The Effect of Illuminance and Color Temperature of LED Lighting on Occupants’ Perception and HRV

1.1. Research background and objectives

With the developments in LED lighting recently, shift from prior lighting towards LED lighting is being done fast. It is because LED lighting has lower power consumption by 20%~70% than fluorescent lamp with life span 5 times longer (fifty thousand hours), and it has also low possibility to be damaged compared to light bulb which uses glass thus lowers much cost in maintenance management such as repair or replacement. With this advancement in lighting technology, traditional lighting market is rapidly changed into LED lighting market helping us expect that 30% of current lighting would be replaced with LED lighting within 2015. One advantage of LED lighting is that it can realize the light in widespread wavelength that includes visible rays freely. This advantage means it can be utilized as emotional or well-being lighting that affects invisible or visible aspects of human by using the light in various ways not being limited to glowing the light simply.

Smart LED lighting was created grafting LED lighting and object Internet technology that links and shares sensor information of objects connected through Internet with nearby devices and people. Unlike prior lighting always fixed, Smart LED lighting enables control over color, brightness users demand possessing variability and availability towards new environment any time.

We perform various tasks indoor with the objectives of various visual works such as rest, talk, reading, watching TV, computer work and the work done continuously and needed whatsoever is rest. Resting is a very important work because it relaxes the body and the mind of occupants. Since this work is done in various indoor setting such as houses and offices, flexible lighting environment design is needed to satisfy occupants along with various works.

This research composed lighting environment condition of resting space combining illuminance and color temperature using LED lighting. We would like to analyze the changes in psychological state and lighting quality that occupants perceive according to the condition of lighting environment through survey then present design conditions for lighting environment of resting space for physical and psychological health of occupants by evaluating physiological reaction through measurements of HRV.

1.2. Literature review on effect of illuminance and color temperature of LED lighting on occupants

As a result of literature review of Yoon, Kyu-Hyun(2014), Kim, Su-Yeon(2013), Park, Hun-Su and 3 others(2012), No, Jung-Rim(2011), Hwang, Tae-Yeon and 2 others(2011), Ji, Sun-Duk and 3 others(2006) as prior researches about the effect of illuminance and color temperature of LED lighting on psychology of occupants, illuminance range suitable for resting was proposed to be between 85 lx and 300 lx and color temperature was from 3000 K to 5000 K. Preference between 150 lx and 200 lx was shown most positively and warm lighting with low color temperature (below 2700 K) was evaluated to give comfort. Advanced researches were all evaluated through survey method and most of items in the survey were evaluated vocabulary such as comfort, preferred, pleasant with 5 to 7 points criterion to evaluate comfort, preferred, pleasantness. This research evaluated changes in perception of lighting quality through total 11 adjective vocabularies with 5 adjective vocabulary about perception on lighting intensity, 4 vocabularies on comfort perception, 1 on pleasantness and intends to evaluate in-depth psychological change rather than simple lower measure vocabulary test applied in advanced research through comparative evaluation survey of mood.

As a result of literature review of Jun, Woo-Suk and 3 others (2007) as advanced researches on effect of illuminance and color temperature of LED lighting on the physiology of occupants, HRV that measured changes in automatic nervous system showed low activation in parasymsympathetic nerve in warm lighting and high one in cool lighting. This research measured HRV according to the color change of lighting, not color temperature and illuminance, and expects different result from LED lighting that intends to evaluate in this result using halogen color filter, not LED as lighting as well. Also, it evaluates changes in automatic nervous system depending on the changes of illuminance and color temperature and in-depth physiological change on occupants more than advanced researches did by analyzing changes in mean and standard deviation in pulse and ratio of symsympathetic and parasymsympathetic nerve not to mention parasymsympathetic activation and symsympathetic activation.

2.1. Organization of laboratory for lighting environment evaluation

Psychological, physiological reaction laboratory of K university was organized into resting place as laboratory to propose resting place design conditions for physical and psychological health by evaluating changes in automatic nervous system activation, mood, perception of indoor lighting of occupants regarding indoor lighting environment organized of LED lighting. Laboratory is composed of 4.5 m (W) x 6.2 m (L) x 2.5 m (H) like Figure 3 and 4 and just like Figure 2, every indoor opening is closed with black curtain and blind to block the outside light and nearby environment is controlled so that no foreign noise can be heard.

Like Figure 1, LED lighting is an overall lighting method that lights the whole room with the ceiling light and the subject did HRV measurement and survey located below the light. To realize resting place, sofa, table, bookshelves are allocated with indoor all painted with white paint. There are books in various colors inside the bookshelves and the height of the ceiling is 2.7 m. The size of the table is 1.5 m (L) x 0.5 m (W).

Fig. 1.

Smart LED lamp and lighting fixture

Fig. 2.

Laboratory for Psychological reaction assessment

Fig. 3.

section of psychological reaction laboratory

Fig. 4.

plan of psychological reaction laboratory

Lighting device is composed of 12 HUE, Smart LED lighting of Phillips company on 12 lighting devices in the size of 0.95 m (L) x 0.7 m (W). Smart LED lighting HUE can express more than 16,000,000 colors. Moreover, it can be on/off automatically at the time set by users and enables free direction of brightness, color that lighting users want. To organize lighting environment conditions, application that could save illuminance and color temperature case was used.

2.2. Experiment condition and evaluation method

Conditions of laboratory lighting environment are realized with illuminance and color temperature and they were contemplated on illuminance conditions presented in JIS of Japan, IESNA lighting Handbook of USA, KS of South Korea like <Table 1-3>. Illuminance standard by room where occupants mainly rest was examined and living room in house was between 30 lx to 60 lx, bedroom was between 15 lx to 50 lx, office lounge was from 60 lx to 100 lx, hospital ward was between 50 lx to 300 lx. The standard of space where resting is done is between 15 lx to 300 lx, so illuminance conditions were classified into 3, 50 lx. 150 lx, 300 lx within this range.

Table 1.

KS A 3011 Illuminance standard

Table 2.

IESNA Lighting Handbook 10th Edition Illuminance standard

Table 3.

JIS General rules of recommended lighting levels

Kim, Su-Yeon(2013) examined physical quantity of lighting in 18 model houses in Seoul and Kyunggi region and as a result, there was no trend by space type in case of color temperature but they could be divided into three groups. below 3000K, from 3000 K to 4000 K, more than 4000 K. No place used only constant color temperature by space type and it can be seen that various color temperature is used in the space because there is no recommendation standard for color temperature according to the space. Thus, among three groups that are below 3000 K, from 3000 K to 4000 K, more than 4000 K used the most, 2000 K as warm lighting, 3800 K as medium lighting, 5600 K as cool lighting were selected. <Table 4> shows 9 lighting environment conditions putting illuminance (50 lx, 150 lx, 300 lx) and color temperature (2000 K, 3800 K, 5600 K) together and experiment condition was organized in random order so that subject cannot expect next order.

Table 4.

Indoor environment condition

Evaluation method is divided into subjective evaluation and physiological measurement according to the conditions of lighting environment. Subjective evaluation is done through mood survey and perception survey regarding indoor lighting quality, physiological measurement is done through HRV measurement.

3.1. Perception survey on indoor lighting quality

Perception survey of indoor lighting quality is evaluated with 7-point criterion and is a meaningful criterion about 10 vocabularies. This vocabularies were re-organized so that subjects can more easily understand after translating English vocabularies used in the research of Kuller & Wetterberg (1993,1996), prior researchers, into Korean. M.Johansson et al.(2013) divided 10 vocabularies into 3 groups, perception on lighting intensity, comfort perception, pleasantness perception.

Profile of mood states

Profile of mood states is developed by McNair DM et al.(1992). This test can be divided into 6 subgroups that are nervousness-anxiety, depression - disappointment, fury –aggression, vitality-vigor, fatigue-lethargy, confusion-embarrassment. Evaluation items are composed of total 65 items that are 9 nervousness items, 15 depression items, 12 fury items, 8 vitality items, 7 fatigue items, 7 confusion items, 7 pile items. To convey more exact meaning of English evaluation vocabularies, they were translated into Korean and evaluation paper was re-organized. Each item was made to be evaluated in self-report means with 5-point criterion from not at all(0) to Yes, very much(4).

3.2. HRV measurement

Physiological measurement measured HRV to evaluate changes in activation of automatic nervous system of subjects depending on the changes in lighting environment. HRV was measured using uBioMacpa pulse measuring device of bio sense creative company. This device measures pulse in the way that changes in aorta pressure is transferred to periphery arteriole upon supply of blood towards the whole body through aorta whenever heart contracts. Pulse wave expresses changes in conveyed vascular dimension in wavy pattern. This device measures pulse change with noninvasive measures putting index finger of subject to this device and analyzes standard deviation in pulse, average deviation in pulse, ratio of sympathetic and parasympathetic, activation of parasympathetic, sympathetic activation from software of relevant device.

Sympathetic activation shows high figure in nervous and excited state and parasympathetic activation is shown high in relaxed state such as enough rest and low in angry, worried, scared state. Ratio or parasympathetic and sympathetic show the ratio of activation of sympathetic and parasympathetic showing immunity index. Standard deviation in pulse shows adaptability of body and is better the higher, showing adaptability towards stress. Average deviation in pulse is an index that shows stability of heart function and is shown low in angry, worried, scared state.

3.3. Experiment process and subject composition

The experiment lasted for 5 weeks from Oct, 14, 2014 to Nov, 21, 2014. Subjects were undergraduates and graduates with various majors of K university at the age of 23.6 on average, subjective measurement was done targeting 50 subjects (50 males, 50 females) physiological measurement was done targeting 36 subjects who answered there is low physical fatigue thanks to enough rest and sleep on the measurement day among 50 subjects who participated in subjective measurement (18 males, 18 females).

Subjective measurement was done when subjects were made to stare the front sitting in the middle of the laboratory. After adjusting to the dark room turning all the indoor lightings off for 3 minutes before applying each lighting condition, the condition was applied and survey was done under it after adjusting to lighting condition. The experiment was repeated 9 times, spending approximately 80 minutes.

Physiological measurement was done supposing subjects are doing resting work in comfortable state sitting on the sofa in the laboratory and the measurement was done when the subject was in stable state after adjusting to the room resting in the laboratory for about 10 minutes. In compliance state to dark room for 2 and a half minutes, pulse change was measured then repeated after adjusting to the lighting condition for 2 and a half minutes again thus to statistically analyze the increase in automatic nervous system when dark room is changed into lighting condition. 5 minutes was taken for 1 time measurement, so about 45 minutes was consumed in total for total 9 sessions.

3.4. Data analysis method

Using SPSS Ver.21, dual variance analysis of effect test of interaction (Two-way ANOVA) was done to examine the changes in variables as a result of following the level change in illuminance and color temperature to independent treatment variables and since there are two treatment variables, they were applied to examine the treatment effect. To test if the effect of changes in illuminance on result variables is different according to the level of color temperature, interaction effect was tested. We could check the significance in significance probability 95% and post-mortem - dual analysis method used Scheffe law.

Effect test between entity induced effect size through partial eta squared. In variance analysis, effect size is a standard value that shows average difference between groups so it is considered that the effect size is small if partial eta squared value is lower than 0.01 for it is the value that determines the effect size in dual analysis and middle if 0.06, big if higher than 0.14. We confirmed average difference of dependent variables through this partial eta squared value.

4.1. Quality evaluation of indoor lighting

<Table 5> is the result of statistic analysis regarding perception change in indoor lighting by illuminance. Perception of lighting intensity was positive in order of 300 lx> 150 lx> 50 lx and comfort perception was positive as well in order of 150 lx> 300 lx> 50 lx , pleasantness perception was positive too in order of 300 lx> 150 lx> 50 lx.

Table 5.

Perceived indoor lighting quality Two-way ANOVA analysis result

When comparing effect size through partial eta squared value among items that showed meaningful difference according to the illuminance, partial eta squared value was shown big, more than 0.14, in perception of lighting intensity 'clear-soft', 'strong-weak', 'bright-dark', 'clear-dim', in comfort perception 'soft-rough', 'not dazzling-dazzling', and in pleasantness perception 'pleasant-unpleasant'. It is decided that 7 items above show sensitive reaction according to relative illuminance.

In perception change of indoor lighting quality according to the color temperature, perception of lighting intensity was shown the most positive in 5600K and comfort perception in 3800K, pleasantness perception in 5600K. It was shown that perception on lighting intensity and pleasantness are more positive as s illuminance and color temperature become higher, and that people perceive the most comfortable state in illuminance 150 lx and color temperature 3800K.

When comparing effect size through partial eta squared value among items that showed meaningful difference according to color temperature, partial eta squared value was shown higher than 0.14 in 'clear-soft' of perception on lighting intensity and 'soft-rough' , 'warm-cold' of comfort perception. 3 items above are determined to have sensitive reaction according to the relative color temperature. It is judged that 'clear-soft' and 'soft-rough' items are the ones that react the most sensitively according to the illuminance and color temperature change in perception of lighting quality for partial eta squared value of those in illuminance and color temperature were all shown high.

4.2. Result from profile of mood states

<Table 6> is the result of statistical analysis regarding mood change by illuminance. Mood change by illuminance showed meaningful difference in every 6 items that are 'nervousness', 'depression', 'fury', 'vitality', 'fatigue', 'confusion' and also positive as they became lower which are negative feelings as illuminance increases. But in case of vitality which is a positive mood, it was shown to decrease as illuminance increases, rather. Mood change by color temperature showed meaningful difference only in vitality and confusion but that is judged not to have much effect since it was shown really small compared to changes by illuminance.

Table 6.

Mood stats Two-way ANOVA analysis result

Among items that showed statistically meaningful difference by illuminance and color temperature, partial eta squared value in 'depression', 'vitality', 'fatigue', 'confusion' items were shown higher than 0.14 with big effect size thus showing big average difference by illuminance in above items. Thus, 'vitality', 'fatigue', 'confusion' emotion rather than 'nervousness', 'fury' are judged to be more sensitive by illuminance. Items that showed meaningful difference by color temperature all showed small effect size.

Correlation between illuminance and color temperature about mood showed meaningful difference in 'vitality', 'fatigue', 'confusion' state and positive mood was shown as illuminance level by each color temperature gets higher. Thus, mood was shown more positive as illuminance level gets higher (300 lx> 150 lx>50 lx) and there was no big difference in mood by color temperature.