# A Guide to Visual Comfort: An Overview of Indices and Its Applications

Christos Tzouvaras, Asimina Dimara, Alexios Papaioannou, Kanela Karatzia, Christos-Nikolaos Anagnostopoulos, Stelios Krinidis, Konstantinos I. Arvanitis, Dimosthenis Ioannidis, Dimitrios Tzovaras

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## To cite this version:

Christos Tzouvaras, Asimina Dimara, Alexios Papaioannou, Kanela Karatzia, Christos-Nikolaos Anagnostopoulos, et al.. A Guide to Visual Comfort: An Overview of Indices and Its Applications. 19th IFIP International Conference on Artificial Intelligence Applications and Innovations (AIAI), Jun 2023, León, Spain. pp.183-194, 10.1007/978-3-031-34171-7_14. [https://doi.org/10.1007/978-3-031-34171-7_1](https://doi.org/10.1007/978-3-031-34171-7_1)

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# A Guide to Visual Comfort: An Overview of Indices and Its Applications

Christos Tzouvaras¹,²(✉), Asimina Dimara²,³, Alexios Papaioannou³,⁴, Kanela Karatzia¹, Christos-Nikolaos Anagnostopoulos², Stelios Krinidis³,⁴, Konstantinos I. Arvanitis¹, Dimosthenis Ioannidis³, and Dimitrios Tzovaras³

¹ Watt and Volt S.A., Athens, Greece

{ch.tzouvaras,k.karatzia,k.arvanitis}@watt-volt.gr

² Department of Cultural Technology and Communication, Intelligent Systems Lab, University of the Aegean (UoA), Mytilene, Greece

canag@aegean.gr

³ Information Technologies Institute (ITI), Centre for Research Technology Hellas (CERTH), Thessaloniki, Greece

{adimara,alexiopa,krinidis,djoannid,Dimitrios.Tzovaras}@iti.gr

⁴ Management Science and Technology Department, International Hellenic University (IHU), Kavala, Greece

**Abstract.** The COVID-19 pandemic has impacted the lives of millions of people around the world. During this period, many nations have imposed strict "stay at home" restrictions to mitigate the spread of the virus. Consequently, people spent more time at home, and the topic of their comfort and well-being in indoor environments came to the forefront. Prompted by the lack of clarity surrounding the topic of visual comfort, this paper aims to provide a detailed guide on visual comfort, its affecting factors, the way it is assessed and its assessment indices. As it became obvious, visual comfort depends on a lot of factors, is studied through many different lenses, has a variety of indices, and most importantly, is perceived differently from person to person.

**Keywords:** Visual comfort · Visual comfort indices · Visual comfort estimation

## 1 Introduction

As Aruga et al. state in [1], during the emergence of COVID-19, several nations imposed strict lockdown rules that forbade people from leaving their houses unless they needed to acquire basic necessities, resulting in a dramatic increase in the amount of time people spent indoors. In [2], Choi et al. argue that social distancing methods, while vital to restrict the virus's transmission, raise the chances of loneliness, isolation, and anxiety. As a result, the individual's experience in an indoor environment came into the limelight as a critical factor in their health, comfort, and well-being, which are key aspects of Indoor Environmental
Quality (IEQ) according to Rohde et al. [3]. The overall experience of the user in an indoor environment, hereinafter referred to as “comfort”, is determined by the user’s thermal, acoustic, and visual comfort.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) defines thermal comfort as the mental state in which contentment with the thermal environment is conveyed [4]. N. Djongyang et al. [5] add that its evaluation incorporates various inputs impacted by physical, physiological, psychological, and other aspects. Furthermore, Rindel JH defines acoustic comfort as a notion that can be distinguished by the lack of undesirable sound and chances for auditory activities without disturbing other people [6]. Similarly to thermal comfort, Rindel JH highlights the subjective nature of the concept of acoustic comfort by noting that acoustic comfort for an individual is tied to the individual not only as a receiver of sound but also as a generator of sound. Visual comfort shares the subjective nature of the concepts of thermal and acoustic comfort while its definition and quantitative determination of are less standardised than those of thermal and acoustic comfort because they are dependent on various external factors such as the amount of available external sunlight, the glazing and screens that “filter” it, the indoor environment (optical properties of walls), and, of course, the visual task itself [7].

While researching the topic of increasing comfort and well-being in indoor environments, two high-level observations were made. Firstly, in order to improve indoor acoustic comfort, some form of major building intervention, such as installing insulation material [8], prefabricated wall modules [9], or false ceilings [10], must take place during the construction or renovation of a building. As a result, the focus shifted to the topics of thermal and visual comfort, where minor interventions can lead to significant results. Secondly, while extensive review literature can be found on the topic of thermal comfort [11–16], the user’s visual comfort is a topic that stands in need of further review. When searching “‘thermal comfort’ in buildings” on the Google Scholar platform, one can find more than 28,700 search results from 2017 to this day. On the other hand, search results for the keywords “‘visual comfort’ in buildings” are only 12,800. Moreover, many of these results represent research that includes both topics of thermal and visual comfort.

These facts led to the rough estimation that only one-third of the research effort dedicated to the topic of thermal comfort has been dedicated to the topic of visual comfort. Further research revealed that this deficiency in the literature lies not only in the volume of work but also in the clarity of the topics within it. Within this context, in this paper, a thorough examination and review of the literature surrounding the topic of visual comfort will be attempted to provide a detailed guide on visual comfort.

The rest of the paper is structured as follows: In Sect. 2, an overview of the factors that affect the resident’s visual comfort will be presented along with the factors that are taken into consideration when assessing visual comfort. Section 3, presents eight of the most prominent visual comfort indices corresponding to the previously mentioned assessment factors. Finally, in Sect. 4, conclusions are drawn regarding this work, the state of the visual comfort literature and the future endeavours of the authors.
## 2 Visual Comfort Definition

According to the European standard EN 12665 [17], visual comfort is defined as a subjective condition of visual well-being induced by the visual surroundings. In this section, the key elements that affect visual comfort will be presented, along with the factors that are taken into account when assessing visual comfort. This description aims to assist in a better understanding of the main visual comfort indices that will be presented in the next section.

### 2.1 Factors Affecting Visual Comfort

As stated previously, the perception of visual comfort is highly subjective, differing between individuals since it depends on various factors including the physiology of the human eye, the spectral emission of the light source and the physical quantities describing the amount of light and its distribution in space [18].

*The Physiology of the Human Eye:* As Grzeczkowski et al. point out in [19] visual processing occurs in the eye and more than 30 visual cortical regions, which account for one-third of the neocortex. At each one of these regions, there are variations that may affect visual processing between individuals. Moreover, according to F. W. Campbell and R. W. Gubisch’s research [20], numerous reasons can account for the differences in findings between physical and psychophysical techniques of judging optical quality including the optical function of the fundus as a poor optical screen or a smooth mirror as well as the light dispersion in optical media.

*The Spectral Emission of the Light Source:* Visible light is light that can be seen with the naked eye. Estimates of the range of wavelengths visible to the human eye vary, but the lower end of the range is around four hundred manometers (400 nm) which correspond to a frequency of about seven hundred and fifty terahertz (750 × 10¹² Hz). The upper end of the range is around seven hundred manometers (700 nm), corresponding to a frequency of about four hundred and thirty terahertz (430 × 10¹² Hz). In well-lit environments, the photopic luminosity function V(λ) best represents the human eye’s sensitivity to visible light at different wavelengths [21]. This function models the ratio of the energy of a light source of the wavelength to which the eye is most sensitive (λmax) to the energy of a light source of wavelength λ that elicits the same reaction in the test subject. The function was based on test data acquired from a variety of laboratories using a variety of approaches [22].

In low-light conditions, the sensitivity of the human eye to visible light at different wavelengths changes, and is best represented by the scotopic luminosity function, based on measurements obtained by US scientists George Wald and B.H Crawford. This is due to the way the rods and cones in the retina of the eye function. Cones are generally utilized for daylight vision. They are not extremely sensitive to changes in light intensity, but they can distinguish between red, blue, and green wavelengths. Night vision is dominated by rods. They are significantly
more sensitive to light than cones (and far more numerous), but far less sensitive to colour. They have a peak sensitivity of five hundred and seven manometers (507 nm) in the blue section of the visible light spectrum and are not especially sensitive to red light. As a consequence, the scotopic luminous efficiency curve is shifted towards the blue end of the spectrum [22]. The photopic (black) and scotopic (green) luminosity functions can be observed in the below Fig. 1 [23].

<figure><img src="image_4.png" /><figcaption>Fig. 1. The photopic and scotopic luminosity functions</figcaption></figure>

The Physical quantities describing the amount of light and its distribution in space:

I Luminous flux ($\Phi$), also referred to as luminous power, describes the total radiance emitted by a light source as perceived by the human eye and is measured in lumens (lm):

$$ \Phi = k_m \int_{380}^{780} V(\lambda) \phi_{e,\lambda}(\lambda) d\lambda \quad (1) $$

where $k_m$, $V(\lambda)$ and $\phi_{e,\lambda}(\lambda)$ are a constant of maximum photopic luminous efficacy of radiations, the spectral luminous efficiency in photopic vision, and the spectral concentration of radiant flux measured in watts per nanometer.

II The luminous intensity ($I$) represents the luminous flux ($\Phi$) emitted within a certain angle in a three-dimensional space, and its unit, the candela ($cd$), is defined as lumens per steradian ($lm/sr$):

$$ I = \frac{\Phi}{\Omega} \quad (2) $$

where $\Omega$ is the solid angle, measured in steradians (sr).
III Illuminance *E* is defined as the total luminous flux incident on a surface in lumens per square meter (lm/m²). It is a measure of how well the incident light illuminates the surface. The SI unit of illuminance is lux (*lx*):

$$E = \frac{I \cos \theta}{d^2} \qquad (3)$$

where *θ* and *d* are the angle of incidence (the angle between the light and the normal to the illuminated surface) and the distance from the light source to the target point in meters (*m*) respectively.

IV The amount of luminous intensity that passes through (or is emitted by) a certain area from a certain angle is described by luminance (*L*). Luminance indicates how bright an emitting or reflecting area appears by using the solid angle of a human's eye as a metric of the spatial light distribution. The SI luminance unit is candela per square meter (cd/m²):

$$L = \frac{d^2 \Phi}{d\Sigma d\Omega_{\Sigma} \cos \theta_{\Sigma}} \qquad (4)$$

where *d²Φ* is the luminous flux in lumens leaving the area *dΣ* in any direction contained within the solid angle *dΩΣ* and *θΣ* is the angle between normal to *dΣ* and the specified direction.

V The accumulated luminous intensity (*I*) applied to a certain area during a given time period is referred to as Luminous Exposure (*H*). In other words, it is the illuminance (*E*) per unit of time in seconds. The unit of Luminous Exposure is, therefore, *lx* × *s* (Fig. 2):

$$H = E \times t \qquad (5)$$

<figure><img src="image_5.png" /><figcaption>Fig. 2. Visual representation of *L*, *I*, *Φ* and *E*</figcaption></figure>
## 2.2 Assessment Factors of Visual Comfort

The EN 12464-1 lighting standard [24] states that a person's ability to perceive and perform a visual activity quickly, safely, and comfortably depends greatly on the amount of light and how it is distributed in the task area and the surrounding environment. Visual comfort is often investigated by assessing factors that reflect the relationship between human needs and the light environment. According to [25] some of these factors are the amount of light, the uniformity of light, the quality of light in the rendering of colours and the prediction of the risk of glare for the occupants

**The Amount of Light:** According to the definitions of the physical quantities of light that were presented in Sect. 2.1, the physical quantity that is utilized to measure the amount of light reaching a certain surface is Illuminance (E) (3). The EN 12464-1 lighting standard [24] defines in clause 5 the minimum illuminance values, referred to as “maintained illuminance” ($E_m$), that must be maintained over the task area for various interiors and tasks. These values were determined considering, psychophysiological aspects such as visual comfort and well-being, requirements for visual tasks, visual ergonomics, practical experience, safety, and economy. Specifically, for office spaces, as Ballarini et al. note in [26], according to the EN 12464-1 lighting standard, the amount of light required to perform visual tasks in office settings should be 500 lx.

**The Uniformity of Light and Luminance Distribution:** [27] indicates that the luminance distribution in the field of vision influences the eye's adaptation level and its capacity to adjust to different degrees of darkness and light, which affects the visibility of the task and visual comfort. Poor luminance distribution can result in suboptimal visual conditions. Too high luminances can cause glare, too high luminance contrasts can induce fatigue due to frequent eye re-adaptation, and too low luminances and low luminance contrasts can create a dull and uninteresting working environment. A well-balanced luminance distribution, on the other hand, results in visual acuity (sharpness), contrast sensitivity (detection of small relative brightness variations), and ocular function efficiency (such as accommodation, convergence, pupillary contraction, eye movements, etc.). In conclusion, the task area should be illuminated as uniformly as possible.

**Colour Appearance and the Quality of Light in Rendering Colours:** A lamp's "colour appearance" relates to the apparent colour (chromaticity) of the light emitted. It is quantified by its correlated colour temperature ($T_{CP}$). $T_{CP}$ is measured in Kelvin (K) and depending on its value, light is referred to as "warm", "intermediate" and "cool" according to Table 1. Colour rendering is also critical for visual comfort and performance. The colour rendering of a light source refers to its ability to correctly display the colours of different objects compared to an ideal or natural light source [28]. Natural colour rendering aids in a sense of well-being and is crucial when it comes to accurately recognizing safety colours. Boyce et al. mention in [29] that daylight is more likely to lead to optimal visual
performance compared to electric light sources. The reason for this lies in both the tendency of daylight to be delivered in high quantities, but also in the wide spectrum that exists in daylight during different times, seasons, and climatic conditions guaranteeing optimal colour rendering.

<table><caption>**Table 1.** Lamp colour appearance groups</caption><thead><tr><td>Colour appearance</td><td>Correlated colour temperature TCP (K)</td></tr></thead><tbody><tr><td>Warm</td><td>below 3300 K</td></tr><tr><td>Intermediate</td><td>3300 to 5300 K</td></tr><tr><td>Cool</td><td>above 5300 K</td></tr></tbody></table>

*The Prediction of the Risk of Glare for Occupants:* The European standard EN 12665 [17] defines “glare” as a state of vision in which there is discomfort or loss in the ability to perceive details or items, induced by an unsatisfactory distribution or range of luminance, or severe contrasts. CIE (International Commission on Illumination), in its International Lighting Vocabulary, [30] classifies glare into two categories:

1. Disability Glare, defined as “glare that impairs the vision of objects without necessarily causing discomfort”

2. Discomfort Glare, defined as “glare that causes discomfort without necessarily impairing the vision of objects”

Disability glare is seldom an issue in interior environments, but is commonly caused by oncoming vehicle headlights at night and in the day by sunlight. The experience of annoyance or pain induced by high levels of luminance in the field of view is known as discomfort glare. Discomfort glare, by definition, does not impair visual performance but does inflict discomfort. The desire for a model to assess whether a lighting installation will end up causing discomfort glare has resulted in the emergence of a number of empirical prediction systems in various countries, including the Visual Comfort Probability (VCP) system in North America. Given the trade hindrance created by different discomfort glare forecast methods in different countries and the overall lack of variation between them in terms of prediction accuracy, the CIE developed a consensus system. This system, known as the “Unified Glare Rating” system (UGR) [31], will be presented in the next chapter.

## 3 Visual Comfort Indices

In this section, eight of the most prominent visual comfort indices will be presented corresponding to the assessment factors presented in Sect. 2.2.
*Daylight Factor, DF:* This index describes the ratio of internal illumination $E_{inside}$ at a given point to external horizontal illumination $E_{outside}$ [32].

$$DF = \frac{E_{inside}}{E_{outside}} \times 100\% \qquad (6)$$

This index is the most straightforward and widely used way to assess the daylight permitted indoors by a window, since it indicates the prospective illuminance within a room in the worst-case scenario, under overcast sky conditions when there is less external daylight, offering a static metric that does not fluctuate with time or building direction [33].

*Useful Daylight Illuminance, UDI:* This index was derived in response to DF to include the wide range of light levels present in a visual plane. UDI, defines the range of useful daylight as 100lx–2000lx [34]. Levels less than 100 lx are generally regarded as insufficient, while levels greater than 2000 lx are likely to cause visual or thermal discomfort, or both. In [26] the authors define two further categories of UDI, supplementary UDI, UDIs, when daylight illuminance ranges between 100 and 500 lx and autonomous UDI, UDIa, when daylight illuminance ranges between 500 and 2500 lx.

*Daylight Autonomy, DA:* This index is defined as the percentage of the year when a minimum illuminance threshold is met by daylight alone. The level of illuminance is specified by referenced documents for lighting such as the IESNA Lighting Handbook [35] and is defined as the amount of daylight illuminance required for that specific task over the occupied times of the year. DA addresses the need for a long-term approach to the assessment of the visual environment.

$$DA = \frac{\sum_{i} \kappa_{i} \times t_{i}}{\sum_{i} t_{i}} \quad \epsilon[0, 1] \quad \kappa_{i} = \begin{cases} 1 & E_{daylight} > E_{lim} \\ 0 & E_{daylight} < E_{lim} \end{cases} \qquad (7)$$

DA, Similarly with UDI, is also divided into further more specialized indices, spatial DA, DAs, and continuous DA, DAc. DAs introduces a spatial element to DA. It was introduced by the Illuminating Engineering Society (IES) and is used to evaluate if a certain fraction of space receives a sufficient amount of daylight during a certain fraction of the year. According to [26] a minimum threshold of 300 lx should be maintained for at least 95% of the occupied time (sDA$_{300|95\%}$), while a minimum illuminance of 500 lx should be maintained for at least 50% of the time. In [32] the authors point out that the value added by sDA to DA is that it provides a single value that describes the daylight illuminance over the space in question. In contrast to spatial DA, continuous DA, DAc, considers fractional levels of illuminance that might not be enough to independently illuminate a fraction of space, but still contribute to its luminosity. For example, if the required maintained illuminance for a specific fraction of space is 500 lx, and the current illuminance is 300 lx, this level of illuminance is completely ignored when calculating sDA. On the other hand, DAc credits this level of illuminance as 300/500 = 60%.
*Annual Sun Exposure (ASE):* DA, like UDI, does not provide an upper limit for daylight illumination. Excessive amounts of sun exposure can lead to discomfort and suboptimal visual performance. As a result, a new daylight index by the name of Annual Sun Exposure (ASE) was created to describe the maximum fraction of space that should receive a certain amount of sunlight during a certain time period. The U.S. Green Building Council (USGBC) advise in LEED v4 [36] that during 250 occupied hours, no more than 10% of the space in question should receive more than 1000 lx.

*Colour Rendering Index (CRI):* The rendering ability of a light source is measured by its (CRI). The appropriate name for the value often referenced as “CRI” in commercial lighting products is CIE $R_a$, “CRI” being a general term and CIE $R_a$ being the international standard colour rendering index. The highest attainable $CIE R_a$ value is 100, which would only be assigned to a source whose spectrum is equal to that of daylight [37]. According to the EN 12464-1 lighting standard [24], lamps possessing a colour rendering index of less than 80 should not be utilized in areas where people work or remain for extended periods of time.

*Illuminance Uniformity (U):* The EN 12464-1 lighting standard [24] defines U as the ratio of minimum illuminance to average illuminance in a visual plane.

*Unified Glare Rating (UGR):* Many indices have been developed over the years for individual countries to evaluate the discomfort caused by glare, such as the British Glare Index (BGI) and the CIE Glare Index (CGI). However, a unified glare index based on commonly accepted parameters was needed. This resulted in the development of the “Unified Glare Rating” (UGR) [31].

$$ UGR = 8 \log_{10} \left[ \frac{0.25}{L_b} \sum \frac{L^2 \omega}{p^2} \right] \qquad (8) $$

where *p* is the position index, *ω* is the solid angle, *L* is the illuminance towards the observer's eyes and $L_b$ is the background illuminance.

*Daylight Glare Probability (DGP):* Despite the many benefits of daylight for the health, wellness, and productivity of users, too much exposure to daylight can cause glare and disturb the user. Wienold and Christoffersen [38] proposed a new index, the Daylight Glare Probability (DGP), which uses the probability that a person is disturbed as a glare metric. DGP is a function of the vertical eye illuminance as well as the luminance of the glare source, its solid angle, and its position index.

$$ DGP = 5.87 \times 10^{-5} E_v + 0.098 \log_{10} \left[ 1 + \sum_{i=1}^{n} \frac{L_{s,i}^2 \times \omega_{s,i}}{E_v^{1.87} \times p_i^2} \right] + 0.16 \qquad (9) $$

where $E_v$ is the vertical eye illuminance, $L_{s,i}$ is the luminance of the source, $\omega_{s,i}$ the solid angle between the observer and the source, and *p* is the position index.
# 4 Conclusions

In this short study, an effort was made to provide a guide to the topic of visual comfort in indoor environments. The authors attempted to present this topic holistically prompted by the lack of clarity in the literature, beginning with the factors that affect visual comfort, continuing with the way it is assessed, and finally presenting its assessment indices. As it became obvious, visual comfort depends on a lot of factors, is studied through many different lenses, has a variety of indices, and most importantly, is perceived differently from person to person. The authors aim to continue this work, defining a holistic practical approach to determining the appropriate index to assess visual comfort and proposing innovative AI models to optimise the visual comfort of users in indoor spaces.

**Acknowledgment.** This work is partially supported by the PRECEPT project, funded by the EU H2020 under Grant Agreement No. 958284.

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