How to illuminate indoor plants sustainably? Tips and tricks to bridge the gap between research and design.

The increasing popularity of biophilic design in architecture and interior design has led to a widespread integration of indoor ornamental plants and green walls. While numerous studies have demonstrated the benefits of such integration for people, only a few have focused on the well-being of the plants themselves. Our latest research project aims to address this gap by exploring suitable lighting conditions for indoor plants, seeking an optimal spectral composition that ensures their well-being, robust growth, and visual appeal, while also optimizing maintenance. Building upon previous applied research, we conducted experiments to analyze the lighting requirements of four species of ornamental plants commonly used indoors. Seven identical room boxes were employed, each illuminated with different light spectra falling within the CCT range of 2800K-5000K. The specific light spectra were meticulously tuned with a focus on the red/blue and red/green ratios, while keeping the photosynthetic photon flux (PPFD) and average illuminance consistent. As a result of this research project, practical guidelines were developed to help lighting designers navigating the intricate world of indoor ornamental plants effectively. By carefully considering the lighting spectrum in the suggested ranges, architects and interior designers can create spaces that not only benefit human occupants but also provide an optimal environment for the well-being and flourishing of indoor plants.


Introduction
In recent years, biophilic design has gained significant traction in the fields of architecture and interior design.Rooted in the concept of biophilia, which denotes the innate human affinity for nature, this design approach seeks to integrate natural elements into constructed spaces, thereby enhancing the physical and mental well-being of occupants [1][2][3][4].Architects and designers employ various strategies, such as incorporating natural materials, patterns, colors, optimizing daylighting, creating artificial lighting conducive to the human circadian rhythm, and integrating ornamental plants in the space.
Extensive research has already demonstrated that the implementation of biophilic design principles can substantially improve the overall experience of interior spaces, promoting better health outcomes, reducing the risk of diseases, and alleviating stress [5][6][7][8][9][10][11][12][13].Moreover, it has been found to enhance productivity, creativity, and overall user well-being, while aligning with green building strategies and contributing to energy savings [14][15][16].The presence of indoor plants has been recognized for its capacity to purify and humidify the air, regulate indoor temperatures, serve as acoustic absorbers, and significantly uplift moods and reduce stress levels, particularly in places where people spend extended periods, such as offices and schools.
In response to the growing significance of indoor plants for the biophilic design approach, the latest version of the WELL Building Standard, a prominent rating system for sustainable and healthy 1320 (2024) 012018 IOP Publishing doi:10.1088/1755-1315/1320/1/012018 2 buildings, has integrated many biophilic design principles, including the use of indoor plants.The new standard highlights the importance of direct connections to nature through the presence of plants, water features, daylight, and natural views.In particular, Chapter M09 'Enhanced access to nature' [17] requires a combination of indoor plants and water features in direct sight of at least 75% of workstations and in shared recreational spaces to encourage a stronger relationship between users and natural elements.
Given the increasing popularity and requirements of international building standards for the integration of plants into the built environment, there is an urgent need to research appropriate lighting techniques for ornamental plants and to identify lighting parameters that influence their health and visual appeal.Surprisingly, this area of research is insufficiently investigated, with existing studies focusing mainly on horticultural lighting and neglecting typical decorative ornamental plants (Fig. 1).Furthermore, the few studies conducted so far have been limited by the equipment used or have examined only a limited set of critical parameters.

Lighting for plants versus lighting for human beings
The lighting of ornamental plants in indoor spaces presents a dilemma of its own: how can lighting for plants be combined with the typical lighting for indoor spaces?The impact of light on plant growth and development goes beyond its practical use for human vision.While humans need light primarily for visual perception and circadian regulation, plants have a more complex relationship with light, relying on it for the vital process of photosynthesis [18][19][20][21][22][23].The spectrum of light that humans find visually pleasing may not always be the most beneficial for plants, as they require specific wavelengths to facilitate photosynthesis and regulate various physiological responses such as germinating, directional growing and flowering.In order to create an environment that promotes the wellbeing of both humans and plants, it is essential to understand their different lighting needs.

The sensitivity to light differs between plants and humans
Both plants and humans rely on photoreceptors containing specific photosensitive pigments to gather information from their environment through light.Humans are able to perceive wavelengths between 400-700 nm, while ultraviolet (UV) and infrared (IR) wavelengths, which are shorter and longer, respectively, that this range, are not visible to the human eye.Plants also use a specific range of wavelengths for photosynthesis called Photosynthetically Active Radiation (PAR), which corresponds to the 400-700 nm wavelengths.However, plants have five different photoreceptors that can detect light outside the PAR range.Research has shown that UV-A radiation can increase plant vigor and affect leaf pigments, while IR radiation can affect the flowering cycle [24][25].In contrast, the Vlambda curve shows the different light sensitivities of humans, with our visual system being most sensitive to green-yellow light (Figure 2).On the other hand, the PAR spectrum highlights the preference of plants for blue and red light, which is crucial for optimizing photosynthesis and various physiological processes [26].While humans thrive in environments rich in green-yellow light, the inclusion of both blue and red light in the PAR spectrum is essential to promote the growth and wellbeing of ornamental plants.In order to design lighting environments that promote the growth and wellbeing of ornamental plants, while considering the visual needs and comfort of humans, it is essential to understand the light sensitivities of both plants and humans.By carefully balancing these light sensitivities, we can create harmonious and biophilic spaces that enhance both human comfort and the flourishing of indoor greenery in our built environment.

Plants and human beings refer to different metrics
Since plants and humans respond differently to light, it is essential to employ distinct metrics when designing lighting for each application.Lumens and lux, which are photometric units related to the human visual system, cannot be used for plants' lighting needs.Instead, the Photosynthetic Photon Flux Density (PPFD) and the Daily Light Integral (DLI) serve as the appropriate measurement units for designing lighting for plants (Fig 3).The PPFD quantifies the number of photons reaching a 1 m2 surface per second and is specified in micromoles per square meter per second (μmol•m -2 •s -1 ), reflecting the instantaneous light intensity received by plants.On the other hand, the DLI measures the number of photosynthetically active photons within the 400-700 nm range delivered to a specific area over a 24-hour period, expressed in (mol•m -2 •d -1 ).This integral metric provides valuable information on the total light accumulation that plants receive during the day in their natural environment, helping to determine the cumulative light exposure necessary for their growth and well-being indoors [27].DLI is crucial for ornamental plants because it directly correlates with their growth, flowering, and overall health.Different species of ornamental plants have varying light requirements.Achieving the ideal DLI for ornamental plants involves considering factors such as the plant's natural habitat, growth stage, light intensity, and duration of exposure.The metrics PPFD and DLI are vital for creating lighting environments that cater to the unique needs of both plants and humans, ensuring optimal growth and well-being in their respective contexts.

Research goals
This research project aims to address the disparity between lighting design for people's needs and the botanical requirements of ornamental plants and to offer valuable insights for architects, designers, and lighting specialists to create healthier and more aesthetically pleasing spaces.
The use of standard indoor LED lighting, developed primarily for human visual preferences, falls short in providing the necessary biological health for plants.Previous studies in this area have examined standard light sources with fixed and default CCTs [28][29][30], but the spectral distribution was not tailored to the specific needs of plants.In order to harmonize lighting for both people and plants, this research aims to demonstrate that typical CCTs used in indoor spaces can be effective for plants if carefully designed for their requirements.The main objective was to identify specific spectra within the typical CCT range of 2800K-5000K and to develop a family of LED chips that can be used in architectural lighting fixtures to achieve a cohesive and a harmonious lighting effect in common spaces.Additionally, considering sustainability, the research aims to investigate the minimum amount of light necessary for plant survival and healthy growth.While previous studies indicate an average illuminance of 2000-2500 lx [28], this project is questioning this reference value and investigating the possibility of ensuring healthy plant development with reduced light levels in line with energy saving requirements.Furthermore, the research aims to contribute to the limited scientific literature on recommended DLI and PPFD-factors for various indoor ornamental plant species, particularly focusing on realistic and energy-efficient possibilities achievable with artificial lighting in indoor spaces.
The research could result in several benefits for the design of indoor environments and the wellbeing of both space users and plants: Overall, the research outcomes have the potential to positively impact indoor environments, plant care practices, sustainability efforts, and scientific knowledge, fostering a more holistic and beneficial approach to lighting design for both humans and plants.

Lighting Experiment -Setting
The lighting experiment was conducted between November 2021 and October 2022 to evaluate the formulated theory.The study focused on investigating two fundamental lighting parameters: the Spectral Power Distribution (SPD), gauged through the analysis of the red/blue and red/green ratios, and the Photosynthetic Photon Flux Density (PPFD).These parameters play a vital role in determining the quality and quantity of light received by plants, affecting their growth and overall well-being.In conjunction with these two light parameters, four distinct plant species commonly found in indoor spaces were selected.By studying these representative plant species, the experiment aimed to gain comprehensive insights into their responses to the different lighting conditions, contributing to the advancement of optimized lighting solutions for indoor environments.

Ornamental Plants
Four plants species were selected for the experiment: Codiaeum Petra, Dracaena Janet, Epipremnum and Spathiphyllum (Fig. 4).Each species represents a unique set of characteristics relevant to indoor greenery.Among these, Epipremnum and Dracaena Janet are categorized as "full sun" plants, thriving in high light intensity conditions, while Codiaeum Petra and Spathiphyllum belong to the "full sun/half shadow" category, capable of adapting to moderate light exposure.Moreover, Spathiphyllum and Codiaeum Petra have the ability to produce flowers.The selection of these four plant species was done in collaboration with a botanist and based on their frequent use in indoor applications by landscape architects.To ensure robust experimentation, we employed two plants for each species for every light configuration, thus a total of eight different plants per light box.Additionally, the garden center responsible for providing the plants ensured appropriate soil quality, supplemented with the necessary nutrients, and provided watering guidelines for consistent care throughout the test duration.

Test Set-up
The experiment was conducted within a controlled laboratory environment.To ensure precise lighting measurements, a physical dark room configuration was established, devoid of any external light sources.The test space was meticulously enclosed with dark cardboard and black opaque fabric, eliminating potential interference from adjacent light.Seven light boxes were arranged in the space, each of them housing one spotlight.The plants were exposed to 12 hours of light and 12 hours of darkness, to simulate the natural light/darkness cycle.The overall dimensions of the test space measured 4.5m x 2.7m x 3.5m, while each light box measured 0.43m x 0.8m x 1.2m, except for Box B, which measured 0.5m x 0.7m x 1.2m.The spotlights were installed at the top of each box, approximately 60 cm away from the top edge of the plants.Notably, the spotlight in Box B was placed off-center to accommodate specific space requirements.All plant specimens were positioned centrally within the test space, with the "full light" category plants placed directly beneath the spotlight positions (Fig. 5).This setup enabled precise control and measurement of lighting conditions, ensuring accurate and reproducible results throughout the experimentation process.

Design of the LED-Chips
The LED chips used for the experiment were custom designed using arrangements of LED modules that were already part of a manufacturer's catalogue for horticultural lighting.Seven distinct LED chip configurations were created to achieve specific values of the investigated lighting parameters.
The design of the LED chips considered the red/blue and red/green ratios, as well as the PPFD values.Each LED chip was composed of 12 LED modules to attain the targeted illuminance levels.
Moreover, individual control of each LED module was enabled through a DMX control system.By independently adjusting each module, the final spectrum of each LED chip was achieved to meet the desired parameter values.This custom-designed LED chip layout allowed for precise control over the light spectrum, facilitating accurate exploration and evaluation of the effects of different lighting conditions on the test plant species.

Measurements and documentations
The different parameters were assessed at regular intervals of two weeks, specifically on the top surface of the plants, positioned at 43 cm from the floor level and directly beneath the spotlight.The measurements were conducted using a spectrometer1 .The stability of the spotlights' spectra was examined through the measurement of several key parameters, including PPFD, R/B ratio, R/G ratio, illuminance, and CCT.In addition to the lighting conditions, humidity and temperature were measured inside the test boxes to take into account their possible influence on the plants' health status.Temperature shifted from 27°C in summer to 20°C in winter, whilst the humidity stayed fairly constant throughout the duration of the experiment at values of 40-50%.
Furthermore, visual documentation of the plants' development and growth was carried out.Photographs of each group of plants were taken every two weeks, allowing for a comprehensive record of their progression over the course of the experiment (Fig. 6).This combination of quantitative data and visual observations ensures a robust evaluation of the effects of different lighting conditions on the selected ornamental plant species, providing valuable insights for the research analysis.

Lighting Experiment -Phase 1
To establish the experiment's parameters, we started with the metrics usually considered by lighting designers and architects for indoor lighting, namely lux levels and CCT.It was decided to explore two illuminance levels, 2,000-2,500 lx and 800-1,000 lx, along with a CCT range of 2800K-5000K, commonly used in various indoor settings.
During the initial phase of the experiment, seven different lighting configurations were designed: four spotlights achieving an illuminance level of 2,000-2,500 lx on the plants' leaves (High output spotlights) and three spotlights achieving an illuminance level of 800-1,000 lx (Low output spotlights).Through multiple tests, it was observed that specific ranges of red/blue (R/B) and red/green (R/G) ratios created a pleasant and natural visual effect on the plants, while maintaining the targeted CCT range.Optimal visual appearance was achieved with R/B ratio < 5.0 and R/G ratio < 2.0, as higher values led to a warm and unnatural visual effect, unsuitable for the desired plants' aesthetics.
To ensure consistent light exposure for all plants, the LED modules were tuned to achieve similar PPFD values for both light levels.
The design of the LED chips aimed to maximize spectral variation, incorporating different R/B and R/G values to investigate their impact on plant growth and development.

Lighting Experiment -Phase 2
After a 5-month duration of the experiment, significant differences in plant growth became evident between the specimens exposed to light levels of 2,000-2,500 lx and those at 800-1,000 lx.The former group exhibited excessive and too rapid growth for two of the plant's species and a prompting consultation with a botanical expert confirmed that these plants were indeed experiencing unhealthy growth, characterized by elongated and oversized leaves, probably weak roots, and overall abnormal appearance.Therefore, it was concluded that an illuminance level of 2,000-2,500 lx (with a PPFD of 30.0 μmol•m -2 •s -1 ) was not beneficial to the normal development of these plant species used in the experiment.
Subsequently, the project entered its second phase, where the light output of the High Output spotlights was decreased to 800-1,000 lx while maintaining an unchanged spectrum.Phase 2 then involved comparing the same seven spotlights with similar illuminance level and PPFD values (800-1,000 lx and 16.0 μmol•m -2 •s -1 , respectively), while preserving the R/B and R/G ratios determined during Phase 1. Table 1 shows the values of the parameters set for Phase 2. This phase of the experiment aimed to evaluate how the modified lighting conditions influenced the growth trajectory and health of the 4 group of plants (Spot A, B, C and G) that experienced a higher light intensity during Phase 1.
Table 1.Values of the relevant parameters set during Phase 2 of the experiment.

Roots Analysis
After a comprehensive 11-month testing period, an analysis of the root systems was deemed necessary to verify whether the visible health status of the plants corresponded to their non-visible part.A The Epipremnum demonstrated more in response to the lighting conditions, with initial rapid growth followed by favorable development and good health status.In contrast, the Codiaeum Petra exhibited heightened sensitivity to the lighting arrangements, as only two configurations led to good health status, while two had detrimental effects, particularly affecting root development.Similar findings were observed for the Spathiphyllum.
In further examination, the health status of each plant species in relation to each spotlight configuration was analyzed, correlating the root analysis results with the lighting parameters of each spotlight.The objective was to identify lighting configurations that consistently promoted good health status across all plant species.However, no single configuration resulted in excellent status for all four plant species.Nevertheless, four spotlights (Spot B, Spot C, Spot G, and Spot H) demonstrated favorable outcomes, yielding acceptable to very good health status across all four plant species.
Particularly, an interesting trend emerged upon inspecting the spectra of these identified configurations.All four spotlights displayed a R/B ratio lower than 3.0 and a R/G ratio lower than 1.0, indicating that these specific ratios might be crucial in promoting optimal growth and health among the diverse set of ornamental plant species under investigation.Further exploration of these spectral characteristics could offer valuable insights for designing lighting strategies that cater to the health and well-being of indoor greenery in various architectural settings.

Conclusions
The primary objective of this project was to establish that light sources with standard CCTs in the 2800K-5000K range can support the healthy development of plants, given careful tuning of their spectrum to provide for plants' specific needs.Through a year-long experiment, it was possible to identify several spectra leading to good health status across all studied plant species while maintaining a pleasing and natural visual appearance.Notably, specific ranges of R/B and R/G ratios indicating favorable impacts on plant health and development were identified.
Furthermore, the research provided evidence that a lower light level of 1,000 lx and a PPFD value of approximately 16.0 μmol•m -2 •s -1 are sufficient to facilitate a healthy growth of the investigated plant species.These findings offer valuable insights into optimizing lighting conditions for indoor ornamental plants, potentially influencing lighting design strategies to promote plant well-being and aesthetics within various architectural spaces.
Once the experiment was completed, it was possible to come up with some guidelines for architects and designers approaching lighting design for indoor plants: • Use the right tools.The quantity and quality of light for plants cannot be assessed through lumens or lux levels using a normal luxmeter, but it must be assessed using a quantum meter able to measure the DLI and the PPFD, ideally in the wavelengths range 300-800nm.CCT is relevant to assess the visual appearance that plants will have together with the surrounding space, but it cannot be used to define the spectrum illuminating the plants.To define the spectrum, it is necessary to refer to the R/B and R/G ratios, along with the PPFD value.• Understand your client.It is important to identify which plants species are going to be used in the space, to understand which needs they have in terms of light, other than water, air and soil.As demonstrated by this research project, the same lighting configuration could be detrimental for some plants species and entirely effective for other species.• Understand the context.Depending on the function of the space in which the plants are integrated, a specific color temperature of light may be preferred to create a harmonious and visually appealing space.It is possible to use a specific color temperature -in the 2800-5000K range -as long as the spectrum is carefully designed.It must be considered, though, that for the same CCT -and thus overall visual effect -it is possible to design spectra that will have different impacts on plants, by changing the proportion of the radiation that makes up the spectra themselves.Spectra with the same CCT could increase or decrease the growth rate, allow, or not allow flower development or lead to more or less compact growth, just to mention a few.According to on-site needs, a specific spectrum of light will need to be selected.• It's possible to be sustainable.As proven by the research project, plants can grow healthily under lower light intensity, if the light spectrum is carefully tailored on their needs.Using the right light spectrum allows the optimization of the light output and thus the reduction of the light intensity needed for plant subsistence, and consequently enables significant energy savings.In the absence of daylight, an amount of light of 800-1000 lx from the right spectrum can ensure proper plant development.It's important to highlight that this value refers to an arrangement of accent lighting on the plants, in a confined environment where the test spotlights were the only source of light in the room.In a real setting, with spill lighting from the surroundings and possible presence of daylight, this value can be further reduced, and further energy savings can be achieved.
Further investigation and refinement of the identified spectra and light parameters may lead to more tailored lighting solutions for enhancing indoor greenery in the built environment.Illumination of ornamental plants indoors is a complex and interdisciplinary topic, involving daylight planning, building physics, interior design, and a collaboration between all the disciplines is necessary to find the optimal solutions.

Figure 1 .
Figure 1.Difference between visual effects of horticultural lighting (left picture) and lighting for indoor ornamental plants (right picture)

Figure 2 .
Figure 2. A comparison of action spectra for plants' photosynthesis and humans' visual perception.Source: Zielinska-Dabkowska et al.Sustainability 2019

Figure 3 .
Figure 3.A comparison between metrics used to evaluate light for plant and for humans.Source: Zielinska-Dabkowska et al.Sustainability 2019

Figure 4 .
Figure 4. Images of the plant's species used for the experiment.From left to right: Codiaeum Petra, Dracaena Janet, Epipremnum and Spathiphyllum

Figure 5 .
Figure 5. Sketch of the test set-up

Figure 6 .
Figure 6.Pictures of the plants under the different lighting arrangements.From top left to bottom right: Spotlight A, Spotlight B, Spotlight C, Spotlight E, Spotlight F, Spotlight G, Spotlight H.

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Enhanced Biophilic Design -By developing lighting solutions that meet specific needs of both people and plants, this research can contribute to the advancement of biophilic design principles.Harmonious and biologically supportive lighting environments can create a stronger connection between occupants and nature, leading to improved mental well-being and productivity.• Optimized Plant Health.Identifying spectra within the typical CCT range that promote healthy plant growth and development can lead to better care and maintenance of ornamental plants in indoor spaces.This knowledge can help architects and designers create conditions that ensure the longevity and vitality of greenery.• Energy Efficiency and Reduction of Maintenance.By questioning the standard reference value of illuminance for plant growth and exploring lower light levels that are still favorable to plants' health, the research can contribute to energy-saving efforts.Reducing the light intensity while maintaining plants' well-being with a natural and compact growth can lead to more sustainable indoor environments with lower energy consumption and lower maintenance costs.• Scientific Contribution.The research's exploration of recommended DLI and PPFD factors for various indoor ornamental plant species fills a gap in scientific literature.This knowledge can serve as a valuable reference for future studies and contribute to a better understanding of optimal lighting conditions for indoor plants.• Practical Applications.The development of a family of LED chips with tailored spectra for plants' needs that can be integrated into standard architectural lighting fixtures, can have practical applications in the lighting industry.Lighting designers can use this technology to create customized and efficient lighting solutions for both the space users and the ornamental plants in indoor spaces.
The analysis of the data presented in Table2reveals significant variations in the responses of different plant species to the various lighting arrangements.The Dracaena Janet displayed robust growth and development across all lighting configurations, maintaining a healthy pace.Minor leaf signs of burnings or water deficiency were observed, but overall, the plants remained in excellent condition.