent lighting fixtures were placed on the ceiling at appropriate distances to meet required illuminance levels for office tasks. Figure 2 shows detailed description of the lighting fixture. The fixtures were recessed with direct distribution of light. Parabolic troffers with louvers were used for the fixtures, and the depth of each troffer was 7.5 cm. Four T8 fluorescent lamps were used for each lighting fixture. The input wattage of each lamp was 18 W. The color produced by each fixture was constant over the entire test period once power was supplied. Overall, the lighting fixtures provided a generally uniform distribution of light. The layout of the lighting fixtures is shown in Figure 1. The target desktop illuminance levels were 500 lx and 750 lx in this study, which are considered the minimum and medium illuminance levels for the performance of   Energies   2014 , 7   4119 medium contrast or small size visual tasks in office environments [18]. The mean desktop illuminance generated by the full light output of lighting fixtures was 950 lx. Since this illuminance level by the lighting fixtures is too high for general office tasks, a rotary dimming controller was installed to control the illuminance. Setting points of the rotary dimming controller were exactly determined in order to keep the target illuminance levels. Windows were blocked to prevent the transmission of daylight and the target illuminance was maintained by electric lights only. Figure 1.  Detailed view of full-scale mock-up model space. Figure 2.  Section of fluorescent lighting fixture. 30cm 30cm7.5cm7.5cmT8-lamp BallastLouver Louver T8-lamp  In order to control color temperature and illuminance levels, fluorescent lamps that had three correlated color temperatures (CCTs) were used for the fluorescent fixtures. A spectrometer was used to measure the spectral distributions and color temperatures of the fluorescent lamps. The CIE chromaticity coordinates and color temperature are summarized in Table 1. The color temperatures   Energies   2014 , 7   4120 were 2,999 K, 3,995 K and 6,481 K according to the values on the CIE chromaticity coordinates [18]. In practice, the color temperature measurements were categorized as 3,000 K, 4,000 K, and 6,500 K for lighting design. Table 1.  Measured CIE Chromaticity coordinates and color temperature. Properties Fluorescent Lamp A B C CIE  x 0.4369 0.3803 0.313 Chromaticity Coordinate  y 0.4041 0.3754 0.3317 Color Temperature [K] 2,999 3,995 6,481 For each color temperature condition, the target illuminance levels on desktop surface were kept equally using the rotary dimming controller. The illuminance sensor positioned at the desktop detected illuminance and controlling setting was fixed when the target illuminance was met. For each condition, the light output from lamps was not measured, but the target illuminance level was used as an index for a final setting of lighting conditions. The example of spectral distributions of the lamps is shown in Figure 3. Longer wavelengths that generate reddish colors were stronger in the spectrum of the 3,000 K lamp. For the 6,500 K lamp, the effects of reddish color became significantly weaker, but the short and medium wavelengths that generate bluish and green colors became stronger. Six different lighting conditions were generated by the combination of two illuminance levels and three color temperatures. The six cases were 500 lx with 3,000 K (Case 1), 750 lx with 3,000 K (Case 2), 500 lx with 4,000 K (Case 3), 750 lx with 4,000 K (Case 4), 500 lx with 6,500 K (Case 5), and 750 lx with 6,500 K (Case 6). 3.2. Questionnaires, Subjects, and Pre-Instruction The questionnaires used for visual annoyance tests included three categories: general questions, questions regarding visual annoyance, and questions regarding the perceptions of visual stimulation and mood. The general questions included questions about age, gender, color blindness, and preference and sensitivity to light. The questions used to assess visual annoyance included visual responses such as annoyance with tasks, visual satisfaction with color of light, and visual distraction. The questions used to assess the visual perceptions of stimulation factors included visual responses, such as sensation of glare, and visual stimulation. Seven questions regarding mood according to visual perception were also included. The survey questions and the voting scale used for the annoyance test are shown in Tables 2 and 3. Twenty undergraduate students were recruited for the tests. All subjects had normal vision and were not color blind, and all were familiar with office tasks such as reading and documentation using computers and paper. Pre-instruction regarding the tests were conducted in a room adjacent to the test space before the main annoyance test began. Then, the subjects were asked to answer the general questions. While the subjects followed instructions and answered the general questions, they had enough time to adapt their eyes to photopic vision conditions, which describe the indoor lighting environment used in this study.
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