Measuring Visual Privacy: A Systematic Review of Evaluation Methods, Conceptual Definitions, and Design Strategies

Abstract

With the increasing density of modern cities, visual privacy has become a critical concern in urban communities. The need for privacy arises from religious and cultural beliefs, personal preferences, and security considerations. Urban planners and architects must develop effective strategies to ensure visual privacy in diverse urban settings. This paper aims to systematically review and analyze quantitative and qualitative measurement methods for visual privacy. This study conducted a systematic review of 199 articles retrieved from the Web of Science, Google Scholar, and Scopus databases, covering publications up to April 2025. After applying relevance-based screening criteria, 51 articles were selected for in-depth analysis. The findings categorize visual privacy definitions into three domains: psychological, environmental-physical, and religious-cultural. Various qualitative and quantitative methods for measuring visual privacy have also been reviewed and introduced. Design strategies for enhancing visual privacy are also examined, focusing on urban fabric configurations, residential layouts, façade elements, and building materials in residential structures.

1. Introduction

The term “privacy” encompasses a wide range of meanings and dimensions. In residential buildings, privacy is influenced by various factors, including visual, acoustic, mental, and psychological aspects [1]. Privacy preferences vary among cultures, with some cultures exhibiting stronger inclinations toward it than others [2]. Visual privacy, in particular, is considered a fundamental value in many cultures, and in Islamic cultures, it is rooted in the teachings of the Holy Qur’an and the practices of the Prophet [3]. Visual privacy refers to the ability to carry out daily activities in one’s home without the intrusion of external observation, such as from neighbors, passersby, or others outside the private space [4].

Urban density is a key factor contributing to the reduction of visual privacy [5]. Land use zoning and urban planning regulations play a crucial role in shaping urban density, directly impacting the availability of visual privacy in densely populated areas.

As cities become more densely populated due to limited land availability, urban planning and residential designs need to address the growing challenges related to privacy [6]. Research indicates that many urban residents experience a lack of visual privacy [7], which is often identified as a source of dissatisfaction among residents in high-density areas [2,8,9]. Privacy is closely linked to personal well-being and is a basic requirement for individuals, as outlined in Maslow’s Hierarchy of Needs, with both physical and psychological needs being integrated [10]. While privacy is universally essential, the way it is perceived and approached varies significantly across cultures [7]. The influence of culture, particularly religious beliefs, can shape the significance of visual privacy [11]. A comprehensive review of visual privacy has not yet been conducted.

Despite the importance of visual privacy, there has not yet been a comprehensive review of the various definitions, measurement methods, and design strategies aimed at enhancing visual privacy. Previous studies have explored privacy from multiple perspectives, including cultural and religious viewpoints [11,12]. However, a gap remains in systematically reviewing the quantitative and qualitative methods used to measure visual privacy.

One of the main challenges architects and urban planners face is understanding the required level of privacy for different spaces within urban buildings and determining how to achieve and assess this level. Several studies have been conducted on visual privacy in urban settings [13,14,15], addressing various definitions and measurement methods. This paper aims to fill the gap by presenting a comprehensive review of definitions of visual privacy and systematically analyzing the quantitative and qualitative methods employed to measure visual privacy. Additionally, this review will highlight and categorize the practical design strategies proposed to enhance visual privacy in urban buildings. The study is based on a rigorous search strategy of peer-reviewed articles from well-known databases such as Scopus, Web of Science, and Google Scholar, ensuring the review is grounded in credible academic sources (Figure 1).

The Figure 1 framework was developed based on a comprehensive literature review, outlining visual privacy’s key components and domains. The diagram integrates three main thematic pillars derived from the systematic literature analysis: definitions of visual privacy, methods of analysis, and design strategies. First, the review categorizes definitions of visual privacy into psychological, environmental-physical aspects, and religious-cultural dimensions, reflecting the diverse interpretations shaped by individual perception, spatial context, and cultural or religious norms. Second, it identifies the main methodological approaches for analyzing visual privacy, including quantitative methods (such as spatial simulations and visual exposure metrics) and qualitative methods (such as surveys and behavioral studies). Finally, the diagram outlines the architectural and spatial strategies to enhance visual privacy, including façade elements, glazing types, lattice screens, and spatial configurations, such as urban layouts and shape plans. Together, these components provide a comprehensive framework for understanding how visual privacy is defined, measured, and addressed through design and serve as the foundation for the structure and content of this paper.

2. Materials and Methods

This study employs a systematic review methodology chosen for its ability to provide a comprehensive, unbiased analysis of the existing literature on visual privacy in residential buildings. The systematic review method allows for transparent and replicable research, ensuring that relevant studies are reviewed and analyzed in a structured manner. Data was collected through searches in the Scopus, Google Scholar, and Web of Science databases, focusing on articles related to visual privacy. The search strategy used was: ((TITLE-ABS-KEY (“visual privacy” OR “visual exposure” OR “architecture privacy”) AND (“residential building” OR apartment OR house OR interior)). The selection criteria were based on articles that directly addressed the definition of visual privacy and employed various quantitative methods to analyze it. The articles were chosen for their relevance to the research questions and the quality of their methodologies.

The research process consisted of several steps. Initially, 199 articles were imported into EndNote 21, and duplicates were removed, leaving 174 articles. Next, titles and abstracts were reviewed to exclude irrelevant studies, removing 118 articles. Finally, the full texts of the remaining articles were assessed for quality and relevance, and 51 studies were selected for content analysis (Figure 2). This analysis was conducted based on the various methods of defining and measuring visual privacy outlined in the selected articles. The data collected consisted of peer-reviewed journal articles, primarily in English, which provided insights into the methodologies, findings, and gaps in visual privacy in residential buildings. The review process was updated on 11 April 2025, focusing on articles published in reputable global databases. At the same time, screening and refining the collected records, books, and book chapters were excluded due to their generally outdated content and limited methodological detail compared to peer-reviewed journal articles. Since this systematic review aims to identify and analyze up-to-date and applicable measurement methods for assessing visual privacy in the built environment, priority was given to recent journal publications. Moreover, although listed in the initial search, several titles were unrelated to visual privacy in architecture or urban studies (e.g., articles focused on computer architecture). These were excluded during the full-text screening stage to ensure alignment with the scope of the study. Additionally, when reviewing the titles, articles that did not align with the framework introduced in Figure 1, specifically those that did not address the definition or measurement methods for visual privacy, were also removed from our list.

A limitation of this study is that only English-language articles were included, which may have excluded relevant studies published in other languages.

Figure 2. The Prisma flowchart for the review process method [16].

Figure 2. The Prisma flowchart for the review process method [16].

Ultimately, content analysis was employed to analyze the data from articles and scientific texts. Eventually, 51 studies were fixed for the review, content, and gap analysis. Most studies consist of journal articles, reaching a peak of 7 items in 2024 (Figure 3). While visual privacy is not a novel field of study, the increased number of publications in recent years underscores the significance of conducting new studies in this area.

Moreover, the classification groups, including the definition of visual privacy from various perspectives, methods of visual privacy analysis, methods of creating visual privacy in residential buildings, and environmental conditions, were considered for the content analysis and gap findings in this study.

In the process of this study, an AI-based image generation tool (Midjourney, version 6.1) was employed to create Figure 16 for illustrative purposes. The image was synthetically generated and does not represent any real person.

3. Results

3.1. Definition of Visual Privacy

The reviewed studies concluded that the definition of visual privacy should be introduced from psychological, environmental, and cultural perspectives.

Psychological: In psychological terms, the concept of privacy is intricately linked to the behaviors of individuals as well as the inherent nature and essence of humanity [17]. Privacy, as a concept, entails an informational control mechanism within an individual’s immediate environment. Put differently, it governs the level of proximity and separation between oneself and others [18,19]. Privacy constitutes a bilateral process that aids individuals in achieving autonomy, mitigating harm, and delineating the boundary separating them from others [20]. When the level of visual privacy matches the extent desired by an individual, an optimal level of privacy is achieved, leading to a sense of mental tranquility. Conversely, if it falls short of this, the individual will experience a feeling of being crowded; however, when the level of privacy exceeds what the individual desires, it results in an isolated situation where interactions with the surrounding environment become significantly reduced [19,21,22].

Environmental and Physical Aspects: As living conditions in urban environments improve, people’s expectations regarding the assurance and enhancement of their quality of life are transforming. Visual privacy within urban settings is critical in ensuring that residents can carry out their daily activities inside their homes without external observation [4,7,9,18,22,23,24]. Visual privacy refers to individuals’ capacity to manage the information occurring beyond public scrutiny in their proximity. Visual privacy in urban environments varies based on factors such as the site plan configuration, the height of adjacent buildings, setbacks of city blocks, orientation, the extent of openings, the architectural layout of buildings, the type and material of windows, the manner of use, and the design of balconies [10]. Visual privacy is established when the exposure and openness of a space decrease, allowing residents to feel a sense of control over their personal space [25]. In constructed environments, visual privacy is the capacity of individuals or groups to control visual interactions with others. In architectural spaces, visual privacy is divided into two aspects: between the residents inside a building unit and individuals outside the building. The required level of visual privacy for each of these groups will be different [26].

Religion and culture: Visual privacy is associated with gender, culture, and religion [2]. In Islam, great importance is given to the sacredness and protection of family privacy. Cultural factors, which are shaped by religious elements, can impact the degree of visual privacy. These cultural factors play a crucial role in defining the extent of visual privacy within an Islamic society [11,17,26,27]. The preservation of the family’s sanctity, particularly in shielding it from the scrutiny of non-mahram (stranger), is highly valued across all cultures, with a special emphasis on Islamic societies [28]. Observing visual privacy is a fundamental value incumbent upon Muslims. This principle originates from the Quran and is based on the lifestyle exemplified by the Prophet Mohammad (sunnah) [3,20,29]. In nations adhering to the Islamic faith, women must wear attire that exposes only their faces and hands when they step outside their residences. Moreover, when within their homes and approaching windows or balconies, where they could be seen by individuals outside, they are expected to maintain the same level of clothing coverage [23].

According to the Islamic perspective, visual privacy is described as the protection of the awrat (intimate part) from the gaze of non-mahram [1,7,21,30]. Visual privacy enables individuals to carry out their daily activities inside their homes without being recognized or identified by non-family members or strangers [3]. Visual privacy comprises two main components. The first is the private space within the home among family members, which must be respected. The second is the visual boundary that residents maintain with neighbors and the external environment of the house [31]. Privacy within the family is highly valued in Islam, and the head of the household is entrusted with creating a space where individuals can find comfort for their bodies, minds, and souls. It is within this sanctuary that a feeling of happiness and overall well-being can be nurtured [7]. In Islamic society, achieving visual privacy does not imply complete isolation of individuals from the community, cutting off connections with the surrounding environment, and causing adverse effects on mental well-being and social relationships. Instead, it advocates for striking a balance in this matter, ensuring that an appropriate level of visual privacy exists to meet human needs without severing their connection with the external environment [11]. Islam establishes limits within the family, differentiating between permissible individuals (mahram) and those not (non-mahram). This distinction applies to interactions between men and women and extends to even the most delicate relationships between parents and children. The emphasis is on the principle that entering someone’s privacy without permission is strictly prohibited [3,32].

3.2. Visual Privacy Analysis

The density of urban spaces has resulted in the issue of being seen by others in residential private spaces [33,34], leading to a decline in residents’ interest in these urban environments. Urban planners and architects strive to design environments that reduce visual intrusion into private spaces while maintaining the quality of the indoor spaces. One aspect associated with visual privacy is the level of visual exposure (visual penetration), which significantly influences the quality of the human environment. Shielding oneself from visual exposure in an urban environment enhances individuals’ self-confidence and aids them in retaining command over their personal space [35].

Various qualitative and quantitative methods have been utilized to assess visual exposure in buildings, which is inherently connected to visual privacy. Generally, as visual exposure decreases, the level of visual privacy increases. The following section presents various methods applied to evaluate visual privacy.

3.2.1. Quantitative Analysis

Visual distance is one of the key components that influence visual exposure. In this method, a series of points are considered sight points at 160 cm from the ground, assuming the observer is standing at that point. Lines of sight are drawn between various points, and these lines are depicted on two-dimensional and three-dimensional maps (Figure 4).

Figure 4. Distances between viewing points on the same level and between different levels after [35].

Figure 4. Distances between viewing points on the same level and between different levels after [35].

Then, all the lines are ranked based on the viewer’s standpoint, and the distances between the lines of sight are measured. The distance between these lines is calculated on the horizontal plane (direct measurement on the plan) and across different floors, and all lines are categorized based on visual distance. When the distance between lines of sight is low, the level of visual exposure increases, resulting in a decrease in visual privacy. Conversely, visual exposure decreases when the distance between lines of sight increases, leading to improved visual privacy [35]. This method is among the initial approaches to quantitatively analyzing the level of visual privacy. It has certain shortcomings that future methods have attempted to address, which will be explored further.

The isovist: An isovist is a collection of points visible from a specific point. It is defined by its area and perimeter, indicating the extent of the visible area from that point about its surrounding environment (Figure 5). This method does not focus on disruptions that may affect an individual’s ability to see their surroundings [36].

Figure 5. Isovist area with an obstacle in the viewpoint after [20].

Figure 5. Isovist area with an obstacle in the viewpoint after [20].

The isovist region is the area within an observer’s line of sight and is measured in square meters. One of the software tools available for isovist analysis is DepthMap 10, developed by researchers at UCL (University College London). DepthMap can automatically generate the isovist region in the plan by inputting a two-dimensional plan and specifying the viewpoint [20,37,38]. One of the limitations noted for this method is that it does not consider territorial visual privacy. It mainly focuses on the user’s own space while overlooking the visual exposure of others’ spaces [39].

SOI (Spatial Openness Index): The Spatial Openness Index is a visual analysis method expressed in three-dimensional space. It measures the visible volume within a surrounding sphere (Figure 6).

Figure 6. The measure of the visible volume from a viewpoint after [40].

Figure 6. The measure of the visible volume from a viewpoint after [40].

Spatial openness correlates with the degree of visual exposure, and a higher index of SOI indicates a stronger visual connection and less visual privacy. This method can be referred to as a three-dimensional isovist. In this approach, three-dimensional visual permeability is examined. Computer models are used in this method to measure the visible volume from a single point. They classify spatial configurations based on the volume level and assess the permeability of spatial configurations from a specific point [40].

Visibility Graph Analyses: These graphs indicate how much of the specific point in space is visible from other parts of the same space (Figure 7) [41].

A good tool for generating such graphs is DepthMap software, which works in a spatial syntax. Hillier and Hanson first introduced space syntax, which is utilized to evaluate and measure the shape and configuration of space. This theory can assess how individuals perceive the built environment and also examine the social relationship between humans and space [20]. Space syntax helps to assess the visual features of a space quantitatively [37]. Various studies have utilized this method to assess the degree of visual penetration and privacy in a space [1,20,25,41].

3D LOS (Line of Sight): Utilizing isovist, this approach quantifies visibility and integrates human perception into the assessment. In this method, initially, various views are presented to a group of individuals through interviews, and different opinions are rated based on a specific objective. Subsequently, a virtual model is generated, and analyses are performed to assess visibility, considering the visible volume from a particular point as the observer’s viewpoint (Figure 8).

For instance, the observable volume from a window towards the outside space is measured. This observed volume includes various elements such as trees, ground, sky, and urban space [23].

Figure 8. LOS 3D line from the window to the urban environment after [42].

Figure 8. LOS 3D line from the window to the urban environment after [42].

During the interviews, assessments were made for all elements present in the urban image, such as trees, buildings, roads, etc. Each element received a score based on the interviewees’ opinions. Subsequently, these scores were multiplied by the observed volume of each component, ultimately resulting in the final visibility score [42].

As shown in Figure 9, the ultimate visibility rating for a window is computed, allowing for comparisons with alternative viewpoints. This technique helps create 3D visual projections that closely match human perception. It can be used to determine the level of visual privacy by considering the conditions associated with human-defined visual privacy views and their impact on 3D visual analysis [14]. This method allows for evaluating factors such as trees and furniture that influence visual exposure and access to the building.

Figure 9. Full-depth view refers to the view through the window [42].

Figure 9. Full-depth view refers to the view through the window [42].

Viewshed analysis: This approach is based on sightlines, and currently, through view analysis, Geographic Information System (GIS) software 3.10 is employed to assess visual exposure across extensive urban areas [43,44]. This assessment investigates the visibility potential originating from a designated point, referred to as the observer, extending to all other points within a specified radius. It identifies impediments that obstruct visual penetration (Figure 10).

Figure 10. Viewshed analysis based on the human field of view after [43].

Figure 10. Viewshed analysis based on the human field of view after [43].

In this approach, after the GIS analysis, maps are produced wherein designated points, regardless of their visibility, are assigned numerical codes of 1 and 0. Various constraints have been identified, such as the juxtaposition of the body and the environment, observer characteristics, weather and climatic conditions, and spatial configuration. Through research on this method, attempts have been made to mitigate and expand upon these limitations. This approach has been integrated as a plugin named “r-viewshed” into GIS software, facilitating its utilization across urban and landscape environments [43,44].

PVEI AND I-PVEI Index: An emerging approach in recent years for evaluating visual privacy is the PVEI (Potential Visual Exposure Index) method. This approach diverges from three-dimensional isovist and viewshed models, which assess the surface area and volume surrounding an observer and encompass the entire surrounding environment. It entails employing a function of sightline analysis from a single point to another location, thereby enabling the assessment and determination of the visual privacy level of a building’s facade by an observer positioned in neighboring buildings or at ground level and streets [5,24]. The PVEI index indicates the degree of visual penetration into the building from public spaces and neighboring openings. This index depends on viewing distance, viewing angle, and the extent of the opening area.

As Figure 11 shows, a target within the window of one building can be observed by different individuals at various locations in the adjacent building. The level of visual penetration to the target depends on its distance from the observer and the vertical and horizontal angles of the observer relative to the target. The closer the distance and the closer the horizontal angle (α) to the target is to 90 degrees, and the closer the vertical angle (β) is to 0, the lower the level of visual privacy and the higher the visual penetration. The α angle concerns the line perpendicular to the window on the horizontal plane (doA), while β pertains to the line perpendicular to the window on the vertical plane (doB).

$$\mathrm{the} {\mathrm{PVEI}}_{\mathrm{i}}={\sum }_j^n{\displaystyle \frac{A_{i}Aj}{d_{\mathrm{i}j}^2}}\times \sin \alpha \times \cos \beta$$

(1)

The formula for the PVEI index [24].

To calculate the PVEI index, we need to use Equation (1), where A_i is the area of the opening in Building I, A_j is the area of the opening in Building J, d_ij is the distance between the observer and the target in the two buildings, α is the horizontal angle, and β is the vertical angle. To determine this index, it is necessary to calculate the formula for every visible point to both the observer and the target. The resultant values are then aggregated to derive the index for the window of the target building. It is rare to find a residential setting where visual privacy is maximally protected (PVEI = 0). Instances where the PVEI value approaches zero are typically observed in specific scenarios, such as openings facing uninhabited areas like the sea or being obstructed entirely by urban structures or trees. While these situations may ensure visual privacy, they are uncommon in reality. Moreover, openings with a low PVEI value may also entail drawbacks, such as reduced sunlight and ventilation. In urban settings, individuals tend to select residences offering relatively better visual privacy that aligns with their expectations, rather than opting for a residence with a PVEI value of 0 [24].

Figure 11. The sight lines between the target and the observer are between two adjacent buildings after [24].

Figure 11. The sight lines between the target and the observer are between two adjacent buildings after [24].

According to Figure 12, the PVEI index remains relatively constant as the distance between the observer and the target increases, showing no significant change. It converges to a continuous value with increasing distance between the observer and the target. It is crucial to prioritize achieving adequate visual privacy for all building openings, particularly in residential and hotel settings. This entails adhering to the Standard Potential Visual Exposure Index (SPVEI), a metric architects and urban planners should integrate into their designs.

A constraint of the PVEI index is its failure to account for human visual perception. The I-PVEI (Improved Potential Visual Exposure Index) index, as introduced by [4], can assess how alterations in both horizontal and vertical dimensions affect visual privacy. This index effectively controls the impact of building morphology on visual privacy within spatial layouts. The visual perception of residential building facades depends on the limitations of human vision. The horizontal field of view (FOV) for humans is 124 degrees, with a central angle of 60 degrees, allowing for simultaneous observation of a target with both eyes. In terms of the vertical field of view, the viewing angle is 120 degrees, comprising 50 degrees upward and 70 degrees downward relative to the vertical axis from the eyes (Figure 13). Beyond a viewing angle range of 25 degrees upward and 30 degrees downward, color recognition and target identification become more challenging. Targets outside the human field of view require individuals to turn their heads toward the target for observation [4].

Figure 12. The relationship between the PVEI index and the distance [24].

Figure 12. The relationship between the PVEI index and the distance [24].

Figure 13. The angle of the human field of view after [4].

Figure 13. The angle of the human field of view after [4].

When two buildings face each other, maximum visual penetration decreases visual privacy. However, the opposite effect happens when the buildings are rotated relative to each other. In this case, the target of interest falls into the second level of the human field of view, making target detection more difficult (Figure 14). Naturally, the focus is on the customary observation during the routine engagements and daily existence of individuals, rather than a purposeful and deliberate standpoint.

Figure 14. (a) The observer’s position relative to the target in two buildings facing each other and (b) with an angle relative to each other after [4].

Figure 14. (a) The observer’s position relative to the target in two buildings facing each other and (b) with an angle relative to each other after [4].

The same scenario also applies to the vertical angle, as shown in Figure 15.

Figure 15. The situation of the target and observer in different vertical FOV after [4].

Figure 15. The situation of the target and observer in different vertical FOV after [4].

Regarding the overall visible lines of sight from observers at both building and pedestrian levels within the constructed environment, the improved Potential Visual Exposure Index (I-PVEI) for each opening can be accordingly delineated:

$$\mathrm{I}-\mathrm{PVEI}={\sum }_j^n{\displaystyle \frac{A_{i}Aj}{d_{\mathrm{i}j}^2}}\times w_h\times \sin \alpha \times w_v\times \cos \beta$$

(2)

I-PVEI formula [4].

The difference between the PVEI and I-PVEI formulas lies in the coefficients of w_h and w_ν, which assign weights to the horizontal (α) and vertical (β) angles, respectively.

$${\mathrm{w}}_{\mathrm{h}} =\left\{\begin{array}{c}{\displaystyle \frac{s{h}_1^{-1}}{s{h}_1^{-1}+s{h}_2^{-1}}},{\alpha }^{\mathrm{\prime }}\in \left[-{30}^{°},{30}^{°}\right]\\ {\displaystyle \frac{s{h}_2^{-1}}{s{h}_1^{-1}+s{h}_2^{-1}}}{,\alpha }^{\mathrm{\prime }}\in \left[-{62}^{°},-{30}^{°}\right)\cup \left({30}^{°},{62}^{°}\right]\end{array}\right.$$

(3)

The formula for the coefficient weight of the horizontal angle (α) [4].

$${\mathrm{W}}_{\mathrm{v}}=\left\{\begin{array}{c}{\displaystyle \frac{s{v}_1^{-1}}{s{v}_1^{-1}+s{v}_2^{-1}+s{v}_3^{-1}}},\mathrm{ǀ} {\beta }^{\mathrm{\prime }}\in \left[-{15}^{°},0^{°}\right]\\ {\displaystyle \frac{s{v}_2^{-1}}{s{v}_1^{-1}+s{v}_2^{-1}+s{v}_3^{-1}}},\mathrm{ǀ} {\beta }^{\mathrm{\prime }}\in \left[-{30}^{°},-{15}^{°}\right)\cup \left(0^{°},{25}^{°}\right]\\ {\displaystyle \frac{s{v}_3^{-1}}{s{v}_1^{-1}+s{v}_2^{-1}+s{v}_3^{-1}}},\mathrm{ǀ} {\beta }^{\mathrm{\prime }}\in \left[-{70}^{°},-{30}^{°}\right)\cup \left({25}^{°},{50}^{°}\right]\end{array}\right.$$

(4)

The formula for the coefficient weight of the vertical angle (β) [4].

In Equations (3) and (4), the letters “Sh” represent the area of the observable sector on the horizontal plane, and “Sv” means the area of the observable sector on the vertical plane, as detailed in Figure 13, illustrating the details of the angles of the observable sections.

3.2.2. Qualitative Analysis

Most qualitative articles have been conducted through interviews, observations, and questionnaires [13,22,45].

Elgardra and Fotios (2023) examined various scales of clothing coverage for women in three Muslim-majority countries [23]. This study demonstrated that women express visual satisfaction and ensure visual privacy with certain types of clothing in the presence of family members and strangers. Libyan women tend to opt for clothing with higher coverage levels when interacting with strangers, preferring garments rated at level 6 while choosing level 2 clothing for family settings (Figure 16).

Figure 16. The scale was developed to examine the effect of clothing levels on perceived privacy after [23].

Figure 16. The scale was developed to examine the effect of clothing levels on perceived privacy after [23].

Upon determining the levels of clothing coverage to ensure minimum visual privacy, two samples of windows were examined in a laboratory setting, one with a horizontal blinds curtain and the other with frosted glass. Various images were displayed on a wall, with viewing times ranging from 0.3 to 3 s. The viewers’ distance from the wall showing the photos is 4 m. For windows covered with curtains, the percentage of free space between them ranged from 3% to 60% in the experiment, while for frosted glass, a range of 4% to 20% was considered using Photoshop’s blending tool. After each performance, test participants had to use a clothing scale to indicate the actors’ clothing level.

Visual privacy was assessed and compared in a controlled laboratory environment after establishing how the free percentage between curtains and the level of frosted glass affect the perception of clothing coverage [23].

The research [10] focused on the impact of setbacks on visual privacy and thermal comfort. The methodology involved field observations and spatial analysis, and the relationship between visual privacy and thermal insulation was examined using an online questionnaire administered to 245 respondents. Survey data were analyzed using multiple regression analysis and the Kruskal-Wallis H test to investigate residents’ perceptions of visual privacy in Ibadan, Nigeria. He employed a combination of questionnaire surveys and interviews with a statistical community of 565 individuals. For data analysis, the ANOVA and chi-square tests were used. Visual comfort was assessed [46] through questionnaire surveys and interviews. To gauge the influence of factors identified during the interviews, the Likert questionnaire was employed to determine the significance level of each factor. The research [29] focuses on examining the privacy aspects of female students in dormitories in Indonesia. Given the significant number of boarding schools in Indonesia, a snowball sampling technique was employed, focusing on the western region of Indonesia, particularly Java. Privacy aspects were assessed and evaluated using direct observation and on-site interviews.

The study [8] examining the observable features of windows and their impact on interior spaces has incorporated visual privacy as one of its criteria. An online survey and human assessment in a controlled experimental environment using an artificial window displaying images via a screen have been employed to achieve the research objective. Eighteen images were evaluated by 91 participants in the online questionnaire, and eventually, eight photos were selected for the controlled experimental phase. In the online survey stage, participants were asked to rate their visual satisfaction with the pictures on a scale ranging from very satisfied to very dissatisfied. The rating scale in the survey comprises seven labels. Three different surveys with fewer questions were utilized in the assessment to prevent response bias on subsequent questions.

The data analysis encompassed using hypothesis testing (NHST) to assess the significance of hypotheses and multivariate analysis of variance (MANOVA) for reference purposes.

Numerous investigations conducted across diverse nations have scrutinized visual privacy [9,18,21,26,30,32,47,48], employing methodologies including interviews and questionnaires for assessment purposes. The questionnaires were distributed through both traditional face-to-face interactions and digital online platforms.

The study [6] examined the levels of visual privacy and the perception of density within compact residential units. In this investigation, multiple computer-generated models of small-scale apartments were created, and participant surveys were administered via virtual reality simulations. Each apartment included designated areas for evaluating perceived density and visual privacy, consistently positioned near the entrance and seating area. One hundred five individuals, representing a diverse cohort including architecture students and professionals from various fields, participated in the study. The interview and questionnaire sessions were brief, lasting less than thirty minutes, and experimental procedures were conducted using virtual reality technology within a controlled setting. The modeling process was exclusively conducted using SketchUp software.

Ordinal regression was employed to compare the apartment ratings assessed on an ordinal scale. The Glimmix procedure within the SAS software was utilized to conduct this analysis. Spearman correlations were computed to explore the associations between the questionnaire items. In another study [13]. Virtual reality was used to assess the impact of window shapes on residents’ visual privacy. Pre-designed scenarios were presented to users through a VR headset, immersing them in a virtual environment. A questionnaire based on a 7-point Likert scale was then administered to evaluate their perceptions. The study ultimately measured how different window shapes influence visual privacy.

3.3. Approaches for Establishing Visual Privacy

This paper delves into diverse strategies for attaining visual privacy, which encompass interventions added to buildings to manage visual perception or exert influence on the fundamental planning and configuration of buildings and urban density. This section categorizes the approaches to ensuring visual privacy examined in the literature.

3.3.1. Spatial Morphology and Configuration

Urban layouts: [10] examined visual privacy within residential environments. Findings revealed a direct relationship between residents’ satisfaction and more significant building setbacks, affecting both thermal comfort and visual privacy (Figure 17).

Figure 17. The setback of a building from adjacent buildings and streets after [10].

Figure 17. The setback of a building from adjacent buildings and streets after [10].

Residents utilized passive strategies, such as natural ventilation, to address thermal discomfort while adhering to cultural norms regarding visual privacy preferences. Enhanced setbacks positively impacted privacy and thermal comfort, particularly when complemented by landscaping, although hard surfaces elevated temperatures, thereby diminishing comfort and satisfaction. [17] stated that residential areas should feature delineated open spaces, segregating private and communal areas to encourage social interaction while preserving neighbors’ privacy rights. To augment privacy, reduce external openings by orienting architectural elements inward towards a central courtyard and shielding them with lattice structures crafted from metal, wood, or plastic.

Ref. [9] suggests dividing the land into skewed plots and arranging buildings staggered next to each other. Meanwhile, opposite plots on the other side of the street are vacant to avoid direct alignment. The boundaries of the plots are recommended to be designed in a tooth-like pattern without changing the setback distance, ensuring uniformity on both sides of the street. This configuration results in open spaces that can serve multiple purposes, including planting trees and shrubs to prevent visual intrusion and improve the street’s aesthetic appeal.

The research [32] states that houses with windows facing green spaces, vegetation, roads, or urban open spaces tend to provide a higher sense of visual privacy than those facing other residential buildings.

Shape plan: The research [1] assesses the visual privacy of residential plan layouts in Iran from 1970 to 2010 using space syntax analysis, focusing on factors such as permeability and wayfinding.

It observes that deeper spaces are associated with heightened visual privacy, with integration values declining over time, indicating a growing emphasis on privacy. Traditional designs prioritize visual access at the expense of privacy, while contemporary layouts emphasize spatial segregation and clear navigation pathways. The study proposes organizing spaces hierarchically according to depth to enhance privacy. Emphasizing clear visual access to public areas and restricted access to private spaces improves both wayfinding and privacy. Recommendations advocate for including corridors and semi-public spaces like kitchens to delineate between public and private areas. In sum, the research highlights the critical role of spatial design in achieving visual privacy within residential contexts.

In research [3], the study focused on analyzing visual privacy in various layout designs of residential apartments in the city of Khartoum, Sudan. It indicates that modern apartments, in comparison to traditional courtyard houses in Sudan, pay less attention to gender segregation in spatial arrangement.

Traditional Arab courtyard houses feature two separate external entrances, one designated for males and the other for females. The courtyard is meticulously designed to maintain privacy, often incorporating a short corridor or screening wall. These elements serve as spatial filters, preventing visitors from viewing the house, even if the door is left open to create an indirect entrance. This study, based on the examined case studies and the culture and religion of the Sudanese people, provides recommendations. It suggests the presence of at least three zones for private spaces within residential areas. When entering the apartment, an indirect entrance is preferred, which can be achieved through a small corridor, a screen, or a balcony. The kitchen should be placed in a semi-private area, away from spaces designed for hosting guests, to minimize the smell of cooking. Properly dimensioned balconies can serve as suitable alternatives to the courtyards of traditional Sudanese houses, fulfilling residents’ need for an outdoor area with adequate privacy. Designing two balconies in apartment layouts is preferable to a single balcony, as it can accommodate various functions.

The study [20] explored the interplay between visual privacy and architectural characteristics, particularly in the traditional settlement of Ghadames. The settlement, comprised of interconnected houses with shared walls and narrow, spiral streets, prioritized visual privacy due to religious and cultural considerations. This was reflected in the design of multi-story dwellings, aimed at segregating genders. Strategic placement of entrances and avoiding direct sightlines further emphasized this privacy concern. Findings from isovist measurements confirmed these observations, indicating that traditional house spaces were typically compact and shielded from public view. This long-standing architectural feature, developed over generations, should be a key consideration in future house designs.

The research of [41] suggests that attention to visual privacy has become a symbolic aspect of residential space design. Through an analysis of the floor plans of middle-income apartment complexes in Dhaka, the capital of Bangladesh, it was found that when the dining and living spaces are contiguous and uninterrupted, visual privacy for homemakers is lower than when these spaces are separated. The homemakers’ activities outside the home have increased visual connection with their surroundings, consequently increasing the visual interaction between the indoor workspace and the surrounding environment and reducing the visual privacy of spaces such as the kitchen.

The study [30], focusing on the cultural and religious aspects of Malaysian society, revealed that the configuration of interior spaces and architectural elements influences behavioral adaptation. It indicates that the presence of non-family guests in the house necessitates the segregation of entrances, interior spaces, and bedrooms. Guests are not permitted to enter the house until authorized by the male homeowner. Furthermore, the study suggests that a shared bedroom for children may be suitable only when their genders are the same; otherwise, separate rooms should be provided for each gender.

The research [21] stated that the issue of visual privacy extends beyond being seen from building windows, encompassing visibility from any point into the interior of a home. One of the challenges of modern buildings is the lack of intermediary space in the entrance area, leading directly into the main living space. Traditional homes connected public and private spaces through entry halls and corridors. Conversely, front-facing doors in modern apartments reduce visual privacy. Findings indicate that the size of entry doors has increased, resulting in heightened visual penetration. The study suggests using double doors to reduce daily visual intrusion, with the second leaf opened only when the furniture needs to be moved. Regarding balcony placement, the research recommends positioning them facing internal courtyards to maximize privacy, while balconies facing urban spaces are less utilized due to reduced visual privacy.

3.3.2. Façade Elements

Architects utilize a variety of elements on building facades to attain visual privacy and regulate the extent of visual intrusion from pedestrians and neighboring structures into residential areas. These features are positioned on openings to diminish visual penetration into the interior space. Various elements, from traditional to modern architectural styles, have been employed to safeguard visual privacy. This study presents several of these elements based on acquired sources.

Mashrabiya: Architects strive to regulate visual privacy by designing openings and balconies, ensuring that neighbors cannot peer into the residence while permitting adequate lighting and sky views. A mashrabiya is a dual-layered façade found in historical architectural designs. It consists of a wooden lattice window usually positioned in front of primary windows. Some accounts trace the earliest appearance of mashrabiyas back to the 12th century in Baghdad, under the Abbasid dynasty. These components frequently showcase wooden embellishments adorned with geometric motifs [28]. Mashrabiyas are employed to secure visual privacy, diminish direct sunlight, and enable natural airflow [49]. Mashrabiya is also called Shenashir in the southern parts of Iran (Figure 18) [50].

Figure 18. Wooden Shenashir in Bushehr city [51].

Figure 18. Wooden Shenashir in Bushehr city [51].

The research [18] on visual privacy in Malaysian homes asserts that the mashrabiya serves as a valuable element in traditional architecture for ensuring visual privacy and fulfilling critical social functions. The mashrabiya allows for ventilation and enables individuals inside the house to have views outward while preventing visual intrusion from people outside the home. The findings of the study [49] suggest that mashrabiyas fulfill the recommended illumination level for residential spaces and reduce glare. However, if mashrabiyas are redesigned or modified considering the uniformity and color of light in the space, they could perform better.

The research [26] proposes that the limited use of mashrabiyas in modern architectural designs could stem from their incongruence with contemporary aesthetics, elevated construction and manufacturing expenses, diminished natural illumination, and maintenance complexities. Individuals typically favor mashrabiyas renovated and reimagined for contemporary architectural settings, employing materials such as wood and glass.

Rowshan: Rowshan is another name for the mashrabiya. It is one of the types of wooden coverings used in traditional Islamic architectural façades, serving various purposes such as providing visual privacy, natural ventilation, and reducing daylight glare (Figure 19). However, in contemporary architecture, due to the lack of skilled craftsmen in constructing rowshan screens and insufficient knowledge about them, they have gradually disappeared from architectural practice. Instead, windows with aluminum or iron frames and transparent glass have replaced them. Consequently, residents of buildings have been compelled to use fabric curtains to ensure their visual privacy, given the use of these new types of windows [47].

Figure 19. Wooden rowshan in the old houses of Jeddah [52].

Figure 19. Wooden rowshan in the old houses of Jeddah [52].

According to [47]. There is a significant interest in using rowshan with a light brown color in modern architecture, provided that the beauty of this traditional screen is preserved. However, challenges associated with its use in contemporary architecture must also be addressed.

Glazing type: As construction advances and larger glass windows become more prevalent, there has been a decline in visual privacy, leading to alterations in transparent glass to address this concern. Frosted glass is utilized to offer visual privacy and manage visual perception [23]. Nonetheless, this glass type’s drawback is its reduced visibility from indoors to outdoors.

Another type of glass is reflective glass, which reflects light during the day and acts as a mirror, reducing visibility from the outside. According to [47]. In the city of Medina, reflective glass with interior curtains has been statistically proven to be the most suitable method for ensuring visual privacy within the community. However, they argue that reflective glass does not meet people’s aesthetic preferences.

Smart or tunable glasses are a new type of glass that can control and adjust the amount of daylight passing through, as well as alter the transparency of the glass to ensure visual privacy inside homes. Electrochromic glasses, polymer-dispersed liquid crystal glasses, and suspended particle devices are currently available as commercial smart glasses. However, due to their high cost, they are not cost-effective, and more affordable technologies are being researched and developed [53].

Opening size: The size of building façade windows plays a crucial role in establishing visual privacy. As the dimensions of these windows increase toward urban spaces and larger neighboring areas, the level of visual exposure rises, leading to a decrease in visual privacy [4,24,42].

Ref. [29] indicates that the height of windows OKB from the floor significantly influences visual privacy. If this height exceeds human eye level, visual privacy is ensured, but it restricts the view of individuals inside to the outside.

4. Discussion

In previous review studies, much of the focus has been on the strategies employed in Islamic residences to establish visual privacy [27], while other works have primarily discussed various definitions of visual privacy from Islamic and cultural perspectives [18]. However, this paper adopts a more comprehensive approach. Our central objective is to classify and examine the methods for measuring visual privacy, both quantitatively and qualitatively, offering a broader perspective on the topic. Different perspectives, including social, climatic, cultural, and religious factors, contribute to various definitions of visual privacy, as explored in this article, with these definitions being influenced by specific conditions within each domain. For instance, within the Islamic view, regulations regarding visual privacy and clothing for women in the presence of non-mahram individuals are outlined based on teachings from the Quran and the practices of the Prophet Muhammad. In quantitative evaluation techniques, all methodologies function based on observers’ lines of sight toward the subject, and research endeavors in recent decades have aimed to enhance the accuracy of these quantitative assessments to reflect real-world scenarios better. As per scholarly literature, the latest quantitative approach—the I-PVEI index—proposed by [4] is designed to evaluate the visibility of a target by incorporating parameters such as distance, viewing angle, window size, and the human field of vision; however, limitations remain, as ambient light levels and weather conditions (e.g., the reduced visual penetration during daylight when external brightness surpasses interior illumination, or the increased visibility at night due to higher interior brightness) are not accounted for. Visual penetration may substantially differ from sunny conditions in certain climates with frequent rainfall or fog. Additionally, these methods typically assume static observers with a fixed perspective oriented perpendicularly, which does not reflect the dynamic and evolving nature of human movement and gaze in practice. Regarding strategies for establishing visual privacy in residential dwellings, the literature has explored only a restricted range of methods; this review, based on the available resources, demonstrates that although a broad spectrum of approaches is lacking, architects and building designers can still devise numerous strategies tailored to their cultural, religious, and climatic contexts by considering fundamental privacy definitions and available measurement techniques. It should also be noted that when establishing visual privacy, creating barriers to prevent inward visual penetration must be balanced with the need for adequate natural light, sky visibility, and proper ventilation [54].

5. Conclusions

This study was conducted to address three core research questions:

(1)

What are the key definitions of visual privacy across various disciplines?

(2)

What qualitative and quantitative methods have been applied to measure visual privacy?

(3)

What architectural strategies have been proposed to enhance visual privacy in residential environments?

To answer these questions, a systematic review was performed, analyzing 51 selected articles that discuss visual privacy from multiple perspectives, including psychological, environmental, physical, and cultural and religious aspects. The findings highlight that although many studies address visual privacy, a comprehensive and structured classification of definitions and evaluation methods has been missing from the literature.

This review classified visual privacy definitions into thematic categories and further distinguished the methods used to measure visual privacy into qualitative and quantitative approaches. In addition, the review outlined various architectural and design strategies applied across case studies to enhance visual privacy, considering the cultural and contextual needs of different societies.

Ultimately, this paper highlights the lack of a universally accepted framework for evaluating visual privacy and emphasizes the need to integrate measurable techniques and culturally sensitive design strategies. While the review focused primarily on measurement methods and conceptual definitions, it also acknowledges that other factors, such as urban morphology, vegetation, lighting conditions, and climatic variations, influence visual privacy and should be the subject of future research.