1 Introduction

The circadian clock of humans is synchronized with symmetric exposure to day and night light [1]. It causes advancement and delays in our biological clock, making it later or earlier to go to bed and wake up [2, 3]. The intervention of unsymmetrical light exposures can cause an imbalance in the light and dark pattern. This light balance will influence the physiology, health, emotional and cognitive responses [4, 5]. People at the workplace are exposed to variant light patterns and intensity, and one of the most significant factors influencing health and productivity is workplace lighting. Various researchers showed the effect of frequent interruption in light sources on variations in their biological clock [6, 7]. Circadian interruption interferes with the balance in hormone secretion, majorly melatonin secretion, which affects people's sleep patterns, healthiness and wellness. Therefore, the workplace must integrate the person's health, mood and visual comfort. Daylight indoors is a better- preferred work environment due to the presence of ambient daylight and the possibility of outside views [8,9,10]. As mentioned earlier, daylight is the perfect source of synchronizing the human circadian system, provided a proper amount of other photometric quantities exist. Even though there are many discussions and arguments, the widely accepted correlated factors on circadian effectiveness are (i) corneal/vertical eye illuminance (Ev, illuminance at the eye level) and (ii) blue wavelength content in the spectral power distribution (cool white/higher CCT) [11, 12]. Various studies have been put forward to establish the impact of vertical eye illuminance and correlated colour temperature (CCT) on circadian entrainment [13,14,15]. The CCT of a light source is defined as a point on the black body locus that matches most closely to its chromaticity coordinates, called its colour temperature, measured in degrees Kelvin (K). Lower CCT values (1800–2700 K) are considered warm tones, neutral white is about 4000 K, and the higher values of 5000 K and above are the coolest CCTs. Optimizing interior light availability requires the controlled availability of illuminance and CCT [16, 17]. Manual control or a closed-loop control scheme can control an interior daylit space. Climate-based artificial light–daylight integrated models are widely used to augment the overall comfort of occupants [18,19,20]. In the author's previous works, the models were designed only to achieve visual comfort, thermal comfort and energy optimization. These automated building controls lack the circadian entrainment of occupants. Optimizing circadian stimulation without compromising overall comfort and energy saving is the major challenge of current research. However, the arrival of tunable LED luminaires and the incorporation of artificial intelligence leads to the possibilities of a closed-loop control scheme for providing circadian enhancement, commonly called '‘human-centric lighting’ systems [21, 22]. Artificial intelligence applications are widely distributed in various engineering fields [23, 24]. A detailed description of this is beyond the scope of this article.

Many standards, regulations and recommended actions are put forward in the world lighting community to enhance the non-visual activities of the occupants. Based on the spectral power distribution and the α-opic illuminance, lighting researchers initiated the quantification of non-visual effects [25,26,27]. Based on the α-opic illuminance, the spectral sensitivity functions of five known photopigments in the eye and the incident light on the cornea, a particular quantity is evaluated for each photopigment. A 32-year-old standard observer is considered as the reference. Later utilizing the spectral sensitivity functions, the national standards committee ‘Deutsches Institut für Normung’ (DIN) established melanopic factor of luminous radiation amel and melanopic daylight equivalent illuminance Evmel, D65 [28, 29]. These metrics represent the effect of light on a single photopigment called melanopsin. But the circadian system's sensitivity also depends on the signals from other photoreceptors, i.e. cones and rods. The ‘International WELL Building Institute’ (IWBI) put forward melanopic ratio (Rmel) and equivalent melanopic lux (EML) [30,31,32], which are based on the melanopsin sensitivity function. However, the metrics mentioned above do not integrate the neuroanatomy of the retina and the signal transfer in the brain. The Lighting Research Institute (LRC) developed a new metric called circadian stimulus (CS), which incorporates complex neuroanatomy, neurophysiology and the circadian system's characteristics [33]. Besides hormonal suppression, other visual effects like brightness, clarity, scene preference and colour preference can be enhanced by considering the neurophysiology. Moreover, recommendations regarding CS for academic and office purposes have been defined and applied in various field studies [9, 10, 34, 35]. CS varies from 0 to a saturation of 0.7. A minimum of CS.0.3 is recommended for circadian entrainment at least for two continuous hours during the daytime. The average CS can be calculated by Eq. (1). Circadian light (CLA) is the spectrally weighted irradiance to the human circadian system.

$$ CS = 0.7*\left( {1 - \frac{1}{{1 + \frac{{CL_{A} }}{355.7}^{1.1026} }}} \right) $$
(1)

CS represents the effectiveness of CLA or the amount of nocturnal melatonin suppression after an hour of exposure to light. The potential of circadian light is defined as a minimum of 0.3 CS at the occupant's eye level during daytime for at least two hours.

From the above literature, the light level reaches our eyes, and its CCT speaks a lot about the distribution of CS, which helps assess the circadian influence based on Ev and CCT. A higher level of vertical eye illuminance always contributes to higher CS, which is suitable for the deliverance of circadian lighting. However, this can also lead to glare sensations for the occupants in the interior space. Without controlled lighting, focusing only on the non-visual aspects is obsolete. Due to the growing awareness of circadian lighting, dimmable and CCT tunable sources are also important. Here the variation in the spectrum at different CCTs is meant to mimic the daylight spectrum across the day to reduce the interruptions by light on the circadian clock. Despite the predominant effects of colour-tunable sources, people sometimes prefer uniform illuminance with static light that exhibits constant white light. People's light perception varies with the appearance of the ambience.

CS mainly changes due to the variations in Ev. By following the changes in Ev, the CS can be well predicted. The vertical eye illuminance (Ev), or the light level reaching the human eye, varies with changes in building geometries like room depth, ceiling and wall reflectance factors, window size, shading elements, light sources, whether daylight and electric light and other external conditions like sky types, seasonal variations, etc. Also, as the distance from the windows changes in only daylit spaces, Ev and CCT vary. Unless the interior space does not have uniform lighting or a controlled daylight–artificial light climate-responsive system, there will be variations in the above photometric quantities at different locations.

Consequently, in those scenarios, the CS also changes. Hence, we need to know about the variations in the lighting metrics mentioned earlier to incorporate human-centric lighting design properly. Also, here we are considering an office having a proper layout, with varying venetian blinds and tunable and dimmable luminaires. We have not undertaken any constructional variations for analysing the circadian effect on occupants. Since the sky conditions vary continuously, it significantly impacts incoming daylight in daylight–artificial light integrated systems. Therefore, it leads to variations in light level and circadian light in an interior space.

Simulations are done with an open office plan integrating the venetian blinds, daylight, static and dynamic (colour-varying) light sources. Real-time experimentation was performed in a test workbench with venetian blinds, fixed and tunable light sources and a market-available HCL system. Overall, this work considers the variation in CS under different lighting ambiences and is obtained from simulation and real-time experimentation in a test room with venetian blinds and dimmable sources and with a commercially available human-centric lighting system. We have considered an open office for the simulation, and a test room is considered for real-time experimentation. This article aims to study the impact of spectrally tunable and fixed light sources on CS in daylight–artificial light integrated systems via simulations. Also, this research focuses on the implications of human-centric lighting systems under varying lighting conditions and shading control strategies.

2 Experimental Methods

2.1 Simulation of an Open-Plan Office

For the simulation study, we considered an open office workspace in Manipal (Latitude 13.350, Longitude 74.790). Here we used the software DIALUX EVO to simulate the workplace. The vertical illuminance (Ev) is estimated at eye level (1.2 m above ground level), whereas the horizontal illuminance (Eh) is obtained at 0.8 m height. The drafting tool AutoCAD 2015 was used to know the exact dimensions of the ceiling, floor, furniture, etc. and the scaled version of each site area. Figure 1 shows the layout of the entire floor and the open office space. Here we have considered the open workspace, as shown in Fig. 2. There are several rooms like Cabins, 6Pax meeting rooms, private offices, 4Pax meeting rooms and 12Pax meeting rooms with typical dimensions. Recess-mounted light fixtures are considered. There are 15 windows for this open workspace. The 3D room view is given in Fig. 2, and the climate change considerations are also below.

  • February–June: summer season

Fig. 1
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Layout of the entire floor plan a and the layout of the open office space b

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Fig. 2
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3-D view of the open office plan

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July–September: rainy season

October–January: winter season

  • Working hours are from 8:00 am to 6:00 pm.

The number of luminaires was selected to achieve an average illuminance of 500 lx.

2.2 Real-Time Experimentation in a Test Workbench

Experimental investigations were carried out in a test room at MIT-Manipal (13.3525° N, 74.7928° E) with a climate-responsive automated control facility for venetian blinds and luminaires [36]. The air-conditioned test room has a dimension of 4 m × 4 m × 2.3 m with clear glass windows of size 1.3 m × 1.3 m on all four wall sides, which can be operated one at a time or multiple windows. Figure 3 shows the system with motor-controlled venetian blinds and two dimmable LED luminaires. The binds and the static luminaires are automated using a climate model-based algorithm implemented on an embedded control platform using LabVIEW. The system is designed to adjust the blind position to achieve visual and thermal comfort and energy saving by keeping the room temperature and the interior illuminance to given set points. The work plane of the room (2 m × 2 m) has two zones. The first zone is considered one window head height from each window, and the second is away from the window. In their previous works, the authors developed fuzzy-based [20] and predictive models [36, 37] to predict the blind position for achieving visual comfort, thermal comfort and energy saving.

Fig. 3
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Interior view of the test room

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2.3 Real-time Readings from a Faculty Cabin with a Commercially Available Human-Centric Lighting system (HCL)

We have implemented a commercially available human-centric lighting system with a controller and sensor in a test cabin. The system is meant to provide adequate light that simulates central characteristics and the dynamic progression of natural daylight to improve the quality of life in the workplace significantly. This technology provides automated circadian entrainment and energy efficiency using intelligent sensors. This system has a pattern of light level and CCT variation and also incorporates automatic on and off based on the occupancy and light level control based on daylight availability. The appearance changes from warm to cool white following the day's timings. Both manual and automatic mode of control is possible with the system. Three zones are considered for sensing a person starting from sitting in the place, next moving hand and then moving forward, taking distance for total coverage as 8m for 360 degrees. The wireless communication between the module and the sensors is carried out via Power over Ethernet (POE). The sensor communicates with the driver accordingly. The delay time starts; then, it reduces the lights with 10% off and finally switching it off. The light is switched on immediately when the person enters the room and turns off after a delay of 5min.

3 Results and Discussion

3.1 Simulation of Daylight–Artificial Light Integration Scheme Incorporating Venetian Blinds (Closed with Slat Angle Open) and Fixed Light output

Daylight does not provide adequate visual and non-visual effects in the interior as a single source. However, integrating sunlight and the artificial source provides a better solution for synchronizing the human clock and providing visual comfort. Daylight glare, interior thermal performance, etc. are some of the major issues that appear while merging electrical light with natural light. Various shading elements are available to control the glare and light level inside the room. Here, we have chosen venetian blinds to optimize visual comfort, thermal comfort and energy savings, a climate-responsive approach. Simulations were carried out with various sky types and seasons from morning 8.00 am to 6.00 pm. Figure 4 shows the average Ev and average CS variations observed under different sky types and seasons in a day. The combinational CS was estimated using the enhanced CS calculator CS 2.0 developed by Lighting Research Centre (LRC) [38]. The direct light source (5700 K, 36 W) is considered to simulate this case. While considering all the circumstances, it was observed that as Ev approaches 400 lx, the probability of an average CS of 0.3 is more. Hence, the integration of venetian blinds as shading devices does not limit the required average CS in the open space for the circadian entrainment of occupants. Also, the variation in the pattern of CS with and without blinds seems similar. The average ratio of CS in both cases was found to be 1.085, and for Ev, it was 1.134; this helps to know the impact of fully closed blinds on the circadian entrainment of occupants in different daylighting conditions. Simulations show that the minimum criteria of CS as 0.3 are met in all cases, except in winter overcast sky. Under the Overcast sky in winter, the CS was on the verge, and the average Ev was found to be less. So in these lighting conditions, an auxiliary light or without shading conditions can enhance the CS level. Incorporating blinds reduces the availability of Ev, especially during the morning time in the rainy season. Also, during the winter, the light level (Ev) is less in the morning in both cases. A light source with higher CCT may provide the required CS with less value of Ev. Replacing fixed luminaires with dimmable/tunable sources can perform better circadian entrainment. A controllable light output provides circadian effects, visual comfort and energy optimization. Figure 5 indicates the uniformity in work plane illuminance in both cases (with and without blinds). Figure 6 shows the view of the room. Shading provides uniformity, but it is suggested not to use shading to give CS for the specific sky and climate conditions.

Fig. 4
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Ev and CS with seasonal variations and sky type conditions with and without window blind. a With blinds and b without blinds

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Fig. 5
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(Left) Uniformity in work plane illuminance with blinds and (right) without blinds

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Fig. 6
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View of the room with Blinds closed and slat angle open

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3.2 Climate-Responsive Integration of Daylight–artificial light using Tunable light source and Venetian Blind

Ten occupants were considered to analyse Ev's distribution at their seating positions in this case. A dimmable and spectrally tunable luminaire with varying CCT is replaced with the source considered in Sect. 3.1. Figure 7 shows the measured spectral variations with an integrating sphere in different CCTs. For the simulation, we used the climate-based adaptive fuzzy control algorithm for the venetian blind developed by the author [36]. The percentage of blind opening and the luminaire dimming was used based on the algorithm. Regarding the tunable sources, higher CCTs were chosen during the morning and gradually lower CCT's afternoon based on the timing.

Fig. 7
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Tunable luminaire spectra at various CCT measured with integrating sphere

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Figure 8 (left) shows the distribution of light levels on polar coordinates; this information in electronic file format, i.e. IES files, is used for the simulation using Dialux Evo. Figure 8 (right) gives the variation of CS on polar coordinates, estimated using the CS2.0 calculator. The simulation was done from 8:30 am to 6:00 pm with a time interval of ½ an hour. Table 1 shows the lighting conditions obtained from the adaptive control algorithm. The blind position from 8:30 am to 12:00 pm is 70% open, and from 12:30 pm to 6:00 am, it is 100% open. To achieve uniformity, the number of luminaires estimated using the lumen method is 65. Figure 9 shows the occupant position and Fig. 10 gives lighting distribution and Ev at each occupant position. The person sitting in the east direction receives less Ev than all others. A supplementary light source is required to augment their circadian effectiveness. Maximum Ev appeared for the person sitting on the west side. In all other directions, a prominent presence of Ev was observed. The distribution pattern of Ev in all orientations was evaluated as the same. Ev is highest on the west side during the morning, especially between 9.00 am and 10 am. There was a drastic change in light level from morning to afternoon on the west side. East side perceived stable Ev throughout the time duration. South-east and north-east directions show a similar pattern in light level.

Fig. 8
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Polar plot of the tunable luminaire (left) and variation in CS on polar coordinates (right) (estimated using CS2.0 Calculator)

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Table 1 Lighting conditions based on the control algorithm
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Fig. 9
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Layout showing occupant position

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Fig. 10
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Ev at occupant positions (left) and light distribution (right)

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Similarly, south-west and north-west directions perceive almost identical Ev. The north and south directions also show the same pattern in Ev during the entire period. Finding the appropriate seating position is a prerequisite in preoccupancy daylight evaluation for circadian enhancement.

Figure 11 shows the variation in CS and Ev with an increase in room depth. As the distance from windows increases, the CS level reduces due to a reduction in Ev's at those locations. Occupants 1, 2, 3, 4, 5 and 6 faced the north window. Those positions were observed to have an adequate level of CS to promote circadian entrainment. Occupants 7, 8, 9 and 10 received light from the west- and north-facing windows. These locations were also found to be satisfactory for meeting circadian criteria. The occupant from 5.484 m from the west and 3.298 m from the north (position 10) has the minimum CS. So these seating positions and the locations beyond this distance required auxiliary light sources to enhance the CS.

Fig. 11
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Variation in CS and Ev with room depth. N indicates the distance from the north window, and W indicates the distance from the west

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Compared to an occupant sitting nearer to the west window and facing the north window, the reduction in CS at the centre is almost 50%. This percentage reduction will increase as the seating moves towards the farthest point from the windows. Table 2 indicates the impact of tunable sources on CCT variation and energy optimization. Fixing a single CCT throughout the day requires more luminaires to meet the uniformity and standards in Eh. Previously the observed Ev for each occupant was higher compared to this case. Daylight incorporation provides more Ev, which helps to increase the CS level. Figure 12 indicates the Ev at each occupant without the intervention of daylight and the light distribution in the open space. For higher CCT, 5136 K, CS was greater than 0.3. In all other CCT variations, CS was lesser than the required level. The highest CS was found at occupants 8 and 3, similar to the previous simulation.

Table 2 Ev at each occupant position using tunable luminaire
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Fig. 12
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Light distribution (left) and estimated CS with LRC toolbox at each occupant position (right)

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However, for higher CCT, 5136 K and 3646 K, whenever Ev is nearly 400 lx, CS was greater than 0.3. It shows a strong correlation between Ev variations rather than CCT deviations. Designing lighting space only with tunable sources increases the number of light fixtures compared to the former (with daylight). Integration of daylight with tunable sources reduces the number of luminaires and minimizes the energy consumption (KWh/m2) compared to the simulation with static sources, providing the required CS.

3.3 Real-Time Experimentation in a Test Workbench

In this section, experimentation was done in the test workbench with the dimmable and non-tunable lamps and the venetian blinds. Here, three weeks of readings during office timings, 10 am to 5 pm, were considered under clear sky conditions. The eye level/vertical illuminance (Ev), horizontal work plane illuminance (Eh), CCT and spectrum of the light falling at the eye were measured in all orientations. As per IESNA standards, a sufficient horizontal illuminance of 300–500 lx is required in office spaces. The occupant sits on all four sides of the work plane for the same experimental setup. The illuminance, CCT and spectrum was measured with a CL-500A spectrophotometer. The circadian metric CS was evaluated using the excel-based LRC toolbox [38]. The horizontal work plane was considered at the height of 0.8 m from the floor level, and vertical illuminance was measured at the eye level height of 1.2 m (Fig. 13). Here, the investigations were done with different lighting scenarios, which were previously done by the same authors [39]. The other cases are as follows: only daylight (DL), i.e. blinds open; only artificial light (AL), i.e. blinds closed; daylight and artificial light (DAL), i.e. not controlled, blinds open; and finally, daylight and artificial light integrated system (DALIS), controlled.

Fig. 13
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Floor plan of the measurement setup [39]

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The east-oriented window was set in the controlling condition in this experimentation; the occupant's facing direction was changed to determine the relevance of the test room's seating/observer position. The suitability of different lighting schemes in the test room was also evaluated based on Ev, Ev/Eh, CS and CCT. Ev/Eh gives the vertical and horizontal illuminance ratio and compares vertical and horizontal light under various lighting conditions. The data were recorded on the same day at the exact time. As mentioned in the previous section, each case of lighting scheme is as follows. AL-1, DL-2, DAL-3 and DALIS-4. Different colour shading shows the occupant's seating position (sitting in the south, west, north or east) looking towards the monitor (Fig. 14). Here, the west side occupant receives the highest CS in all lighting schemes because the occupant is facing towards the opened east-oriented window.

Fig. 14
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Average values of measured CS, CCT, Ev and Ev/Eh in the test room in various lighting conditions [39]

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Consequently, more light reaches the eye. However, this position is not preferred to be occupied in a workspace due to glare, especially in the afternoon. The north side provides minimum CS for circadian entrainment compared with other seating positions. The DAL scheme gives the highest Ev and, consequently, the high CS among all the lighting schemes. Nevertheless, the Ev/Eh was 37.5% lesser than the DL scheme. Hence, it indicates the presence of more Eh in the DAL scheme. Compared to the DL scheme, there was a 50% increase in Eh in DAL due to artificial light. In DAL mode, higher Ev may lead to discomfort glare conditions. In the DALIS mode, except the east side, all other sides were suitable for working with minor settings dimming for a minimum value of CS as 0.3. Under these lighting conditions, the Ev/Eh ratio was almost similar to the DL scheme, but Ev was 25% lesser than the latter. Hence, in terms of CS and glare, daylight and electrical light's integrated scheme is closer to ideal workspace conditions than other schemes.

3.4 Real-Time Readings from a Faculty Cabin with a Commercially available Human-Centric Lighting System (HCL)

The readings were taken from 8 am to 5 pm from the cabin room. We measured the vertical and horizontal illuminance, CCT and spectrum using spectrophotometer (CL-500A), and CS values were estimated using LRC Tool. Measurements were taken at the centre point of the work plane. Figure 15 gave the pattern of variation of horizontal illuminance at the centre point of the work plane, vertical illuminance of the occupant facing east, CCT and estimated CS of commercially available human-centric lighting placed in the test room. Analysis was carried out for different points. Eh, Ev and CCT are increasing up to 1 pm and then reducing and after 4 pm, again increasing. As in this case, using four luminaires, uniformity was perfect. The CCT maintained was around 3000 K-5000 K. The highest CCT was around 5031 K at around 11.30 am, and the lowest CCT was about 2826 K around 3 pm. The CS maintained at noon, which was 0.36, was large, and 0.28 of CS is also recorded, the smallest value. The system responds to occupancy, and Ev and CS's pattern looks similar, indicating the correlation between both variables. Also, the CCT variation for all days observed is the same. It suggests that the system is not responding to actual climate, only tracking the reference curve. Visual comfort, thermal comfort and energy savings via effective use of daylight and circadian entrainment are possible with climate-responsive controlled daylight and controlled spectrum.

Fig. 15
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Analysis of commercially available human-centric lighting. a Horizontal illuminance (top left); b CCT pattern (top right); c vertical illuminance at the eye level (bottom left); d estimated CS (bottom right)

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4 Conclusions

A few inferences are made using simulations and experimentation to establish a 'good' lighting environment in daylight indoor work areas. This lighting design helps to have no detrimental effects on human health and comfort and fewer energy expenditures. Our inferences and suggestions are as follows:

  • Preoccupancy and post-occupancy evaluations of daylight in work areas are necessary to obtain the tenants' circadian effectiveness and visual comfort. Preoccupancy evaluation using simulation helps find suitable shading elements, electrical light sources, seating positions and viewing directions.

  • Uncontrolled daylight and higher Ev cause glare sensation and non-uniformity; manual/automated shading elements reduce visual discomfort and regulate sunlight. A computerized system is required to control the shading equipment based on climatic conditions.

  • The integration of venetian blinds for shading does not limit the CS in the interior for the circadian entrainment of occupants. Incorporating blinds avoids glare and also provides thermal comfort. An additional light source with higher CCT as a supplementary source or tunable with high CCT may give the required CS with less value of Ev.

  • The study shows that the impact of a varying light source on circadian entrainment is more compared to a fixed light source. A tunable LED luminaire that provides spectrally varying dimmable light output can eliminate the need for higher Ev in lower CCT. In lower CCT, the warmness of light will be more, which is unsuitable during the daytime. As per the simulation, to have circadian effectiveness at higher CCTs (shorter wavelength content), nearly or greater than 400 lx as Ev is required, this will be very high in the case of lower CCTs.

  • During uncontrolled lighting conditions, the vertical eye illuminance is more than the controlled scheme, leading to visual discomfort, where a light source with fixed CCT is used, providing a uniform colour appearance throughout the day.

  • Observation with the commercial HCL system shows that the tunable LED luminaires provide a varying colour appearance in the ambience, from cool white to warm white, by maintaining the average illuminance of around 500 lx without compromising circadian stimulation.

Sustainable development in buildings incorporated with healthy indoor spaces is a multi-objective problem for developers and designers. Simulation at the earlier stages of building design is necessary to evaluate and estimate the possibilities of implementing a healthy interior and net-zero entity. In addition, the colour appearance of the lighting ambience varies from person to person, either static or dynamic lighting fixtures. Improving the overall wellness of building occupants makes them 'happier' and increases their working productivity. Hence integrating wellness, health and energy minimization is the challenge we need to answer. A well-designed daylight–artificial light integrated scheme can maintain visual comfort, thermal comfort and energy savings. Only uncontrolled daylight situations lead to discomfort glare and more heat load to the air-conditioning unit. Climate adaptively Controlled tunable source with controlled shading is the perfect solution to achieve visual comfort, thermal comfort and circadian entrainment with energy savings. Future research work can be extended to study the impact of various dynamic shading elements in HCL design. It will help the lighting community to design interior spaces with a more aesthetic appearance.