UNDERSTANDING VISUAL NEEDS
Good daylighting can achieve a variety of results, including creating great architecture and inspiring a range of emotions in people. Creative, experiential ambitions of daylighting, however, should not be prioritised over ensuring that tangible visible needs are met – rather, appropriate visible needs should form the foundation from which to design your inspiring daylighting outcomes.
Buildings must be capable of being used for the purposes they are designed for, and daylighting is an important element shaping whether that is the case. If the lighting in a space is unsuitable or inadequate, making it difficult, uncomfortable, or even unhealthy to use the space, it not only negatively influences the intended use, but can significantly reduce the value of the building to its users.
UNDERSTANDING VISUAL NEEDS
KEY LEARNINGS
- Become familiar with guideline interior light levels.
- Gain an appreciation for what constitutes visual comfort.
- Describe the three main types of glare.
How much illuminance is needed?
To orientate ourselves and move freely within rooms and buildings, interior light levels should not impede our ability to see. Poor brightness and inadequate contrast, high luminance differences, and flickering light can all cause unnecessary eye strain, eye irritation, fatigue, headaches, and lead to accidents and/or injuries.
The absolute level of illuminance needed for a visual task depends on the character of the task, and on the visual environment where the task is performed. Sector-specific guidance should always be sought where appropriate.
As an example of recommendations for ‘general’ day-to-day tasks, the Chartered Institution of Building Services Engineers, CIBSE provides the following interior light levels.1
Evidence in literature indicates that natural illuminances in the range of 100 to 3000 lux are likely to result in significant reduction of electric lighting usage.2
Visual comfort
Visual comfort in relation to daylight covers a range of factors including, but not limited to, intensity of light, contrast/uniformity, direction of light source, colour and glare.
As with comfort of any type, what conditions create visual comfort is at times subjective and therefore difficult to define universally. However, a range of metrics can be used to assess, at an objective level, different aspects of light in buildings that can help ensure reasonable comfort for most users. Selecting appropriate factors to target and measure can help to create a definition of visual comfort for an individual project, or multiple definitions for different areas of a building when relevant.
The distribution of light in a space, and how that light is perceived by the eye, is perhaps one of the most fundamental aspects of visual comfort. When the distribution of light is not balanced, it often results in glare – an experience that anybody can relate to in terms of understanding when visual comfort is not achieved.
Poor visibility and visual discomfort, including glare, can occur if the eye is forced to adapt to a wide range of light levels too quickly and/or too often. Glare can occur when luminance variation in the range of 20:1 to 40:1 and, when it occurs, the eye adapts to the high luminance level of the glare source, making it hard to perceive details in the now too-dark focus area3.
Photography: Iwan Baan. Architects: SANAA
DID YOU KNOW?
The human eye can generally accept greater luminance variations when spaces are lit by daylight. For dwellings, luminance variations of around 10:1 are considered suitable for general daylighting design.
What are the three main types of glare?
A good daylighting design provides large amounts of glare-free light. A poor daylighting design provides either inadequate amounts of daylight, so that electric lighting must be used frequently, or too much daylight in certain areas, problematic contrast, or reflections, which can lead to excessive glare, heat gain, and/or unhealthy interior environments4.
Glare from daylight may be caused by the sun, bright skylight and clouds, and surfaces reflecting the daylight. It directly relates to the luminance ratio in a field of view. There are three main types of glare.
Discomfort glare
Discomfort glare is an irritating or distracting, but not necessarily impairing, effect. In most cases, its perceived magnitude is lower than for disability glare.
Indoors, discomfort glare is influenced by the full visual environment, including windows, reflections (especially specular), external surroundings and/or interior surfaces. Discomfort glare can cause later side-effects or after-effects, in the form of headaches or fatigue.
Example: the field of view from an office desk includes a small facade window with no shading device, while the rest of the room is below the 'daylit' threshold. The contrast between the external light levels perceived through the window, and the internal light levels, creates visual discomfort for the person working at the desk.
Disability glare
Disability glare is the effect of scattered light in the eye, reducing visibility and visual performance. It occurs when glare sources of high luminance (e.g., the sun or specular reflection of the sun) are in the field of view. In daylit interiors, discomfort glare is usually reported before disability glare becomes an issue.
Example: specular reflection from a glass building opposite a workspace creates a blinding effect that prevents a person from completing the task they are trying to perform.
Reflection or veiling glare
Reflection glare reduces the contrast between background and foreground for the visual task, and therefore reduces readability. This can occur on display screens, or materials such as paper. Reflections occur when bright light sources (e.g., sun through windows) are in the reflected field of view of the screen.
Example: struggling to read the on-screen image when using a laptop outside or by a window on sunny day.
Views to the outside Whilst visual comfort is important, there is immense valuable in extending beyond this to include our deeper human needs - visual ‘delight’5. Building users have a natural visual need for a view to the outside. A view supplies information about orientation, gives experience of weather changes, and generally makes it possible to follow the passage of time over the day – something that is particularly critical for our circadian rhythms. Wherever possible, views should incorporate ’layers’ for the greatest visual interest, and preferably include a view of the sky, horizon, and ground6.
Thinking beyond visual needs Good and bad daylighting impacts on building users in different ways. Visual needs encompass only one aspect of an occupant’s experience, and it’s also necessary to think about the non-visual impact of daylight. Understanding more about the human response to daylight can strengthen the case for pursuing particular outcomes to deliver maximum comfort in buildings.
» Read on: Understanding non-visual needs
1 CIBSE (2006), Guide A: Environmental Design. Chartered Institution of Building Services Engineers. · 2 Mardaljevic, J. (2008), Climate-Based Daylight Analysis for Residential Buildings. Institute of Energy and Sustainable Development, De Montfort University · 3 Rea, M.S. (2000) The IESNA Lighting Handbook: Reference and application, New York: Illuminating Engineering Society of North America. · 4 Boyce et al. (2003), The Benefits of Daylight through Windows, Lighting Research Center, Rensselaer Polytechnic Institute. · 5 Heschong, L. (2021) Visual Delight in Architecture - Daylight, Vision and View. Taylor & Francis LTD. · 6 CEN European Daylight Standard (EN 17037:2018), Assessment of views.