Effects of General and Task Lighting on Subjective Perception and Work Performance: Empirical Study in an Office

. Lighting accounts for a large proportion of building energy use. Task lighting is effective in saving lighting energy consumption and found to improve productivity in factories, but its effects in offices remain unknown. This study aims to investigate the effects of general and task lighting on office occupants’ satisfaction, alertness, mood, and performance in simple and complex tests. A within-subject design involving 2 lighting condition (100% general lighting vs. 70% general + 30% task lighting) × 2 task type (paper-based vs. computer tasks) was adopted. The work-plane illuminance and the equivalent melanopic illuminance were controlled at the same level in two lighting conditions. The lighting power was reduced by 16.7% when introducing task lighting. 28 subjects participated in this empirical study. The results showed that different lighting conditions had no significant impact on alertness. Introducing task lighting would suppress positive mood, but improve work performance. The respond speed in simple tasks was significantly improved by 4.3%-8.5% and the correct rates in complex assessments increased by 6.2%. These findings highlight that the combination of general and task lighting reduces power consumption and benefits work performance. However, its suppression on the positive mood also needs to be considered in the lighting design.


Introduction
Buildings are responsible for 28% of global energyrelated CO2 emissions [1] and lighting accounts for a large proportion of building energy consumption, approximately one-third of electricity in commercial buildings in the US [2], and 20%-40% in large office buildings in China [3]. Task lighting is effective in saving lighting energy consumption as it efficiently lights up the working area. Even for the dimmable general lighting systems, adding task lighting could bring an extra 10% energy saving without compromising visual comfort [4]. In the mean time, lighting has impacts on occupants' satisfactory, mood, and work efficiency [5,6]. Setting task lighting luminaires has been proven to increase the productivity by 4.5% in a factory [7], improve the overall assessment of the light environment [8], and improve the performance in some tasks [9] in offices. However, there is a limit to the percentage of task lighting, as too bright task lighting may cause glare and uncomfortable contrast for visual work. A field study found the task/general ratio between 20/80 and 40/60 is appropriate for visual appearance [10]. Although these studies have investigated some effects of task lighting, the impacts of introducing task lighting on different work types and task difficulty remain unknown. Paper-based work used to be the common type of tasks, but visual display terminals such as computers are * Corresponding author: linbr@tsinghua.edu.cn ubiquitous nowadays. In addition, different work requires distinct abilities, such as arithmetic, memory and reasoning. Therefore, this study conducted an experimental design involving two lighting conditions and two task types (paper-based vs. computer tasks) to explore the effects of the lighting conditions on subjective perception and performance in different tasks. The lighting conditions included 100% general lighting and 70% general + 30% task lighting as suggested in previous study [10].

Lighting conditions
The experiment was conducted in an office with two kinds of luminaires, panel lamps for general lighting and downlight for task lighting (Fig. 1). All the windows were covered by blackout blinds to avoid daylight and the interior lighting was provided entirely by luminaires.
a Layout of the office.
b Photo of the luminaires. Two lighting conditions were set up as shown in Fig. 2. In one condition, the illuminances were completely provided by the panel lamps as general lighting. In the other condition, 70% of the illuminances were provided by the panel lamps and 30% by the downlights as task lighting with a 60 beam angle. To focus on the impact of general and task lighting, the work-plane illuminance, the eye-level illuminance and the equivalent melanopic lux (EML) were controlled at the same level in two lighting conditions. Fig. 3 demonstrated the spectral power distributions (SPDs) of the panel lamp and downlight. The illuminances and EML of two lighting conditions were calculated from the integration of the SPDs and the sensitivity curves, and were illustrated in Table 1.   In addition, the electrical consumption of the luminaires were measured by a power meter. In general lighting condition, the lighting power was 23.3 W for a panel lamp. In general and task lighting condition, the lighting power was 19.4 W for a panel lamp and two downlights. The lighting energy consumption reduced by 16.7% after introducing task lighting with consistent illuminance and EML settings, as the task lighting was more efficient in lighting up the personnel area.

Measures
The effects of the lighting conditions were assessed by subjective questionnaires as well as objective task evaluations. In the questionnaire, participants were asked to evaluate the light environment, including their satisfactions with the light environment and light distribution, a standard Karolinska Sleepiness Scale (KSS) vote, and a standard Positive Affect and Negative Affect Scale (PANAS) vote. The satisfactions were rated by seven-point scales from 1 (extremely unsatisfied) to 7 (extremely satisfied). The KSS was a ten-point scale rated from 1 (extremely alert) to 10 (extremely sleepy and cannot keep awake). The PANAS included ten types of positive and ten types of negative moods, and the participants should vote from 1 (little or not at all) to 5 (very much). As for the task evaluations, tests of varying difficulty were chosen to fully explore the impact of lighting conditions on work performance. Two tests, verbal memory and sum of numbers were the representatives of simple work. In verbal memory test, a combination of six letters in the alphabet was randomly generated. After 2 s, the letters disappeared, and the participants needed to fill in the six letters from memory. The sum of numbers evaluated participants' arithmetic performance.
The participants had to add three single-digit random numbers from 1 to 9. Apart from the simple tests, complex tests were also set up to assess the participants' performance in logical reasoning, reading comprehension, etc. One test contains four arithmetic questions, four logic questions, four analogy questions, four reading questions and five analysis questions. The tests were randomly generated from a pool of questions with similar historical correctness. The personnel performance was evaluated by test scores within 30 minutes.

Participants and procedure
All 28 participants were recruited from Tsinghua University, Beijing, China, of which 13 were males and 15 were females. Their ages ranged from 19 to 30 years (mean=23 years, SD=2 years). Except for two participant who dropped out after completing two session due to a personal schedule, all other participants followed the complete experimental protocol. In addition to the two lighting conditions mentioned in section 2.1, another explored variable was the work types, respectively the paper-based work and computer work. Therefore, each participant joined in four sessions (two lighting conditions two work types) in random order as shown in Fig. 4. In each session, participants completed questionnaires and simple tasks at the beginning and the end. Another questionnaire was added in the middle. The complex tests were completed in the last 0.5 h, and in the rest time, participants did reading work either on paper or on computer.

Data analysis
SPSS version 25 was used for all analyses. General linear model (GLM) was adopted with two withinsubject factors, respectively lighting conditions (general lighting vs. general + task lighting) and work types (paper-based task vs. computer task). All of the above measures were analysed separately. Statistical significance was indicated by p<0.05. Table 2 demonstrates the estimated marginal means, the standard errors, and the GLM analysis results of subjective assessment under different lighting conditions. No significant effects were observed (p > .05) for the satisfaction with light environment, the KSS votes, and the negative mood votes, but the lighting conditions significantly affected the light distribution satisfaction and positive mood. As for the satisfaction votes, Fig. 5 illustrates the changes in light distribution satisfaction after introducing task lighting. The dots and numbers above represent the percentage of increased satisfaction, while the dots and numbers below indicate the percentage of decreased satisfaction. It can be seen that a larger proportion of participants (approximately 50% in the first two votes) felt less satisfied with the light distribution under general and task lighting. This may be attributed to the reduced light uniformity as the downlights for task lighting only illuminated the personnel area. As for the positive mood vote, Fig. 6 shows the estimated marginal means and standard errors of ten types of positive mood, which all weaken in the general and task lighting condition. Equations should be centred and should be numbered with the number on the righthand side. This indicates the task lighting causes suppression of personnel positive feelings. Analysis results of the other influencing factor, the work type, are presented in Table 3. No significant effects were observed (p > .05) for all the subjective assessments. There is a weak correlation between the work type and light environment satisfaction, and between the work type and the KSS vote. Participants felt more satisfied (F = 2.096, p = .066) and less sleepy (F = 2.059, p = .070) when conducting computer work. Therefore, it is more important to promote the feelings and performance through suitable environment with paper-based work.

Performance of simple tests
For the two simple tests of verbal memory and sum of numbers, the lighting conditions had no significant effects on correctness, but significantly affected the completed time (p < .001), as shown in Table 4.

Performance of complex tests
The complex tests further explored the influences on logical reasoning, reading comprehension, etc. The test time was set at 0.5 h, and the performance was determined by the total score of the correct answers.   8 presents the score distributions. After introducing the task lighting, the number of participants scoring 80 and above was increased. As a result, the average score increased by 6.2% as shown in Fig. 9. The combination of general and task lighting is conductive to better performance of complex tasks.

Conclusions
An empirical study was conducted in an office under two lighting conditions (100% general lighting vs. 70% general lighting + 30% Task lighting) and with two work types (paper-based work vs. computer work), to investigate their effects on subjective perception and work performance. After analysing the experimental results, the following conclusions were drawn.
(1) The combination of general and task lighting reduces the satisfaction with light distribution and the positive mood. Although the satisfaction with the overall light environment was not significantly different, nearly 50% of the participants felt less satisfied with the light distribution in the combination of general and task lighting. Besides, the votes of all kinds of positive mood were lower. (2) The combination of general and task lighting improves the work performance in both simple and complex tests, including a 4.3% to 8.5% improvement in completed time in memory and computing tasks, and a 6.2% score improvement in comprehensive performance tests. The most pronounced improvement (8.5%) was seen in verbal memory performance in paper-based tasks. (3) The work types rarely have significant effects on the experimental results, except in the subjective assessment, participants felt more satisfied and less sleepy when conducting computer work. Therefore, introducing task lighting significantly benefits work performance while saving the lighting energy consumption. However, its suppression on the positive mood also needs to be considered in the lighting design. Future research could explore different combinations of general and task lighting, to achieve the optimum among energy saving, work performance, and subjective satisfaction.