1. Introduction
Lighting is a fundamental component of school classrooms. The parameters affected by lighting in a classroom include the optimal visibility of all information to the students, the mood or behaviour of the students, and the learning ability and performance of students. Despite its vitality, there have been few empirical studies on the effects of lighting on the academic performance or well-being of school students [
1], and not many studies on more specific lighting factors such as correlated colour temperature.
A significant amount of research has focused on the effects of lighting in the work environment. Thus, it can be assumed that these effects already identified in the work environment may also apply to the school setting, and especially to university classrooms. The literature review in this study is based primarily on research conducted in workplace settings.
The two most important characteristics of light that must be considered regarding human perception are correlated colour temperature (CCT) and illuminance level [
2]. The effects of illuminance on human perception have been thoroughly investigated, and many governments have established their own recommendations or standards for illuminance levels [
3,
4,
5]. However, the effects of CCT on human perception and learning performance remain less understood compared to the effects of illuminance level. Thus, it is critical to investigate students’ subjective lighting preference in an actual classroom, so as to enable an increase in their learning performance and to enhance their mood to facilitate further learning.
Kruithof [
6] conducted a study on human perception of combinations of illuminance and CCT and found that people prefer high colour temperature sources at high illuminance levels and low colour temperature sources at low illuminance levels. Unfortunately, his work has not been extensively reported. What is apparent from the details that are available is that different light sources such as tungsten, daylight, and tubular fluorescent were used to produce the different CCTs. Thus, the light distribution and the colour of the light varied with the CCT. As both spectral distribution and colour temperature are known to affect human perception, the applicability of Kruithof’s curve may be limited [
7]. Comfortable indoor lighting conditions have been developed using lamps that are presently available [
8]. The effects of CCT on human sensation, perception, and cognitive performance have been studied and the results are still unclear.
Reported effects of CCT on lighting sensation and perception are controversial. Brightness is the sensation obtained by the eyes, and comfort or satisfaction is the process of perception by which the brain selects, organizes, and interprets brightness. These two concepts should be carefully considered.
Multiple studies reveal that rooms illuminated with higher CCT lamps appear brighter than rooms illuminated with lower CCT lamps, assuming other characteristics like illuminance and luminance distribution are held constant [
9,
10,
11,
12]. Akashi and Boyce [
13] found that the lamp with CCT 6500 K was the most effective in increasing the brightness in an office setting, especially compared to lamps with CCT 3500 K and 5000 K. Wei et al. [
14] used a 2 x 2 factorial design in their study, which comprised two levels of CCT (3500 and 5000 K) and lumen output (2330 and 3000 lm) with 26 participants. The luminous conditions at 5000 K were rated to be brighter than those at 3500 K. However, the increase in spatial brightness resulted in lower satisfaction and a decrease in visual comfort. Baniya et al. [
15] examined nine configurations combining three illuminance levels (300, 500, and 750 lx) and three CCTs (3000, 4000, 5000 K) with 53 participants belonging to different ethnicities. They found that the impression of brightness increased with a higher CCT at 500 and 750 lx. They further reported that the European group preferred a lighter environment at 4000 K for office lighting, whereas Asian and African groups preferred between 4000 and 5000 K depending upon illuminance levels. Yu and Akita [
16] also examined the effect of illuminance (150 and 300 lx) and CCT (2800, 5000, and 6700 K) with 18 participants in a capsule hotel, where the illuminance levels were lower than the two previous studies. They found higher CCT to be associated with higher spatial brightness, and lower CCT with a higher sense of security, restfulness, and positive feelings. Higher CCT was associated with higher brightness and lower CCT was associated with higher satisfaction, visual comfort, restfulness, positive feelings, and self-reported productivity [
14,
15,
16]. Lower CCT was associated with a better mood at lower illuminance levels, while higher CCT was preferred at higher illuminance levels [
17]. CCT of 4000 K was found to be preferable as compared to CCT of 6500 K at 500 lx in the office lighting environment [
18]. Wang et al. [
19] concluded that CCT has a significant impact on the subjective comfort and preference of individuals. The preferable illuminance and CCT vary according to the activities of the users [
20,
21]. Kocaoğlu [
22] compared two different lighting settings, 4000 K and 6500 K, with 14 university students and concluded that 6500 K was better than 4000 K for sustained attention and mood.
On the other hand, other studies have reported that the CCT of lamps had no effect on the brightness of the room’s lighting [
7,
23,
24,
25]. Boyce and Cuttle [
7] also reported that once the participant has fully adapted to the conditions, the CCT of good colour-rendering lamps in the range of 2700 K to 6300 K had little effect on the participants’ impressions of the room lighting. Davis and Ginthner [
23] found that subjective ratings of preference were influenced only by light level and not by colour temperature. Fotios [
26] concluded that the one condition to avoid is low illuminance and that the variation in CCT (within the range of approximately 2500 to 6500 K) does not affect pleasing conditions and can be chosen by other criteria. Therefore, it was suggested that the Kruithof graph should show a single curve—a straight line.
There is a dearth of research on the effects of CCT on cognitive performance. Shamsul et al. [
2] found that a CCT of 3000 K was detrimental to the alertness level and typing performances in their study on 47 undergraduate students. Mental activity was considered to be more activated under lighting with higher CCT than lighting with lower CCT [
27,
28,
29]. Navvab [
30] found that word reading and letter acuity were significantly better at CCT 6500 K compared to 3500 K in their study on 101 young adults. Boyce et al. [
31] found that visual task performance was better at CCT 6500 K compared to CCT 3000 K at 500 lx. Yamagishi et al. [
32] examined the effects of CCT 2500 K, 5000 K, and 8200 K at 470 lx on the numerical verification performance of 12 elderly people. They found that the LED lighting with 5000 K and 8200 K was better for visual performance. In the recent decade, evidence which supports the effects of CCT on cognitive performance has been reported by Huang et al. [
33], who found significantly better focused and sustained attention levels at 4300 K among CCT 2700 K, 4300 K, and 6500 K at 500 lx in a study conducted with 210 undergraduate students. Luo et al. [
34] found that the work performance of a youth group was the best at CCT 4000 K as compared to 3000 K and 5000 K at 500 lx. A recent classroom field study by Pulay et al. [
35] concluded that the CCT of 4100 K lighting was better for the on task behaviours of students rather than the CCT of 3000 K lighting.
However, other studies did not find any significant effects of CCT on cognitive performance. Boray et al. [
36] examined three lighting conditions, 3000, 4150, and 5000 K fluorescent spectra at 500 lx illuminance projected on a table. They found no significant differences in simple verbal and quantitative tasks in the three CCT conditions involving 117 university students. Davis and Garza [
37] compared two task lightings with 2700 and 4100 K (1290, 323, and 54 lx) and no differences on cognitive performance were found in their sample of 17 elderly people. Ru et al. [
25] concluded that CCT did not have statistical significance regarding subjective alertness and task performance across various cognitive domains, but it did affect the participants’ negative mood.
The purpose of this study was to investigate the optimal CCT of LED lighting for university classrooms considering the students’ sensation, perception and cognitive performance. Sixty university students were exposed to the six lighting configurations (CCT: 3000 K, 4000 K, and 5700 K; illuminance: 650 lx and 1050 lx). The lighting in most school classrooms in Korea has a high colour temperature. Thus, it is necessary to determine whether the typically installed high CCT level of lighting compared to an optimal CCT of lighting influences student sensation, perception, and cognitive performance in a real university classroom.
5. Conclusions
The effects of a correlated colour temperature of lighting on the brightness sensation, lighting perception, and working memory performance of students were investigated under moderate steady-state luminous conditions.
Brightness sensation was found to be affected by CCT. An increase in CCT led to an increase in brightness sensation; however, increased CCT did not linearly increase lighting comfort. Among three CCT levels, 3000 K, 4000 K, and 5700 K, the CCT of 4000 K was considered as the optimum level for lighting comfort in the educational settings used in this study. However, in comparison to comfort, higher levels of perceptual properties, satisfaction, and acceptance, were not affected by CCT from 3000 K to 5700 K. The scores on the working memory test were significantly affected by CCT and the illuminance level only for men. The effects of gender appeared in glare sensation and the working memory test. Women were sensitive to glare sensation and had a lower mean score in the working memory test than men. However, these phenomena have not been analysed and interpreted at this stage of the study.
The optimal CCT is more beneficial than increased illuminance in moderate ambient indoor lighting, as it provides better lighting comfort. Further research can look at the long-term effects of CCT on lighting perception depending on brain processing levels and diverse and in-depth cognitive performance, with more precise measurements at the human eye level.