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Daylight Glare: Back to Basics

By: Eloïse Sok-Paupardin
Apr. 5, 2021
Daylight Glare: back to basics

Glare is a major source of visual discomfort in our buildings. But how exactly is this phenomenon defined? What are the determining factors and mechanisms at play?

Definition and Mechanisms

Glare is commonly defined as discomfort to the eye caused by bright light or extreme contrasts. We can, however, distinguish between two main types of glare found in buildings

  • Disability glare: this causes a reduction in visibility of the visual task or visual acuity. In other words, the observer is no longer able to make out the details of the objects or people in their field of vision. This type of glare is mostly physiological: the change in visual capacity results from a loss of contrast in the image on the retina caused by intraocular light scatter2. The time taken to recover characterizes the resistance to glare and varies from person to person3.
     
  • Discomfort glare: this may distract or bother the observer without necessarily preventing them from performing a visual task. But it can cause negative effects in the medium to long term, such as visual fatigue or migraines. It is a phenomenon that is essentially subjective, and the physiological and psychological mechanisms involved are still poorly understood4. This makes it harder to characterize. Discomfort glare continues to be the subject of numerous studies, unlike disability glare, which is more quantifiable5
     

Sources of Glare

Glare in our buildings can be caused by different sources:

  • Direct view through windows of the sun or sky when it’s very bright
     
  • Sunlight reflected from windows onto neighboring buildings, the ground (e.g., snow) or water 
     
  • Reflections of light (natural or artificial) on screens (computers, projector screens, notice boards) or other work surfaces. They cause a decrease in contrast between the visual task and the immediate surroundings, a phenomenon also known as “veiling glare”
     
  • Reflections of light (natural or artificial) on any shiny or reflective surfaces in the space directly facing the observer’s eye
     
  • Lamps and light fittings with a very strong luminous intensity
     
  • Major contrast between direct (sun, lamps) or indirect (illuminated surfaces) light sources and adjacent surfaces

These different sources can cause discomfort glare, disability glare, or both simultaneously.

Situations in daylit spaces causing a risk of glare (source: (a) https://patternguide.advancedbuildings.net/ (b) Critical Investigation of Common Lighting Design Metrics for Predicting Human Visual Comfort in Offices with Daylight, Kevin Van Den Wymelenberg, 2014)

 

Physical Characteristics of Glare

The first key parameter involved in characterizing glare is luminance. It refers to the luminous intensity from a point on a given surface in a given direction, and is measured in candela per m² (cd/m²). It can come directly from a light source (sun, lamp) or the light reflected by an illuminated surface. In this case, it depends on the illuminance on the surface and its reflective properties. Luminance is the physical quantity closest to what the human eye perceives, unlike illuminance, which refers to the quantity of light falling on an area (measured in lux). It is experienced subjectively as the perception of brightness3.

The following table shows luminance in a few different orders of magnitude6:

Source

Luminance

The sun at its highest point

1,600,000,000 cd/m²

Fluorescent light

7,000 cd/m²

Overcast winter sky

3,000 cd/m²

PC flat screen

200 cd/m²

Star in the night sky

0.000001 cd/m²

The perception of glare is generally caused by excessive luminance or contrast of luminance in the field of vision, or a combination of the two.

Specifically, the sensation of discomfort occurs when the luminance in one part of the field of vision is much higher than the adaptation luminance (i.e., the luminance to which the visual system has adjusted at a given moment). The human eye is sensitive to an extremely wide range of variations in luminance — of a magnitude of 108. However, in a given visual scene, the visual system operates on a much more limited range of luminance1 — of 105. When significant or rapid changes in lighting conditions occur, the eye has to readjust. The same applies to major variations in luminance within the scene. This constant readjustment is a source of discomfort and fatigue for the eyes7.

The position of the source of glare in relation to the observer is also very important in the perception of glare. The closer the source of glare to the line of vision, the more intense its effect. Finally, the apparent size of the source is another significant factor affecting the sensation of glare. These two parameters depend directly on the position of the observer in a given space. That’s why the interior layout of spaces plays a key role in preventing the risk of glare8. For instance, occupational health and safety (OHS) codes recommend placing workstations perpendicular to windows9.

 

Subjectivity of Glare and Other Determining Factors

Some individual characteristics may influence the perception of glare and the discomfort it causes, such as age, eye color, chronotype10 and certain eye problems11. However, a recent study12 showed that cultural factors have little effect on its perception. 

Other psychological factors may also impact our perception of glare. For instance, multiple studies have shown increased tolerance to glare in the presence of a pleasant or interesting view of the outdoors1. Environmental factors such as temperature, the ambient light spectrum, air quality and acoustics may also have an impact on our individual perception at any given moment14. Some transient factors may also play a role in our sensitivity to glare at a given moment, including fatigue, mood, consumption of food, caffeine or alcohol, or past levels of light exposure4.

The type and difficulty of the visual task involved also have an impact. For instance, a person involved in a task requiring more cognitive resources will feel less discomfort when exposed to glare15.

Finally, recent studies have shown that visual discomfort changes depending on the direction of our gaze16. We might expect that our attention and therefore our gaze stays on the visual task that we are currently performing. In reality, however, the direction of our gaze is dynamic and shifts, particularly in the presence of appealing elements in our field of vision — interplay of light, an interesting view outdoors, shiny objects17

All the factors described above contribute to the difficulty of characterizing and assessing glare. However, further studies are required to help us better understand their impact10.

 

Conclusions

Glare is something that most of us are familiar with. But it is a complex phenomenon influenced by many different factors — environmental, individual and temporal. It is important to be aware of these when aiming to ensure occupants’ visual comfort in spaces with natural light.

 

Eloise Sok

 

Eloïse Sok is Concept Creator in the SageGlass Europe & Middle-East Team. She holds a Double-Degree in the Engineering field from Ecole Centrale (France) and Tsinghua University (China). Her main interests include sustainable architecture, daylighting and occupant’s comfort. Her motto: “Passion is our best strength!”.

 


Sources:

  1. The Application of Luminance Mapping to Discomfort Glare: A Modified Glare Index for Green Buildings, M. Hirning, 2014
  2. Overview of glare types and their relationship with macular pigment optical density, O. Putnam, 2017
  3. Eclairage et vision [Rapport de recherche] Notes scientifiques et techniques de l’INRS [Lighting and vision. {Research report} INRS scientific and technical notes], R. Floru, 1996
  4. Temporal effects in glare response, M. Kent, 2016
  5. Maximum luminances and luminance ratios and their impact on users’ discomfort glare perception and productivity in daylit offices, A. C. Linney, 2008
  6. Daylighting handbook, C. Reinhart
  7. Design Guidelines for Glare-free Daylit Work Environments, Osterhaus, 2005
  8. DESIGN RECOMMENDATIONS FOR PERIMETER OFFICE SPACES BASED ON VISUAL PERFORMANCE CRITERIA, I. Kostantzos et al,
  9. www.inrs.fr/risques/travail-ecran/prevention-risques.html
  10. The chronotype is a manifestation of an individual’s natural biological clock, which defines their preference for activities in the morning or evening, such as getting up and going to bed.
  11. Review of Factors Influencing Discomfort Glare Perception from Daylight, C. Pierson, 2018
  12. Discomfort glare from daylighting: influence of culture on discomfort glare perception, C. Pierson et al, 2017
  13. Tuaycharoen, N and Tregenza, P.R., 2007. View and discomfort glare from windows,
  14. Discomfort glare cut-off values from field and laboratory studies, C. Pierson, 2018
  15. An Experimental Study on the Effect of Visual Tasks on Discomfort Due to Peripheral Glare, M. Kent, 2017
  16. HUMAN RESPONSIVE DAYLIGHTING IN OFFICES a gaze-driven approach for dynamic discomfort glare assessment, M. Sarey Khanie, 2015
  17. What attracts our visual attention? A study on saliency mapping for architectural daylit scenes based on virtual reality data, C. Karmann, 2019

 

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