Assessing the Role of Sunlight Exposure in Lighting Performance and Lighting Energy Performance in Learning Environments: A Case Study in South Korea

Abstract

In South Korea, sunlight rights and daylight rights are legally distinguished, yet no standardized methodology exists for their quantitative assessment. Current evaluations of sunlight rights are narrowly defined, relying on the duration of direct solar penetration at the window center during the winter solstice, while excluding reflected and diffuse light. This restrictive approach has led to confusion among both researchers and legal practitioners, as it diverges from daylighting evaluations that account for indoor brightness and energy performance. The recent enactment of regulations to secure solar access in schools has further intensified disputes between educational institutions striving to protect students’ visual comfort and developers seeking to maximize building potential. To address this gap, this study proposes an evaluation framework tailored to the Korean context. A reference classroom model representative of standard Korean schools was developed, and simulations were conducted by introducing obstructing building masses to block direct sunlight. The methodology evaluated key variables, including time of day and solar altitude, and analyzed daylighting performance and lighting-related energy consumption under obstructed conditions. The results show that blocking sunlight through south-facing windows reduces daylighting performance by 89% to 98%, leading to additional reliance on artificial lighting, with energy use increasing between 128 Wh and 768 Wh. These findings underscore the limitations of current legal interpretations based solely on sunlight duration and highlight the necessity of adopting performance-based evaluation methods. Protecting school sunlight rights through such approaches is essential to enhancing classroom visual environments and reducing energy demand.

1. Introduction

The right to sunlight refers to the entitlement to uninterrupted access to direct solar radiation, free from obstruction by surrounding buildings. As urban density increases in both commercial and residential districts, including those in South Korea, the demand for natural light has grown, and the protection of this right has become a pressing concern. Integrating solar and daylight rights into early stages of urban planning is widely recognized as essential for achieving sustainable and balanced development. In low- and medium-density zones, established methodologies and tools support urban design practices that ensure compliance with solar access requirements and maintain adequate exposure to natural light [1,2].

In South Korea, however, the legal framework separates the right to sunlight from the broader concept of daylighting, which encompasses direct sunlight, reflected light, and skylight. Sunlight rights are narrowly defined as the entitlement to direct solar access without interference. This distinction has created significant challenges, as no standardized methodology or quantitative indicators exist to evaluate or compare the relative value of these rights. The Korean Supreme Court has ruled that “if a resident on a neighboring site suffers a disadvantage due to a reduction in direct sunlight caused by the construction of a new building, such construction falls outside the scope of a legitimate exercise of rights. For the construction to be deemed illegal under judicial law, the degree of interference with the right to sunlight must exceed the threshold commonly recognized by social norms”. This ruling has since served as the principal precedent for legal judgments regarding sunlight rights.

The Building Act references “sunlight” primarily in relation to height restrictions intended to secure solar access. However, a statutory definition of a “right to sunlight” does not formally exist. Instead, judicial interpretations generally assess sunlight rights in terms of the number of hours during which direct sunlight enters a window. While some rulings take into account building use and site conditions, evaluations remain limited to measures of total or continuous sunlight duration, reflecting a lack of comprehensive assessment metrics.

The situation has become more complex since the enactment of the Education Environment Protection Act in 2017, which introduced stricter standards for securing solar access in schools. This legislation has heightened conflicts between schools seeking to protect the learning environment and developers pursuing larger-scale residential and commercial projects. Yet, despite these legal developments, there remains a critical absence of scientific and engineering-based evaluation tools capable of quantifying the performance value of sunlight rights in educational contexts.

This study addresses this gap by developing an evaluation framework tailored to the Korean context, focusing on the quantitative assessment of natural and artificial lighting conditions in school classrooms. Through the use of performance simulations and regulatory analysis, the study examines how solar access influences daylighting performance, visual comfort, and lighting-related energy consumption, thereby providing evidence to inform both policy and design practices.

2. Literature Review on Daylighting and the Right to Sunlight

2.1. Daylighting in Schools

The right to sunlight has increasingly been recognized as a critical factor in urban planning and building design, particularly for educational environments. Previous studies have emphasized the technical, regulatory, and human-centered aspects of sunlight access. Lighting is a critical factor in school design, serving as a key enabler of academic performance and playing a vital role in child development [3]. Historically, school buildings have been designed with a primary focus on optimizing classroom conditions. However, the increasing emphasis on environmental sustainability and lighting energy performance has led to more compact architectural designs, often resulting in limited access to natural daylight in learning spaces [4]. Between 2000 and 2007, over 5000 new schools were constructed in the United States to accommodate the needs of K–12 students. During this period, national initiatives promoted the incorporation of daylighting, improvements in lighting energy performance, and the integration of renewable energy technologies in school designs, all of which contribute to a more effective learning environment [5].

A large-scale study analyzing test scores from more than 21,000 students across three school districts in California, Washington, and Colorado employed multivariate linear regression to control for external factors influencing academic performance. In one district, students in classrooms with the highest levels of daylight progressed 20% faster in mathematics and 26% faster in reading compared to those in classrooms with the least sunlight. In the other two districts, students in well-lit classrooms achieved test scores that were 7% to 18% higher than those in less illuminated environments [5].

Similarly, a study of over 1200 students in North Carolina found that natural light exposure increased student participation rates by 5% to 14% [6]. Another study examining health, behavior, and cortisol levels in 90 elementary school children in Sweden revealed that insufficient natural light negatively affected hormone levels, concentration, cooperative behavior, and overall physical health [7]. Additionally, students with greater exposure to natural light demonstrated lower incidences of dental decay. In Canada, research on the effects of school lighting on physical development and academic performance among elementary students showed that classrooms equipped with artificial lighting that mimicked daylight conditions correlated with better attendance, achievement, growth, and development, as well as fewer dental cavities [8]. Collectively, these studies confirm that natural light is closely linked to student health and well-being while also offering energy savings related to electric lighting and heating.

Light is a critical environmental factor that influences various physiological functions, including pulse rate, blood pressure, and brain activity [9]. In Educational Facilities Planning, Tanner highlighted that “poorly lit and windowless classrooms can cause students to experience a daily form of jet lag; furthermore, certain types of fluorescent lighting may negatively affect students and teachers, potentially triggering mild seizures” [10]. Research further indicates that students who learn in daylight-rich environments tend to exhibit improved eyesight, enhanced growth, and stronger immune systems [11]. Increased sunlight exposure also facilitates greater vitamin D synthesis, which enhances calcium absorption, contributing to improved dental health and overall growth, particularly in children. Natural daylight provides the broadest spectral content of any light source, delivering optimal visual conditions while minimizing eye strain [11]. Additionally, windows allow students to view the outdoors, which provides necessary visual relief—requiring at least 50 feet of unobstructed view—and offers subtle, non-disruptive distractions that can enhance focus and well-being [9].

A previous study conducted a comprehensive review of existing regulations and metrics designed to ensure direct sunlight availability in buildings, along with assessment methods implemented across various countries and cities. It critically analyzed the approaches, methodologies, and tools governing rights of access to sunlight. Furthermore, the study examined case studies from former socialist countries, where stringent long-term regulations were enforced to safeguard direct sunlight access in residential buildings [12]. Recent research (2024) has demonstrated that daylight performance can vary significantly under regulatory frameworks, such as the Basque Country’s Habitability Decree, indicating that fixed daylight ratios do not necessarily ensure adequate natural lighting. This emphasizes the importance of context-specific assessments, particularly in dense urban settings such as Seoul, where school buildings are often surrounded by taller developments [13]. This study provides a robust conceptual and methodological framework for analyzing sunlight rights in Korean educational environments. Building on these insights, the present study quantitatively evaluates how sunlight obstruction impacts classroom illuminance and the resulting energy demand for artificial lighting, highlighting both legal compliance and environmental performance as key considerations in sunlight rights protection.

2.2. Right to Sunlight in South Korean Law

As South Korea’s economy, society, and culture continue to evolve, there is a growing demand for improved living conditions, enhanced individual rights, and higher-quality residential and educational environments. However, these demands often lead to conflicts related to privacy invasion, noise pollution, and deteriorating air quality—challenges associated with increasing urban density. Among these concerns, the “right to sunlight” has emerged as a significant social issue.

In response to urban overcrowding, South Korea introduced the Building Act Enforcement Decree in 1971 to establish minimum living standards. By 1976, the term “right to sunlight” was first incorporated into legal discourse, particularly in relation to height restrictions on new buildings. Currently, South Korean regulations regarding sunlight rights are found in Article 86 of the Enforcement Ordinance of the Building Act and Article 61 of the Building Act. These provisions mandate a minimum setback distance between buildings along the north–south axis and require that structures receive at least two hours of direct sunlight between 9:00 a.m. and 3:00 p.m. on the winter solstice.

Internationally, approaches to sunlight rights vary. In the United Kingdom, the Prescription Act of 1832 established a legal framework for sunlight access, granting sunlight rights to buildings that have received uninterrupted sunlight for more than 20 years, provided there is an explicit or implied agreement. The Rights of Light Act of 1959 further reinforced these protections by defining the right to sunlight as an easement, ensuring that property owners have the legal right to receive a specified amount of sunlight. This law protects light entering a building through windows from obstructions on adjacent properties, provided there is evidence of uninterrupted sunlight exposure for over two decades or a legally recognized mutual agreement [14,15].

In Japan, the right to sunlight is a broader concept encompassing not only sunlight access but also ventilation and building pressure regulation. Japan enforces shadow regulations that limit the duration of shadow casting on buildings to ensure adequate daylight access. Additionally, absolute and diagonal height restrictions—including north-side, road-facing, and adjacent branch-line limitations—are codified in Article 56 of the Building Standards Act to mitigate shadow-related impacts. These restrictions are applied at the municipal level and vary by zoning district [16,17].

In European nations such as France, Germany, and Switzerland, sunlight obstruction is addressed under the legal principles of neighborhood disturbance or infringement on quality of life. France and Switzerland recognize excessive sunlight obstruction as an abuse of rights, allowing affected parties to seek legal remedies. Conversely, in Germany, sunlight access is regarded as a passive concern without explicit legal protection [18].

In the United States, environmental rights are broadly addressed in the Fifth and Ninth Amendments, but no Supreme Court rulings specifically protect sunlight access. While lower courts have addressed state-level disputes, these cases generally prioritize property ownership over sunlight conservation. A notable example is the Fontainebleau Hotel Corp. v. Forty-Five Twenty-Five, Inc. case in Miami Beach, Florida. In this dispute, the Fontainebleau Hotel’s expansion cast a shadow over the neighboring Eden Roc Hotel. However, the court ruled in favor of the Fontainebleau Hotel, affirming that the expansion was a legitimate exercise of property rights rather than an unlawful nuisance. This case reflects the U.S. legal system’s tendency to prioritize land ownership over environmental considerations related to sunlight access [19,20].

The legal frameworks governing sunlight rights differ across jurisdictions, influencing how disputes are evaluated and resolved. Determining the extent of damage caused by sunlight obstruction is inherently complex, and in South Korea, legal loopholes have led to frequent disputes. While the Supreme Court of Korea has ruled on cases involving sunlight rights—such as the 2004.10.28 ruling (2002 Da 63565)—the criteria for evaluating sunlight rights remain ambiguous. Consequently, civil complaints and legal battles continue to rise, exacerbated by regulatory interpretations that are open to competing legal perspectives.

In the context of educational institutions, the Educational Environment Protection Act of 2017 introduced stringent regulations to ensure optimal learning conditions for students. Sunlight regulations for school buildings are stricter than those for general buildings, requiring assessments of potential sunlight infringement caused by surrounding structures. However, developers often perceive these regulations as obstacles to maximizing profitability, leading to frequent conflicts between school administrators and private developers. While most construction projects near schools comply with general sunlight regulations, they often struggle to meet the additional requirements set by educational environment protection laws. Therefore, legal and technical solutions are necessary to balance development interests with the preservation of sunlight access in educational settings, thereby preventing future conflicts.

3. Lighting Standards for Schools in South Korea

Illumination standards in South Korea are established by the Korean Standards Association (KSA). For schools, specific standards are outlined in KS A 3011, issued by the Korean Standards Association in collaboration with the National Institute of Technology and Standards and the School Health Act. These standards define appropriate illumination levels based on location and the nature of activities conducted within the space [21].

The luminance standard for classroom blackboards is determined by the required contrast between the blackboard surface and the surrounding general luminance levels. Additionally, luminance requirements for visual tasks involving small objects are considered in defining these standards. A comprehensive overview of school illumination standards is provided in

Table 1. Korean Standards Association Illuminance Standards (KS A 3011: 1998) for Schools [lux].

Location/Activity	Illuminance Level	Location/Activity	Illuminance Level
Auditorium, assembly room	150–200–300	Classroom (blackboard)	300–400–600
Stairs, corridors, elevator	300–400–600	Faculty Office, Office, Guard room, conference room	150–200–300
Labor room	300–400–600	Reading books	600–1000–1500

.

The lux values in Table 1 represent the minimum required illuminance levels for comfortable visual tasks. In South Korea, the standard illuminance for schools is set at 400 lux, with a maximum of 600 lux. Classroom blackboards must receive a minimum of 300 lux, which aligns with the standard for desk surfaces.

In comparison, the Illuminating Engineering Society of North America (IES), which establishes illumination standards in the United States, recommends an illuminance range of 300–500 lux for most classroom visual activities [22]. Additionally, blackboards and specialized classrooms are required to meet a higher standard of 500 lux. Similar minimum standards are outlined in EN 12464-1, which governs illumination requirements in Europe [23,24].

Under South Korea’s Education Environment Protection Act (2017), school buildings are classified as “living rooms”.

Table 2. Korean Standards Association illuminance standards (KS A 3011: 1998): Residence [lux].

Location	Activity	Illuminance Level [min-avg-max]
Living room	Entertainment	150–200–300
	Reading, makeup	300–400–600
	Handicraft, sewing	600–1000–1500
	Overall	30–40–60

outlines the illuminance requirements for living rooms. According to these regulations, the required illuminance range for classrooms with blackboards is equivalent to that for activities such as reading and makeup application, which involve similar visual demands.

Additionally, The Rules on the Standards for Evacuation and Fire Protection of Buildings specify a minimum illuminance level of 150 lux for activities such as reading, eating, and cooking (

Table 3. Korean Illumination standards for living rooms in the Rules on the Standards for Evacuation and Fire Protection.

Location	Activity	Illuminance Level [lux]
Living	Reading, Cooking	150
	Overall	70

). This standard is lower than the illuminance requirements set by the Korean Standards Association (KS A 3011) for classrooms.

South Korea’s Building Act establishes sunlight rights standards, requiring either a minimum of four total hours of sunlight between 08:00 and 16:00 on the winter solstice or two consecutive hours of sunlight between 09:00 and 15:00. However, the Protection of the Educational Environment Act imposes more detailed and stringent regulations.

For school buildings, all windows must comply with sunlight criteria, and the required sunlight hours vary by school level. Schools must ensure at least four total hours of sunlight between 08:00 and 16:00. Additionally, kindergartens and elementary schools must receive at least two consecutive hours of sunlight between 09:00 and 13:00, junior high schools between 09:00 and 14:00, and high schools between 09:00 and 15:00.

For outdoor sports grounds, a minimum of two total hours of sunlight is required between 08:00 and 16:00. Furthermore, kindergartens and elementary schools must secure at least one consecutive hour of sunlight between 09:00 and 13:00, junior high schools between 09:00 and 14:00, and high schools between 09:00 and 15:00.

In South Korea, regulations governing daylighting operate separately from those addressing the right to sunlight, and the absence of provisions to quantify or supplement daylighting values has led to ongoing controversy. As a result, there is a need to analyze the value of sunlight more comprehensively by evaluating its impact over specific time periods and converting it into indoor daylighting performance.

4. Methods

4.1. Overview

In South Korea, the evaluation of the right to sunlight is determined by calculating the duration of direct solar penetration at the center point of a window. This assessment is highly restrictive, as it is based solely on the winter solstice—the day with the lowest solar altitude—and excludes both reflected and diffuse light. Consequently, sunlight rights are recognized only under limited conditions.

This narrow legal interpretation has led to confusion among both researchers and legal practitioners, as it differs fundamentally from the concept of daylight rights, which relate to indoor brightness. For example, the law interprets a one-hour violation as the complete absence of direct solar penetration for that duration. In reality, however, the actual solar flux and indoor illuminance vary significantly depending on solar altitude and azimuth. Furthermore, the associated lighting energy demand also differs according to daylighting performance.

The purpose of this study is to establish an evaluation framework for school environments that reflects the Korean context, thereby addressing the growing disputes between schools seeking to preserve a visually comfortable environment and neighboring developers pursuing construction interests. As noted earlier, the Korean system evaluates sunlight exclusively in terms of direct penetration. To replicate real-world infringements of sunlight rights, this study introduced an obstructing building mass in simulation models to block direct solar access to the reference classroom. Subsequently, indoor daylighting performance and lighting energy consumption were analyzed under obstructed conditions to quantify the impact of reduced sunlight exposure.

4.2. Simulation Tools

This study assessed lighting performance and lighting energy consumption using Radiance 6.0 and ReluxDesktop 2024.2 software, respectively. Radiance is a physically based lighting simulation software widely used in architectural research for daylighting performance analysis. It employs a ray-tracing algorithm to accurately model the transport of light, including direct and diffuse components, through complex geometries and material properties. The program allows for precise calculation of illuminance, luminance, glare indices, daylight factors, and other lighting performance metrics under various sky conditions. In this study, Radiance was used to compute direct sunlight availability and resulting illuminance in classrooms, providing quantitative outputs for evaluating the impact of right-to-sunlight infringements on lighting performance and associated energy consumption.

Ensuring the representativeness of the model space is critical in architectural research. In this study, the classroom selected as the model represents the most common and typical classroom module in South Korea. Although educational facilities may vary according to different operators, such as private or public institutions, the vast majority of schools in South Korea are public and generally adhere to standardized classroom forms and dimensions developed by the Ministry of Education. These standardized classrooms are periodically upgraded in response to curriculum revisions. In this context, the classroom model selected for this study can be considered representative. While this may differ somewhat from concepts of international standardization, it nonetheless reflects one of the typical forms of advanced educational spaces [25,26].

Relux is a widely used lighting simulation software for evaluating both artificial and daylighting systems in architectural spaces. It allows for detailed modeling of lighting installations, including lamp types, luminaires, reflectance of surfaces, and control strategies. The software employs photometric calculations based on the principles of radiometry to compute illuminance distributions, energy consumption, and lighting performance metrics such as uniformity and glare indices. In energy analysis, Relux can simulate the operation of artificial lighting in response to daylight availability, enabling estimation of energy savings from daylighting integration and the impact of different lighting designs. In this study, Relux was used to quantify the energy consumption of artificial lighting required to compensate for reductions in daylight due to right-to-sunlight infringements [27,28,29].

Previous research (2006) examined the validity of sunlight calculations in legal contexts, demonstrating that accurate modeling of window frames, glazing, and interior reflectance is essential for assessing compliance with sunlight access requirements. Such methodological rigor provides a basis for evaluating sunlight exposure in buildings subject to legal protection [30].

The results obtained from RELUX and Radiance simulations were analyzed under identical conditions, including space dimensions, surface finishing materials, and sky conditions. A clear sky model was applied to assess sunlight rights in South Korea, considering only direct solar radiation.

4.3. Target Configuration of Classroom

In this study, the classroom selected as the model represents the most common and typical classroom module in South Korea (Figure 1). Although educational facilities may vary according to different operators, such as private or public institutions, the vast majority of schools in South Korea are public and generally adhere to standardized classroom forms and dimensions developed by the Ministry of Education. These standardized classrooms are periodically upgraded in response to curriculum revisions. In this context, the classroom model selected for this study can be considered representative.

To evaluate sunlight access in school classrooms, a reference window was defined with dimensions of 3.0 m (width) × 1.8 m (height), with its bottom edge positioned at 0.8 m above the floor. The solar geometry was calculated for Seoul, where the solar altitude on the winter solstice reaches 7° at 8:00 a.m. and 29.1° at noon (Figure 2a), and the corresponding sun azimuth angles were determined based on true solar time (Figure 2b). These parameters were used to simulate the temporal variation in direct sunlight penetration into the reference classroom. By integrating obstructing building masses in the simulation, the study assessed the impact of blocked sunlight on indoor illuminance levels and associated lighting energy consumption, thereby quantifying the practical implications of sunlight rights violations under winter conditions.

To evaluate daylighting performance, a grid of measurement points, arranged in 9 rows by 9 columns, was established within the space illustrated in Figure 1. To avoid extremely low illuminance values near the room corners, all points were positioned 0.6 m away from the surrounding walls.

To analyze supplementary lighting energy consumption in cases where indoor illuminance from sunlight was insufficient, the energy required to power a fluorescent light (32 W × 2 bulbs)—a common lighting setup in South Korean classrooms—was calculated. This approach helps evaluate the energy-saving benefits of direct sunlight as well as its impact on the visual environment. A target space based on a typical classroom module in South Korea was configured, as shown in Figure 1.

The target space was designed with beige wallpaper on the walls (reflectance: 45%), an ivory paper-covered ceiling (reflectance: 85%), and a gray concrete floor (reflectance: 31.2%).

Table 4. Material properties for the target space.

Element of Target Space	Property of Materials
Wall	Reflectance	45%
Floor		31.2%
Ceiling		85%
Glazing	Transmittance	82%

presents the material properties used in the simulation.

Considering this current reality, the study was conducted using fluorescent lamps as the reference artificial lighting system for lighting energy simulation. This choice ensures that the analysis reflects typical conditions in existing classrooms.

4.4. Winter Solstice Scenario for Right-to-Sunlight Analysis

The primary objective of protecting the right to sunlight in educational environments is to ensure that adequate direct sunlight is available during the periods when natural daylight is most limited, considering both season and time of day. Accordingly, it is reasonable to evaluate direct sunlight under the most unfavorable conditions—namely, on the winter solstice when solar altitude is at its lowest. Using the Radiance program, accurate illuminance calculations can be performed for this critical period. Therefore, for the purposes of this study, consideration of seasonal or long-term operational variations is not required, as the analysis already reflects the worst-case scenario for daylight availability.

The classroom lighting performance was analyzed based on time periods and orientation (Figure 2). To evaluate the impact of direct sunlight on indoor daylighting performance, comparisons were made between scenarios where sunlight was obstructed by nearby buildings and those where sunlight remained unobstructed. In the absence of sunlight obstruction, direct solar penetration ensures sufficient illuminance in classrooms, minimizing the need for artificial lighting. However, the construction of adjacent buildings can block direct sunlight, thereby reducing illuminance and increasing reliance on artificial lighting. To assess this impact, scenarios were modeled using a daylight rights analysis program, in which new buildings with adequate width and heights exceeding solar altitude were assumed to block sunlight completely. The resulting decrease in illuminance and the corresponding artificial lighting energy demand were quantified, thereby demonstrating the quantitative importance of the right to sunlight.

5. Analysis and Results

The simulation results were analyzed based on time period, solar elevation, and distance from windows by comparing direct sunlight inflow with scenarios where sunlight was blocked by nearby buildings. The analysis was conducted on the winter solstice, the standard reference date for evaluating sunlighting performance.

5.1. Indoor Illumination by Time Period

First, the sunlighting performance over time was analyzed for a south-facing building. The indoor average illuminance showed a tendency to increase both before and after sunlight infringement occurred, followed by a slight decrease around 12:00, as shown in Figure 3.

Before sunlight infringement, the maximum average illuminance reached 9074 lux at 11:00, while the minimum was 714 lux at 8:00. After infringement, the maximum illuminance dropped to 214 lux at 11:00, the same time as before infringement, resulting in a difference of 8860 lux. This indicates that only 2.4% of the original sunlight was available after infringement. Similarly, the minimum average illuminance after infringement occurred at 8:00, with a value of 78 lux, meaning 10.9% of the pre-infringement sunlight remained. Overall, the percentage of available sunlight decreased progressively over time after infringement.

5.2. Indoor Illumination by Solar Elevation

This section examines how indoor illuminance changes with solar elevation when observed through a fixed window. As the sun’s elevation increases, the average indoor illuminance also tends to rise, both before and after sunlight infringement. However, after infringement, illuminance increases with solar elevation but slightly decreases when the sun reaches 29.1°.

Before sunlight infringement, the maximum average illuminance was 9056 lux at a solar elevation of 29.1°. After infringement, the highest recorded illuminance was 374 lux at 27.4°, marking an 8682 lux reduction. At 29.1° elevation, the illuminance before infringement peaked at 9056 lux but dropped to 210 lux after infringement, a 97.7% decrease. The lowest illuminance after infringement occurred when the sun’s elevation was 7.0°, with a drop from 1192 lux to 111 lux, reflecting a 90.7% reduction (Figure 4).

5.3. Indoor Illumination by Distance from Window

Sunlighting performance over time was analyzed in a south-facing building. At 8:00, before sunlight infringement, indoor illuminance showed a decreasing trend as the distance from the window increased (Figure 5). After infringement, the illuminance further decreased with increasing distance from the window.

At a distance of 1.4 m from the window, the largest difference in illuminance before and after sunlight infringement was observed, measuring 601 lux. Conversely, at 6.9 m from the window—where both pre- and post-infringement illuminance values were lowest—the difference was minimal, at 250 lux.

At 9:00, the pattern of indoor illuminance variation with distance from the window differed from that observed at 8:00, as illustrated in Figure 6. Similarly, at 10:00, the distribution of interior illuminance followed a different pattern than at 8:00. Before sunlight infringement, when the distance from the window increased beyond a certain point, indoor illuminance initially rose sharply and then declined again as the distance further increased.

At 10:00, sunlight entered from the southeast, preventing direct light from reaching the center of the classroom—the analysis point—resulting in higher illuminance at shorter distances. Additionally, because the window was not directly front-facing, illuminance remained low even at close distances. However, after sunlight was obstructed, the overall illuminance levels remained low, regardless of the effect of the windowsill height, and continued to decrease as the distance from the window increased.

Between 2.2 and 4.5 m from the window, more than 99% of the sunlight was blocked before infringement, leading to a significant decrease in illuminance. At 3.0 m—the point where the highest illuminance value was recorded before sunlight infringement—the difference in illuminance before and after infringement reached 20,235 lux.

At 12:00, the illuminance pattern changed from that observed at 10:00, with relatively lower illuminance values recorded (Figure 7). Additionally, as the distance from the window increased, illuminance consistently decreased both before and after sunlight infringement. The points with the lowest percentage of retained sunlight at 12:00 were compared to the highest illuminance values recorded during earlier time periods. Because both the windows and the sun were positioned on the south-facing side at 12:00, and the analysis point was located behind pillars between the windows, the measured illuminance primarily resulted from diffused light rather than direct sunlight.

5.4. Artificial Lighting Consumption in Relation to Sunlight Infringement

An analysis of indoor illumination under varying sunlight conditions was conducted to assess changes in artificial lighting consumption when the lighting standard was set to 300 and 500 lux. Using a spatial model developed in the Radiance program, RELUX calculates the number of lighting fixtures necessary to achieve the target illuminance under specified natural lighting conditions.

5.4.1. Impact of Sunlight Infringement on Artificial Lighting Consumption

Table 5. Artificial lighting usage based on time variation: lighting standard (300 Lux).

	8:00	9:00	10:00	11:00	12:00
Number of lighting used before sunlight infringement	6	2	0	0	0
Power consumption (W) according to lighting usage before sunlight infringement	384	128	0	0	0
Number of lighting units used after sunlight infringement	12	12	12	11	11
Power consumption (W) according to lighting use after sunlight infringement	768	768	768	704	704
Differences in the number of lighting used before/after sunlight infringement	6	10	12	11	11
Difference in power consumption before/after sunlight infringement (W)	384	640	768	704	704

presents the analysis results of artificial lighting usage over different time periods, based on a 300 lux standard—the minimum classroom illuminance requirement in Korea (KS A 3011; 1998).

The number of artificial lighting units in use decreased over time both before and after sunlight infringement. Before infringement, the number of active electric lighting units dropped from six at 8:00 to two at 9:00, with no artificial lighting required after 10:00. However, from 8:00 to 10:00, all 12 artificial lights were used to illuminate the space due to low illumination, and 11 lights were used from 11:00 to 12:00. This indicates that prior to sunlight infringement, natural light sufficiently met indoor illuminance standards. After infringement, as sunlight decreased, artificial lighting became necessary to maintain the required indoor illumination levels. Consequently, energy consumption increased from 384 W to 768 W due to the additional use of artificial lighting.

A comparative analysis of artificial lighting usage based on changes in solar elevation is presented in

Table 6. Artificial lighting consumption based on changes in solar elevation: Lighting standard (300 lux).

	7.0°	15.8°	22.8°	27.4°	29.1°
Number of lighting used before sunlight infringement	2	0	0	0	0
Power consumption (W) according to lighting usage before sunlight infringement	128	0	0	0	0
Number of lighting used after sunlight infringement	12	10	10	10	11
Power consumption (W) according to lighting use after sunlight infringement	768	640	640	640	704
Differences in the number of lighting used before/after sunlight infringement	10	10	10	10	11
Difference in power consumption before/after sunlight infringement (W)	640	640	640	640	704

, using an illuminance standard of 300 lux. Before sunlight infringement, artificial lighting was needed only at a solar elevation of 7.0°, with two electric lights in operation; beyond this point, natural daylight was sufficient, and artificial lighting was not required. However, after sunlight infringement occurred, artificial lighting was necessary at all solar elevations, indicating a significant reliance on artificial lighting due to reduced daylight availability.

Before sunlight infringement occurred, when the sun’s elevation was low, sunlight was able to penetrate deep into the classroom. However, due to a wall partially covering the side of the window, illumination levels remained low in that area, necessitating artificial lighting. As the sun’s elevation increased, the combination of direct and diffused sunlight provided sufficient illumination, eliminating the need for artificial lighting.

In contrast, after sunlight infringement took place, sunlight penetration into the classroom was significantly reduced, making artificial lighting necessary regardless of solar elevation. As a result, the availability of natural light decreased dramatically, leading to an additional power consumption of 640 W to 704 W compared to before infringement.

In many schools, a 500-lux lighting standard is recommended to create an optimal learning environment. When this higher standard was applied, power consumption further increased, as shown in

Table 7. Artificial lighting usage over time: lighting standard (500 Lux).

	8:00	9:00	10:00	11:00	12:00
Number of lighting used before sunlight infringement	10	4	2	1	0
Power consumption (W) according to lighting usage before sunlight infringement	640	256	128	64	0
Number of lighting used after sunlight infringement	12	12	12	12	12
Power consumption (W) according to lighting use after sunlight infringement	768	768	768	768	768
Differences in the number of lighting used before/after sunlight infringement	2	8	10	11	12
Difference in power consumption before/after sunlight infringement (W)	128	512	640	704	768

.

Even before sunlight infringement, 10 lighting fixtures were in use during the morning hours, gradually decreasing throughout the day until artificial lighting was no longer needed by 12:00. However, after sunlight infringement, all 12 fixtures were required in the morning. Even near 12:00, when natural sunlight was expected to provide sufficient illumination, power consumption remained high as artificial lighting was needed to compensate for the loss of natural light.

Table 8. Artificial lighting use based on solar elevation change: lighting standard (500 lux).

	7.0°	15.8°	22.8°	27.4°	29.1°
Number of lighting used before sunlight infringement	6	2	0	0	0
Power consumption (W) according to lighting usage before sunlight infringement	384	128	0	0	0
Number of lighting used after sunlight infringement	12	12	10	10	12
Power consumption (W) according to lighting use after sunlight infringement	768	768	640	640	768
Differences in the number of lighting used before/after sunlight infringement	6	10	10	10	12
Difference in power consumption before/after sunlight infringement (W)	384	640	640	640	768

presents an analysis of artificial lighting usage based on changes in solar elevation, using an illumination standard of 500 lux. When the solar elevation was low (7.0°), six lighting fixtures were needed; however, from 22.8° onwards, artificial lighting was no longer required as increased sunlight penetration illuminated the classroom. After sunlight infringement, however, more than ten lighting fixtures were necessary regardless of the sun’s elevation, leading to an increase in energy consumption from 384 W to 768 W.

5.4.2. Variations in Power Consumption Due to Sunlight Infringement

Figure 8 illustrates the additional power consumption for each illuminance standard over time. When the minimum classroom illumination standard of 300 lux was applied, additional power consumption increased until 10:00 before slightly decreasing around 11:00.

As the sun’s elevation and azimuth shifted, the amount of artificial lighting used remained unchanged, as both 10:00 and 11:00 met the required illuminance standard before sunlight infringement occurred. After infringement, the illuminance at 10:00, initially at 258 lux, increased to 313 lux at 11:00 as sunlight entered, satisfying the illuminance standard and eliminating the need for artificial lighting. At an illuminance standard of 500 lux, additional power consumption increased over time. Before sunlight infringement, natural illumination was sufficient, making artificial lighting unnecessary. However, after infringement, sunlight became inadequate, necessitating the use of artificial lighting even during periods when it was previously not required.

As shown in Figure 9, as the sun’s elevation increased, additional power consumption of 640 W was recorded up to a certain elevation for the 300 lux standard, reaching 704 W at 29.1°. Similarly, for the 500 lux standard, additional power consumption increased from 384 W to 640 W up to a certain elevation, eventually reaching 768 W as the sun’s elevation continued to rise.

6. Discussion

Sunlight rights have traditionally been assessed based on the duration of direct sunlight entering a window, without accounting for the quantity or quality of indoor illumination as influenced by the sun’s movement. Beyond the Building Act, the Education Environment Act—enacted to protect school learning environments—incorporates enhanced sunlight protection standards. However, disputes between school officials advocating for a comfortable learning environment and architectural developers prioritizing maximum profitability remain complex. Ambiguous legal criteria and limited scientific data continue to hinder objective decision-making in such cases.

This study emphasizes the importance of preserving the right to sunlight by quantitatively evaluating its impact on Korean school environments. The novelty of the present study lies in linking reductions in sunlight availability due to right-to-sunlight infringements with both decreases in indoor illuminance and increases in artificial lighting energy consumption. Simulation results compared indoor lighting performance before and after sunlight obstruction and quantified the energy demand of artificial lighting required to compensate for insufficient daylight.

The findings indicate that when direct sunlight was obstructed, indoor illuminance decreased by 89–98%, while artificial lighting energy consumption increased by approximately 128 to 768 Wh. The reduction in daylight performance varied with distance from the window, with a sharp decline observed beyond 3.8 to 4 m, suggesting that nearly half of a typical classroom may be significantly affected by sunlight obstruction. These results demonstrate that protecting the right to sunlight can enhance both learning environments and lighting energy performance in schools.

Furthermore, glare in classrooms represents a critical concern, as high luminance caused by natural daylight can produce visual discomfort beyond the scope of legal sunlight protection. While glare from artificial lighting is regulated under light pollution control laws, there are currently no legal measures addressing glare from natural daylight due to its extremely high luminance and the difficulty of attributing responsibility. Consequently, design guidelines recommend mitigating veiling reflections through strategic furniture and luminaire placement, orienting openings to avoid direct views of the sun, and employing blinds or curtains during periods of low solar altitude.

Although social conflicts arising from urban development and the protection of sunlight rights are recognized issues, a detailed analysis of these conflicts is beyond the scope of the present study. Future research will aim to collect and analyze a wide range of dispute cases to provide a foundation for informed social consensus and policy development.

Finally, future studies should investigate the physiological effects of sunlight exposure on students, as well as potential impacts on academic performance and health outcomes, to further substantiate the benefits of preserving sunlight access in educational spaces.