Comprehensive Comparison of Natural Light and Electric Light Sources | Must-Read for Lighting Professionals

Knoop, M., Stefani, O., Bueno, B., et al. (2020). Daylight: What makes the difference? Lighting Research & Technology, 52(3), 423-442.

Daylight is the holistic combination of the luminous characteristics of sunlight from direct solar radiation and skylight from diffuse solar radiation (Figure 1). Unlike electric lighting, daylight is highly dynamic, changing within and across days, throughout the year, and with weather conditions in intensity, colour, diffuseness and direction. Daylighting refers to the illumination of indoor spaces by daylight delivered through openings in the building skin.

This article arose from discussions between the authors at a seminar held in Berlin in June 2018 and is not intended to be a comprehensive review paper. The purpose of the seminar was to reflect an interdisciplinary discussion on the various scientific, technical and creative aspects of the differences between daylight and electric light. As a first step, this overview should provide a basis for further, more specific discussion and research.

Numerous survey-based studies have shown that daylight is preferred to electric lighting in most settings[1-5]. Boyce et al [6] state that ‘There is no doubt that people prefer daylight over electric lighting as their primary source of illumination’ and provide an overview of literature which shows that high percentages of survey respondents prefer to work by daylight. Most studies were performed at latitudes around 50°N [2,3,7]; one study in the tropics indicates that the majority of occupants prefer to work under daylight as well [8].

Many reviews document the importance of daylight for health, well-being, and sustainability and the consequences for architecture [5-13]. Veitch and Galasiu [11] summarise: 'The reviews [5,14] concluded that windows and daylighting are desired by most employees and that they are contributors to health and well-being’. Here we show that the specific characteristics and related benefits of daylight as summarised in Table 1 that produce this human reaction go beyond subjective preferences for natural light, as discussed by Haans [15].

Many review papers document the importance of daylight for health, well-being, and sustainability, and its impact on architecture. Veitch and Galasiu summarize: “Most employees in office spaces desire windows and natural lighting, which contribute positively to health and well-being.” This paper further points out that the specific characteristics of daylight and their benefits (summarized in Table 1) go beyond people's subjective preferences for natural light, as discussed by Haans.

Table 1 Characteristics of daylight and electric lighting

Underlying the human preference for daylight are experiences that transcend immediate physical stimuli, often orchestrated by their nature to be interwoven with context-related knowledge. The sun has been worshiped in many cultures, with sunlight and the qualities of shadows and darkness being generally felt to be a source of spiritual and aesthetic experience as well as of health and well-being.

The underlying reason for people's preference for daylight is experiences that transcend immediate physical stimuli, which are often intertwined with context-related knowledge. The sun is worshiped in many cultures, and sunlight and the qualities of shadows and darkness are generally regarded as sources of spiritual and aesthetic experience, as well as of health and well-being.

Unlike daylight, electric lighting is a controllable man-made light source associated with advances in science and technology that is easier both to study and to engineer to achieve specific outcomes. In contrast, daylight as a natural source is more difficult to control and the daily, seasonal and annual dynamics of daylight produce different outcomes in different locations, additionally modified by weather conditions. Due to these geographical differences, appropriate daylight utilisation can vary from sun- and skylight exposure to complete exclusion of sunlight from buildings.

Unlike daylight, Electric light source is a controllable man-made light source, a product of science and technology, easier to be studied, designed, and engineered to achieve specific results. In contrast, Daylighting as a natural product is harder to control, and its daily, seasonal, and annual dynamic changes produce different results in different locations, additionally affected by weather conditions. Due to these geographical differences, appropriate Daylighting varies greatly: from exposure to sunlight and skylight, to completely blocking direct sunlight.

In addition,the use of daylight openings in the building envelope depends on the function of the indoor space, as well as occupants’ requirements for privacy, view, glare protection and solar heat gain management. Individuals also respond differently to daylight, as for example reviewed by Pierson et al [16]. A complex construct of individual, physiological, cultural, geographical and seasonal preferences and characteristics underlies the desire for daylight, and the subsequent human response, as well as the environmental and monetary benefits.

In addition, the use of daylight openings in the building envelope depends on the function of the indoor space, as well as the occupants' requirements for privacy, view, Glare protection, and solar heat gain management. Individual responses to Daylighting also vary, as reviewed by Pierson et al. The reasons behind people's desire for and response to Daylighting are complex: integrating individual, physiological, cultural, geographical, and seasonal preferences and characteristics, as well as environmental and economic benefits.

2. Visual Performance

Vision is the most developed sense in humans and, therefore, our species significantly relies on the provision of light of adequate quality. Visual performance, defined as the speed and accuracy of processing visual information, is influenced by lighting conditions [17]. Daylight is a very good light source to support visual performance. It is a flicker-free light source with a continuous spectral power distribution covering the full visible range (Figure 2).

Vision is the most developed sense in humans, so we humans significantly rely on light of appropriate quality. Visual performance is defined as the speed and accuracy of processing visual information, and it is influenced by lighting conditions. Natural light is an excellent light source that supports visual performance; it is a flicker-free light source with a continuous spectrum power distribution covering the entire visible spectrum range (Figure 2).

The high illuminances (Figure 3) enable discrimination of fine details supporting visual acuity. Glare must be controlled both for daylight and electric light. The spectral power distribution of daylight offers optimal colour rendering and allows better colour discrimination than most electric lighting, whilst the directionality of both daylight and electric light can produce shadows that enhance details for three-dimensional tasks.

High illuminance (Figure 3) supports visual acuity by enabling the discrimination of details. Glare from both natural light and electric light sources must be controlled. The spectral power distribution of natural light provides optimal colour rendering and offers better colour discrimination than most electric light sources; the directionality of both natural and electric light sources can produce shadows that enhance details, aiding three-dimensional visual tasks.

3. Good Eyesight

Lack of daylight exposure seems to be linked to developing myopia or short-sightedness. Myopia is the most common visual disorder affecting young people; it has reached epidemic levels in East Asia and is increasing elsewhere. Myopia is normally first diagnosed in school-age children. Recent studies have revived the idea that it is the environment in which children learn that determines whether or not they become short-sighted [18]. It seems that children who engage in outdoor activities have lower levels of myopia [19]. Thus, daylight exposure at levels significantly higher than those typically found indoors (Figure 3) may be important in preventing myopia.

Lack of natural light exposure seems to be related to developing myopia. Myopia is the most common visual disorder affecting young people; it has reached epidemic levels in East Asia and is increasing elsewhere. Myopia is usually first diagnosed in school-age children. Recent studies have revived the view that it is the physical learning environment that determines whether they become myopic (iLLab Note: For a long time, academia believed that myopia was caused by excessive eye use, with little relation to the environment.). Children with sufficient outdoor activities also have lower levels of myopia. Therefore, natural light exposure at illuminance levels significantly higher than typical indoor levels (Figure 3) may play an important role in preventing myopia.

The precise biological mechanisms through which being outdoors may protect children's eyesight are not yet fully understood. The hypotheses are that (i) bright light stimulates the release of the retinal neurotransmitter dopamine, which inhibits the axial growth of the eye that causes short-sightedness; (ii) since circadian rhythms in the eye affect ocular growth, disruption of such rhythms by low light levels has also been proposed as a development factor [20] and (iii) there is a geographical, seasonal, component, as both eye elongation and myopia progression increase as day-length shortens [21]. The complex protective effect of daylight may depend on many interlinked aspects including duration and timing of daylight exposure, wavelength and intensity.

The precise biological mechanisms for the protective effect of natural light on children's vision are not yet clear, but there are three related hypotheses: (i) Bright light can stimulate the release of the retinal neurotransmitter dopamine, thereby inhibiting the axial growth of the eye that leads to myopia; (ii) Since circadian rhythms in the eye affect eyeball growth, low light levels disrupting these rhythms are also considered a developmental factor; (iii) There are geographical and seasonal factors, as both axial elongation and myopia progression increase as daylight hours shorten. The complex protective effect of natural light may depend on many interrelated aspects, including the duration and timing of natural light exposure, as well as its wavelength and intensity.

Excessive near-work may also damage children's eyesight; even though evidence for this is inconsistent, a recent review of myopia prevention by Lagrèze and Schaeffel [22] reported that ‘A person with little exposure to daylight has a fivefold risk of developing myopia, which can rise as high as a 16-fold risk if that person also performs close-up work’.

Excessive near-work (iLLab Note: visual tasks) may also damage children's eyesight; although the relevant evidence is inconsistent, a recent review of myopia prevention by Lagrèze and Schaeffel reported: “People with little exposure to natural light have a five times higher risk of myopia than average, and if they also engage in close-up visual tasks, the risk can be as high as 16 times.”

The spectral component of daylight exposure (Figure 2) may affect visual colour performance. Reimchen [23] showed that colour deficiencies are more common in northern latitudes, where twilight occupies a more significant part of the day than at the equator, where colour deficiencies are very uncommon. A study of visual perception in individuals born below and above the Arctic Circle, in different seasons, indicated that a reduction of daylight and an increase of exposure to twilight and electric lighting during infancy changed colour sensitivity; participants born in autumn above the Arctic Circle showed the lowest overall colour performance [24].

The spectral component of natural light (Figure 2) may affect visual efficacy in terms of color. Reimchen's research indicates that color vision defects are more common in northern latitudes and rare in equatorial regions. Compared to equatorial regions, twilight occupies a longer portion of the day in northern latitudes. A study on visual perception in individuals born near the Arctic Circle in different seasons showed that reduced exposure to natural light and increased exposure to twilight and electric light sources during infancy alter color vision sensitivity; infants born in autumn and living outside the Arctic Circle had the lowest color vision efficacy.

4. Circadian Entrainment

Well-timed lighting can entrain the circadian system, which is important for positively affecting an individual's sleep quality, health, mood and cognitive abilities [25]. Daylight, due to its temporal variations in spectral power distribution and intensity (Figure 4), is the natural time cue (‘zeitgeber’) for synchronisation of the circadian system and the sleep–wake cycle. Dawn and dusk are important cues for entrainment with high light levels during the day followed by darkness at night being essential for optimal sleep.

Light input to the circadian system occurs through intrinsically photosensitive retinal ganglion cells (ipRGCs) particularly sensitive to the short-wavelength ‘blue’ component of light. Discovered in 2002 [26,27] these cells are connected to the circadian clock and other parts of the brain, affecting primarily non-visual functions [28]. To support circadian functionality, bright and short-wavelength light exposure during daytime is important together with avoidance of light during nighttime. A study in the Antarctic region showed better sleep quality of base personnel during the period of the year with daylight, with its prevalent higher light levels, compared to sleep quality during the polar winter with only electric lighting. When comparing electric lighting conditions, blue-enriched (17,000 K) light was more efficient than bright white (5000 K) in supporting good sleep–wake cycles [29]. In interiors, reduced exposure to sunlight during the day together with electric light exposure after sunset can delay timing of the circadian clock leading to difficulties falling asleep at night and problems getting up on time in the morning.

The effect of light on the circadian rhythm system is mainly achieved through intrinsically photosensitive retinal ganglion cells (ipRGCs), which are particularly sensitive to short-wavelength "blue light" components. These cells were discovered in 2002; they connect to the biological clock and other brain regions, primarily influencing non-visual functions. To maintain the circadian rhythm, bright light rich in short wavelengths is needed during the day, while light exposure should be avoided at night. A study conducted in Antarctica showed that base personnel had better sleep quality during periods with natural light (higher light levels) and poorer sleep quality during the polar night (only electric light sources). When comparing electric light source conditions, light rich in blue light (17,000 K) supported a good sleep-wake cycle more effectively than bright white light (5000 K). In indoor environments, reduced sunlight exposure during the day, combined with the use of electric light sources after sunset, may delay the phase of the circadian rhythm, leading to difficulty falling asleep at night and waking up in the morning.

Exclusive exposure to daylight synchronises the circadian rhythm system to solar time [30,31]. Roenneberg and Merrow [32] proposed to treat and prevent circadian misalignment by ‘strengthening light environments (more light during the day and less light during the night). This requires taking advantage of dynamic changes in spectral composition, and applying architectural solutions to get more daylight into buildings’. To artificially provide the high-amplitude temporal dynamics of daylight by means of electric lighting requires significant energy use. It is assumed that daylight is the best and appropriate light source for circadian entrainment, though conclusive research evidence for this is lacking.

Exposure to pure natural light can synchronize the circadian rhythm system with solar time. Roenneberg and Merrow proposed treating and preventing circadian rhythm disorders by “strengthening the light environment” (more light during the day, less light at night). This requires utilizing dynamic changes in spectrum composition and adopting architectural design solutions to introduce more natural light into buildings. Providing high-amplitude dynamics similar to natural light through artificial lighting requires high energy consumption. The default premise here is that natural light (iLLab: meaning all natural light including day and night periods, including Zhuge Liang and Zhuge An) is the perfect light source for circadian rhythm regulation, but there is a lack of clear research evidence.

Dawn and dusk signals are the most powerful zeitgebers, not requiring high intensity light but a pattern of diurnal change with sunrise and sunset. They depend on day of year and latitude. Simulation studies have shown a rapid phase advance with a single dawn pulse [33], and exposure to natural dawn and dusk immediately re-positions sleep to within the night [31]. Compared to static lighting, dynamic lighting simulating a natural sunrise through a change of colour temperature (from 1090 K to 2750 K) and illuminance at the eye (0–250 lx) resulted in better subjective mood and well-being [34], better cognitive performance [35] and could be a potential protector for cardiac vulnerability in the critical morning hours [36]. Dynamic lighting that included lower lighting conditions and colour temperatures in mornings and evenings resulted in significantly higher melatonin production 1 hour prior to bedtime compared to static light [37].

Dawn and dusk signals are the most powerful zeitgebers, requiring not high-intensity light but a pattern of change during the diurnal cycle with sunrise and sunset. These signals depend on the day of the year and geographic location. Simulation studies indicate that a single dawn pulse can rapidly advance the circadian rhythm phase; exposure to natural dawn and dusk can immediately reset the sleep cycle to nighttime. Compared to static lighting, dynamic lighting simulating a natural sunrise (through changes in color temperature from 1090-2750 K and illuminance at eye level from 0-250 lx) can improve subjective mood and well-being, enhance cognitive performance, and may protect against cardiac vulnerability during critical morning hours. Dynamic lighting providing low illuminance and low color temperature in the morning and evening can significantly increase melatonin secretion levels one hour before bedtime compared to static lighting.

Daylight outdoors intrinsically provides temporal dynamics. Thus, the simplest solution to getting enough circadian stimulus is to go outside. Nonetheless, people in the modern, industrialised, society spend up to 90% of their time indoors [38–41]. In buildings, the form and façade, as well as the choice of glazing material in the windows and shading system modify intensity, colour and distribution of daylight in the interior. Daylighting conditions available to the occupant of a room also depend on their distance from the window, the geometry of the room and surface reflectances. Depending on the daylighting design, indoor daylight can often provide an adequate stimulus and support to the circadian system, thus remaining as the usual light source for circadian support. Office workers with access to windows have reported better sleep quality than those without windows [42]. Sleep quality increases with higher daylight availability in summer [43], with the duration over a threshold of 1000 lx or 2500 lx at eye level being an indicator for better sleep quality [44].

Outdoor natural light can provide intrinsic temporal dynamic changes. Therefore, the simplest solution to obtain circadian light stimulus is to "go outside". However, in modern industrialized societies, people spend up to 90% of their time indoors. The building type, facade, window glazing materials, and shading systems all modify the intensity, color, and distribution of natural light entering the interior. The natural light conditions available to room occupants also depend on their distance from the window, the room geometry, and surface reflectance. Depending on the daylighting design, indoor natural light is often sufficient to drive and regulate the circadian rhythm, so indoor lighting has always served as a common light source for maintaining the circadian rhythm (iLLab note: meaning that additional so-called "circadian lighting" is not really needed) . Workers in office spaces with windows have reported better sleep quality than those without. If light exposure lasting longer than a certain threshold of 1000 or 2500 lx (illuminance at eye level) is used as an indicator related to sleep quality, then sleep quality improves with increased daytime hours in summer (iLLab note: this is also the principle behind artificial light intervention therapy for seasonal affective disorder in high-latitude regions of Northern Europe during autumn and winter) .

Comparing daylight to electric lighting conditions, Turner et al. [45] state: ‘Typical residential illuminance [on average 100 lux or less, due to electric lighting] is too low for circadian needs even in young adults. Properly timed exposure to sunlight or other bright light sources is vital for mental and physical well-being in all age groups. […] In general, several hours of at least 2500 lux of blue weighted light exposure (ideally sunlight) starting early in the morning benefit most people. Bright light immediately and directly enhances cognition, alertness, performance and mood, so bright environments throughout the day provide additional benefits, especially for middle-aged or older adults.’

Regarding the comparison between natural light and electric light sources, Turner et al. state: “Typical residential illuminance levels (average 100 lux or lower, from electric light sources) are too low for circadian rhythm needs, even for young people (iLLab note: older adults require higher doses of light). Properly timed exposure to sunlight or other bright light sources is crucial for the mental and physical health of people of all ages. […] Generally, several hours of blue-weighted light exposure above 2500 lux (preferably sunlight) starting from early morning is beneficial for most people. Bright light can immediately and directly improve cognition, arousal, performance, and mood; therefore, staying in a bright environment throughout the day can provide additional benefits, especially for middle-aged and elderly people.”

5. Acute, Non-image Forming Effects

Circadian rhythm responses, such as regulation of sleep timing, are related to retinal-mediated responses to light mediated by the ipRGCs. In addition, some acute effects, such as melatonin suppression, increase of heart rate or alertness, can also be realised by light through the ipRGCs or a combination of photoreceptors [46]. Both intensity and spectral composition of light play a role in inducing or avoiding these effects. Daylight can provide high light levels. However, the spectral power distribution of light from specific regions of the sky can vary widely [47]; indoors, since the daylight received depends on the orientation of a room, the color temperature of the light can be considerably cooler than the 6500 K cool white often assumed. The related spectral power distribution and short-wavelength component indicate daylight has a high potential to support acute non-image forming effects (Figure 5).

Circadian rhythm responses (such as regulation of sleep timing) are related to retinal-mediated light responses, which are mainly achieved through ipRGCs (intrinsically photosensitive retinal ganglion cells). In addition, some immediate effects (such as melatonin suppression, increased heart rate, or enhanced alertness) can also be achieved by light acting through ipRGCs or a combination of multiple photoreceptors. Light intensity and spectrum play an important role in inducing or avoiding these effects. Natural light can provide the required light intensity, but the spectral power distribution from specific sky regions may vary greatly; therefore, due to different room orientations, the color temperature of indoor daylighting may be cooler than the commonly assumed 6500 K cool white. The relevant spectral power distribution and short-wavelength components indicate that natural light has great potential to support immediate non-image-forming effects (Figure 5).

Investigations of these acute non-image forming effects of light have mostly been conducted with electric lighting. It has been shown, for example, that self-reported daytime performance, alertness and ability to concentrate, and reduction of daytime sleepiness, improve under static lighting with high correlated colour temperature[48,49]. Smolders et al. [50] found increased subjective alertness and vitality, as well as objective performance and physiological arousal, when offering 1000 lx instead of 200 lx at eye level in the morning. Even though relevant studies with daylight have been limited, daylight would be expected to very effectively produce acute non-image forming effects during daytime due to the availability of high light levels together with the pronounced short-wavelength component in its spectrum. Though lamps have been specifically developed to support circadian rhythm and acute non-image forming effects, daylight is the natural light source to support these effects whilst incurring little, or no, energy use.

Regarding these acute non-image forming effects, most studies are based on electric light sources. For example, two studies based on static artificial lighting indicated that high color temperature can improve self-reported daytime work performance, alertness, sleepiness, and concentration levels. Smolders et al. found that when illuminance at eye level was increased from 200 lx to 1000 lx in the morning, subjects' subjective alertness and vitality increased, while objective performance and physiological arousal also improved. Although there are few studies on acute non-image forming effects based on natural light, natural light is highly expected to play a role in this aspect due to its ability to provide high illuminance levels and short-wavelength spectral components during the day. Although there are currently electric light source products specifically developed for circadian rhythm and acute non-image forming effects, natural light remains the natural light source that can support these effects with zero or near-zero energy consumption.

6. Room, object and human appearance

The multiple characteristics of daylight (both sunlight and skylight) affect room, object and human appearance, providing a specific perceived room ambience that can influence the occupants’ emotional state. There is no conclusive research on the impact of dynamic changes of directionality and diffuseness due to variations of sunlight and skylight entering built environments. However, users of a space are sensitive to the intensity, direction and diffuseness of light in a space [51].

The multiple characteristics of daylight (including sunlight and skylight) affect the appearance of rooms, objects, and the human body, creating a specific perceived atmosphere that influences the emotional state of occupants. There is currently no conclusive research on the effects caused by the dynamic changes in directionality and diffuseness of sunlight and skylight in the built environment. However, people are highly sensitive to light intensity, directionality, and diffuseness in space.

Electric light systems usually deliver light from a number of points distributed over a space leading to light rays of various intensities and directions creating overlapping shadows that can be perceived as visual noise. Conversely, daylight is delivered through a window or a skylight, which has a main direction inward to the room from the opening in the building skin. This creates visual clarity that can provide an impression of serenity of the space. The spatial light distribution also affects room appearance, as well as the perceived representation of objects and human faces. The appearance of faces of people seated near the window, side-lit by daylight, has been shown to be labelled with positive attributes, and high luminance contrasts are not perceived as disturbing [52].

Electric light source systems usually provide light through multiple points distributed in space, with varying intensities and directions, forming overlapping shadows that may be perceived as visual noise. In contrast, daylight enters the room through windows or skylights, projecting light with a main direction from the openings in the building envelope into the interior. This light creates visual clarity, thereby giving a sense of tranquility to the space. Spatial light distribution also affects the appearance of the room, as well as the visual presentation of objects and human faces. Studies have shown that the faces of people near windows, laterally illuminated by daylight, are attributed with positive qualities, and this high luminance contrast does not cause annoyance (iLLab note: the Sichuan-Chongqing dialect term "naohuo" is more precise).

Due to the size of a window, shadows are typically ‘soft’, which is considered appropriate for good modelling [53]. In addition, the light from the side, or a lateral ‘flow’ of light, seems to be preferred in the perception of human faces and objects; daylight through windows is effective in realising such spatial light distributions (Figure 6)[54–56].

Due to the larger size of windows, the resulting shadows are usually ‘soft’, which is considered suitable for good modelling. Furthermore, light from the side or the ‘flow’ of light seems to be more favored when perceiving human faces and objects, and daylight through windows can effectively achieve this kind of spatial light distribution (Figure 6).

Research under electric lighting conditions showed that brightness of room surfaces, preferably greater than 30–40 cd/m² in a horizontal band of 20° above and below the line of sight, give visual lightness and attractiveness to office rooms [57–60]. Also important for perceived spaciousness is the amount of light [61,62], with wall-oriented lighting alone or a combination with a low level of overhead lighting seemingly beneficial for spaciousness [61,63]. A full-scale study (Figure 7) of a series of room quality attributes showed that high levels of daylight from large windows are crucial in order to achieve a more pleasant, exciting, complex, legible, coherent and open room [64].

Studies conducted under electric light source conditions indicate that the luminance of room surfaces (preferably greater than 30–40 cd/m² within a horizontal band of 20° above and below the line of sight) can impart a sense of visual lightness and attractiveness to offices. The amount of illuminance is also important for the perceived sense of space; wall-oriented lighting alone, or combined with low-level overhead lighting, appears beneficial for the sense of spaciousness (iLLab note: sense of spaciousness). A full-scale study on spatial quality attributes (Figure 7) indicates that high illuminance from natural light through large windows is crucial for creating a space that is more pleasant, exciting, complex, coherent, and clearly legible.

Direct sunlight affects room appearance with a light spot as well as clearly defined shadows produced by parallel beams of sunlight. Whilst the light spot is seen as a visual stimulus, research suggests that appropriate sunlight penetration can induce relaxation [65]. Sunlight penetration was found to have a positive effect on job satisfaction and general well-being [66]. A social survey in four different building types by Ne’eman et al. [67] showed that sunshine has ‘a unique non-physical property which induces psychological wellbeing’.

One study used an artificial sky to mimic daylight of a clear sky with defined (blue-toned) shadows, a sun patch (producing a brightness ratio for the sunlight to shaded areas as found outdoors), as well as a bright light source seen through the window (having the appropriate perceived size of the sun) [68,69]. The results indicated that these lighting characteristics had a positive effect on perceptions of room appearance and the mood, stress and anxiety levels of participants. Sunlight falling directly on the occupant or reflected from a surface can cause visual and/or thermal discomfort. This discomfort is linked to blind usage [70], which will then block (part of the) direct sunlight and skylight from entering the building.

Though most research on space and object appearance has been conducted using controlled electric lighting, the results are applicable to daylight conditions. The research included in this section has mainly been performed in temperate climates and indicates that the spatial lighting realised by daylight supports good perception of room and object appearances. Direct sunlight seems to enhance perceived room ambience and the user's emotional state, when visual and thermal comfort are maintained. Façade design considerations to maintain comfort in tropical regions will affect indoor daylight conditions. Both the resulting room and object appearance, as well as the prevalence of sunny conditions might result in different subsequent occupant responses.

Although most studies on the appearance of spaces and objects are based on electric light sources, these results also apply to natural light conditions. Most of the studies included in this section were conducted in temperate regions, so the results indicate that the spatial light distribution achieved by natural light is conducive to forming a good perception of room and object appearance. When visual comfort and thermal comfort are not compromised, direct sunlight seems to enhance the perception of spatial atmosphere and the emotional state of users. For tropical regions, the main consideration in facade design is to ensure comfort, which will affect indoor natural light conditions. The resulting room and object appearance, as well as the probability of sunny days, may lead to different user reactions (iLLab note: compared to temperate regions) .

The specific spectral power distribution and brightness of daylight can also affect human physical comfort. Physical comfort is the feeling of well-being, when an environment's thermal and lighting conditions are experienced as pleasant and associated with satisfaction. The brightness and the strong infrared component of daylight (Figure 2) may be appealing, but can cause visual and thermal discomfort. Nonetheless, interviews in field studies showed that occupants can be satisfied with daylight even though they sometimes experience visual discomfort [71].

Sunlight penetration heats up a room. In addition, windows are a source of heat transfer from and to the exterior. Differences between temperate zones and the extremes of polar or equatorial regions are typically reflected in architectural solutions, as the design approach should be different to give comfortable indoor environmental conditions [72].

Thermal discomfort due to high or low temperatures activates biological cooling (e.g. sweating) or heating (e.g. shivering), respectively. Discomfort can also arise from the thermal asymmetry between the temperatures of the cool internal surfaces of windows and those of warmer walls [73]. A field study by Chinazzo et al [74] indicates that satisfaction with the temperature in the room is affected by lighting conditions, with a lower satisfaction under lower lighting levels. This could suggest a greater tolerance for thermal discomfort in situations with daylight, as previously proposed by Veitch and Galasiu [11].

Visual discomfort, referring to ‘discomfort or pain in or around the eyes’ (according to Boyce and Wilkins [75]), can have several causes, including glare and flicker from the light source. Glare can impact visual performance, but even glare that does not necessarily impair seeing objects can lead to fatigue. Research on discomfort glare due to high luminances or luminance contrast from daylight or electric lighting indicates a greater tolerance when mild discomfort glare arises from daylight [76,77] and/or a diversity of individual requirements for visual comfort from daylight [78] than met from electric light sources with the same luminance. Culture and climate are suggested to influence perceived glare from daylight [79]. Flicker can cause headaches, eye strain or seizures, and reduce visual performance [80]. Electric lighting can be a source of flicker, whilst daylight is flicker-free.

There are many triggers for visual discomfort (defined by Boyce and Wilkins as "discomfort or pain in or around the eyes"), including glare and flicker from light sources. Glare can affect visual performance; some glare, even if it does not hinder object visibility, may cause visual fatigue. Studies show that for discomfort glare caused by high luminance or high luminance contrast from either natural light or electric light sources, people have higher tolerance when it comes from natural light. Meanwhile, individual needs for visual comfort from natural light vary more than those for electric light sources of the same luminance. Cultural and climatic conditions also affect people's perception of glare from natural light. Flicker can trigger headaches, eye strain, or seizures, and reduce visual performance. Electric light sources may be a source of flicker, but natural light is free of flicker (iLLab note: natural light has advantages in terms of glare, and the advantage is even more pronounced in terms of flicker).

A window offers daylight, air exchange, a view, and information on the weather and activities outside. Window material properties, design and usage offer control over outdoor influences, such as smell, sound and heat. In addition, windows may provide an escape route. All these aspects play a role in the feeling of control and safety in indoor environments. Enclosure, privacy, safety and (subconscious) knowledge of escape routes relate to the functionality of a space. Stamps [81] states that lightness of a scene is related to judged safety (‘ability to move and the ability to perceive’). No information about the weather and lack of a view were the reasons female office workers dislike windowless offices, having feelings of isolation, depression and tension (Ruys, reported in Collins [1] ).

The view from a window can affect several aspects of physical and mental well-being. It can, for example, support restorative processes, relieve stress or increase job satisfaction. Research investigating the effects of view content suggests that busy and dense urban areas with obstructions giving a short visual range require constant accommodation and adaptation processes by the eye muscles, to keep an image fixed at the fovea. Conversely, views into a deep space can relieve the eye and the muscle tonus, and free the cerebral cortex from processing information, leading to cognitive relaxation. Looking at a view speeds-up physiological recovery from a stressful experience [82,83].

The landscape outside the window can affect multiple aspects of physical and psychological health. For example, it can support restorative processes, alleviate stress, or increase job satisfaction. Research on landscape content indicates that busy and dense urban areas, if obstructed resulting in a short visual range, require the eye muscles to constantly adjust and adapt to keep the image fixed on the fovea centralis of the retina. In contrast, gazing into the distance can relax the eyes and muscle tension, and free the cerebral cortex from processing information, thereby achieving cognitive relaxation. Viewing scenery can accelerate the physiological process of recovering from stress.

Less information is available comparing the relative restorative benefit of rooms with window views, artificial windows and windowless walls. Office occupants have a preference for real windows or an artificial window with a dynamic view of nature [84], but the restorative effect of artificial windows with dynamic ‘views’ seems to be lower [85]. In windowless spaces, occupants seem to compensate for the lack of windows with nature elements, in the form of plants or pictures of natural scenery. Heerwagen and Orians [86] found that small windowless offices are decorated with twice the number of visual materials than windowed rooms with views. Visual material (in windowless offices) did not represent ‘surrogate’ views, but did include natural themes.

Windows also offer contextual clues about time of day and about weather conditions, that fix ourselves in time and space, both consciously and unconsciously. Patients in an intensive therapy unit with a translucent window had a more accurate memory and orientation and fewer hallucinations and delusions then those in a windowless unit [87]. A questionnaire to understand the preference for windows showed that the view outside that gave temporal information was amongst the most frequently cited favourable factors for residential spaces and a number of non-residential spaces [88]. According to Veitch and Galasiu [11] ‘This information provision is an acknowledged function of a window’.

Both the content and the perceived quality of a view can affect human responses to daylight. The number of view layers, the width and distance of the view, the perceived quality of the landscape elements and the composition of the view are important influential parameters, as shown in Figure 8 [89]. Tolerance of discomfort glare from daylight through a window is partly determined by how interesting the scene outside is [90,91], its attractiveness [92] and its content [93]. An outdoor view enhances the desirable perception of daylight, especially for natural, attractive and interesting views, but the mechanisms for this are not yet fully understood. Even though the contextual clues associated with daylight can be emulated, research indicates that some benefits might not be reproduced by electric lighting.

窗景的内容和感知质量都会影响人类对天然光的反应。景观的层次、视野的宽度和距离、景观元素的感知质量及其构图，都是重要的影响参数（Figure 8）。对天然光眩光的容忍程度，部分取决于窗外景观的有趣程度、吸引力及其内容。尽管窗景可以提高人们对天然光感知的预期，尤其是对自然、吸引人且有趣的景观，但其背后的机制尚不明确。天然光所提供的部分信息是可以通过电光源模拟得到，但仍有研究表明有些天然光的好处是电光源复制不来的。

Daylight provision offers cost-free indoor lighting with a continuous spectral power distribution from 320 nm to 2600 nm, which has implications for the heating, cooling and lighting energy demand of a building [94]. Daylighting can directly reduce the electric energy required to illuminate a room. The extent to which daylight can displace times of use of electricity is obviously specific to the design, location, purpose and use of a space within a building. Care should be given to the most suitable location of activities; for example, highly visual tasks should be performed near a naturally lit building perimeter. A daylight design should be combined with electric lighting controls that switch off or reduce, while maintaining the quality of, electric lighting to reduce electricity use. Lighting energy savings achieved through installing daylight-responsive lighting controls range from 20% to 70% [95–99]. A meta-analysis by Williams et al. [100] showed average savings of approximately 30% in various applications. Tsangrassoulis et al. [101] indicate that a 40% reduction in lighting energy consumption can reduce overall primary energy consumption by 17%.

Only when the full potential of such designed-in approaches has been exhausted should consideration be given to the introduction of technological systems to convey daylight deeper into interior spaces by deflection at windows or the, often costly to install, transmission of daylight from a roof through intervening floors by mirrored pipes or fibre optic cables [102–104]. (iLLab Note: What a long sentence!)

Daylight openings affect thermal conditions in a building. Heat losses in wintertime can increase when the heat resistance of windows is less than walls. Heat gain arises from solar radiation through windows and depends on climate; this might be beneficial in winter but may require additional cooling in summer. The energy saved as well as the cost-effectiveness of daylighting is thus less if cooling energy is required. Modern glazing systems are capable of filtering-out a significant fraction of the infrared component.

Solar heat gains can be modulated with shading devices or switchable glazing systems, which, ideally, should also balance provision of daylight and a view outside, and protection against glare [105]. There are large differences in daylight composition and daylight availability between temperate and equatorial regions for which architectural solutions are usually appropriately defined. The overall energy demand depends on building type, form and construction, occupant activities and patterns together with geographical location, climate, orientation and degree of obstruction [106–108].

Electric lighting requires energy. It may also release heat to the building, depending on the light source that can increase the cooling load but can also decrease heating energy demands. A windowless building is often less energy efficient than one with an appropriate selection and control of well located windows.

Electric light sources consume energy, depending on the type of light source used. At the same time, electric light sources also release heat into the building, thereby increasing the cooling load or reducing the heating load (iLLab note: specifically depending on the season) . The energy efficiency of windowless buildings is usually worse than that of buildings with windows.

Daylight design can bring monetary benefits by reducing the energy cost of electric lighting and by improving the productivity of building occupants. Daylight can increase the latter by a combination of sharpened vision due to better colour rendering or higher light levels, improved visual modelling of objects and faces, reduction of flicker and/or the provision of contextual clues [109]. Productivity has been shown to increase by 5–15% in companies that have moved into buildings with more daylight [110,111]. However, the exact role of daylight on productivity in these kind of studies is still subject to future research, given the many other factors that change simultaneously with such a move. The impact of daylight on productivity and related aspects, such as absenteeism, can only be investigated in field studies and epidemiological studies [112,113], in which experimental control is difficult and interpretation of results is demanding [5]. For now, insufficient results are available to draw conclusions with respect to the impact of daylight availability on productivity; further research is necessary.

An analysis of annual income and expense data for commercial buildings by Kim and Wineman [114] indicated that views have an economic value. In their study, higher buildings, likely to have a skyline and cityscape views, had higher property values. In interviews, the majority of businesses stated that the view was a consideration in setting rents. A study by Heschong [115] indicated that call centre workers with the best possible view processed calls faster and scored better on tests of mental function when compared with those workers without a view. An analysis of sales in stores with and without skylights by Heschong et al [109] indicated that stores with skylights had an increase in their sales index. Interviews indicated that the skylight unconsciously led to the visual environment being perceived as cleaner and more spacious.

As stated above, the detailed mechanisms behind these and other, secondary, monetary benefits are largely unknown. In addition, an increase in productivity can only be achieved when unwanted effects from daylighting, such as glare, shadows, veiling reflections and overheating, are avoided.

As mentioned above, our understanding of the mechanisms behind natural light and its direct economic benefits as well as its secondary benefits is still shallow. Furthermore, the prerequisite for achieving increased production efficiency is to avoid the negative effects brought about by natural light, such as glare, shadows, reflections, and overheating.

Intensity, spectral power distribution, and the spatial direction and diffuseness of daylight are characteristics that support room and object appearance as well as non-image forming effects. The dynamics of changes in the intensity and colour of daylight naturally support circadian entrainment, mood and alertness. Some human responses, such as non-image forming effects, seem to be well defined. Also the role of sunlight on the skin to support vitamin D production is well established. However, many benefits of daylight and windows cannot yet be explained so straightforwardly. The higher onset of visual discomfort glare in daylight conditions as well as the positive effect of the contextual clues provided by a view are induced by mechanisms that are not well understood. Some responses to light seem to be mediated through both visual and non-image-forming pathways that require further research [116–118].

Even though many characteristics of daylight can be mimicked by electric lighting, it has not been demonstrated that all the diverse holistic positive outcomes associated with daylight can be reproduced artificially. Indeed, the characteristics of the complex interaction of the dynamics of daylight with individual human responses have not been readily quantifiable to-date. They remain key areas that require extensive further research.

We suggest that future studies should address the impact of daylight on the following aspects of human performance, health and well-being that might lead to behaviours translating into monetary benefits:

1. Differential impact of variations in the spectral power distribution and light intensity across the day and seasons at different geographical locations, for example through epidemiological studies further exploring the effect of daylight provision on good eyesight and circadian entrainment, restorative sleep and better health;

2. Differences in the impact of the source of light on room and object appearance, comparing electric lighting, and daylight through windows, skylights or light tubes, which includes the differences between static and dynamic lighting;

3. Statistical estimations of the variance in the impact of daylight with concurrent exposure to electric light, to elaborate their interactions including assessments of light history effects, and to obtain a better insight into the acute, non-image forming potential of daylight;

3. Statistical assessment of the variation in impact when natural light and electric light sources are exposed simultaneously, to clarify their interactions (including assessment of light history effects) and to more clearly understand the potential of natural light in terms of immediate and non-image-forming aspects.

(iLLab Note: iLLab is concurrently conducting related research on all three points above: Human centric lighting (HCL), daylight simulation, and integrated daylighting and lighting.)

1. Qualitative assessments of the perception of an (e.g. work) environment to study the role of context and content under different lighting regimes including the absence of light and whether symptoms of such absence can be quantified/operationalised;

2. Quantification of the view and contextual clues from windows. Metrics need to be developed for the quantity and quality of the view out and a measure to evaluate the importance of contextual clues, to balance different window functions, such as glare protection, solar heat gain management and daylight provision;

3. Prevalence of weather conditions and architectural archetypes might influence occupants' expectations and responses, thus the impact of climate and culture on light source preference, room and object appearance as well as comfort aspects should be the subject of further investigation.

And finally, maybe what is most urgently needed and most difficult to devise would be a (set of) metric(s) to measure the ‘naturalness’ of light.

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