The Egyptians used daylight controls to temper the heat of their extreme climate, introducing lattice and screens with different size openings to allow for daylight penetration into space. In Rome, buildings were designed around courtyards surrounded by living space to maximize available daylight. European Renaissance masters revered light as both a practical and aesthetic design tool. The baroque style used indirect light to create mystery and intrigue in buildings, but as electric lighting sources and technologies improved; daylight took a back seat in lighting design.
Today, encouraged by updated building codes, new energy regulations, and a renewed emphasis on sustainability, architects, building owners, and lighting designers are once again embracing daylight as a practical, aesthetic, and symbolic element of good building design. This is known as designing for daylight.
Just using daylight for habitable spaces and affirming the building to be green is not enough in today’s competitive world. That’s where the concept of usable daylight comes in picture. Hence the concept of daylight autonomy was evolved. Spatial Daylight Autonomy and Annual Sunlight Exposure allow designers to quantify and compare the success of daylit spaces. (Kevin Van Den Wymelenberg, 2016) It basically acts as a judge on the qualitative daylight performance of the building.
This concept has been devised by IESNA and applied for the design of many buildings in the USA, where the amount of available daylight is low compared to the tropical climates. However, this concept needs to be emphasized in tropical climates since the available exterior illumination is high. The actual usable daylight times in India are pretty short since most of the time the daylight is too harsh. Therefore, there is a need for correlation of daylight autonomy with Indian climatic context.
Daylighting is an important aspect of achieving energy efficiency in built form. The significant contradiction arises in buildings in the hot climate regions when relating window sizes and shadings to achieve both reduced energy consumption (requiring minimum opening ratios and more shading) and sufficient daylighting (requiring maximum opening ratios and less shading). (Mohamed Amer Hegazy, 2013) Natural lighting has two noticeable effects, Light and Heat. In hot climates, large windows can provide more daylight but higher cooling loads, while small windows can decrease energy consumption but do not offer sufficient daylight. Hence, hot climate is considered one of the most challenging climates when it comes to daylighting (A., 2012)
What is Daylight?
The entire natural light available from the sun and reflecting sky. It is an important feature to be considered for designing an energy efficient building, as it reduces the load on electric lighting during daylit hours.
What does Autonomy mean?
It means independence or freedom.
Daylight autonomy essentially means the autonomy in natural light, meaning that space does not require artificial light to perform activities, which happens to be the core of sustainable lighting design. It means to maximize the amount of useful daylight, thereby minimizing or eliminating the need for supplemental electric light.
What is useful daylight, and how is it defined?
Useful daylight illuminance (UDI) is a modification of daylight autonomy which places illuminance between 100 – 2000 lux. The range 0-100 lux is termed as under-illuminated and insufficient for functioning, whereas above 2000 lux is termed as over illuminated causing glare. There is little research to support the selection of 2,000 lux as an absolute upper threshold. Basically, UDI guarantees visual comfort.
Useful daylight is independent of daylight factor. Daylight factor calculates the percentage of interior illumination with respect to exterior illumination. Exterior illumination differs with latitude and sky conditions. Hence DF cannot be termed as an ultimate measure of interior illumination.
Calculation
Daylight autonomy is calculated as the percentage of time when the architectural project is autonomous with daylighting in functional hours. Illumination on each point throughout the daylit hours of the year will be measured. The percentage of time at which the daylight on that particular point falls between 100-2000 lux (UDI) will be calculated. This is presented as the daylight autonomy if the point. For a successful design, it should at least be 50%. It is a major innovation since it considers geographic location-specific weather information on an annual basis.
In case the basic requirement for the function is 300 lux, the user is free to set the threshold above which Daylight Autonomy is calculated. For the graphs above, we selected a Daylight Autonomy threshold of 300 lux (DA300). The graphical percent values represent the percentage of the floor area that exceeds 300 lux for at least 50% of the time.
Figure 1 calculation of daylight autonomy
Importance
All the criteria in ECBC, IGBC, LEED, GRIHA, etc define the lower threshold of illumination through daylight. Most of the activities have minimum lux levels defined, like 300 lux for office working, 50 lux for circulation, etc. However, knowing the dynamic nature of daylight, buildings and daylight apertures designed for overcast sky conditions, will provide illumination much more than required. Lux levels as high as 3000-4000 lux will be achieved. This can easily cause glare for the occupants and hamper functioning. In such conditions, it is observed that the occupant will close the daylight aperture and will rely on the more adjustable artificial lighting. This will defy the whole purpose of sustainable lighting. Hence there is a need to define the upper threshold of illuminance through daylighting.
Daylight autonomy defines the lower as well as the upper level of illumination. This range is termed as usable daylight illumination. Furthermore, it defines the percentage of time in the whole year where a particular point will receive daylight within the usable range. This can define the annual daylight performance of the building. Since most of the buildings when designed for daylight are designed only for overcast sky conditions, the annual performance is ignored. This results in actually higher usage of artificial illumination in highly daylight hours. Thus daylight autonomy can form a better measuring unit for the annual daylight performance of the building.
Available exterior illumination in the tropical climate of India
India is a tropical climate lying between latitudes 8 degrees to 32 degrees north. With respect to solar position and available exterior illumination, this is a wide range. The available exterior illuminations for different latitude are given in the following table.
(Littlefield, 2012)
The skies are mostly clear except for the months of July and August, resulting in abundant daylight available from 8 am to 4 pm, the primary daylit hours.
Correlation between daylight autonomy and Indian climate
Daylight is the visible part of solar radiation and daylight at these latitudes can also mean thermal gain. In addition, bright sky reflected light from the ground, and direct solar radiation can be causes of glare. To avoid overheating, daylight is normally blocked by shading devices or highly reflective glazing. To manage glare, most traditional diffuser materials and glare management systems reduce the usable output lumens as direct light is blocked and spreading increased. As a result, interior spaces can be dark and many buildings depend entirely on artificial lighting for illumination, even when external illuminance levels are very high.
The significance of daylight autonomy in higher exterior illuminance of India
Daylight autonomy gives a measure of total illumination on the work plane available throughout the year in daylit hours. It considers the lower as well as upper threshold. The lower threshold can be modified according to the function and activity requirements. Daylight is dynamic, thus there is no one common solution for all scenarios. It is more critical in the tropical climate where the sky is predominantly intermediate; with inconsistent clouds formations which will influence the presence of direct sunlight and daylight availability. Daylight autonomy is a dynamic measure of daylight illumination. Therefore it is proposed that instead of any static measures like daylight factor or illumination in overcast conditions, daylight autonomy or continuous daylight autonomy with the help of useful daylight illumination will form a better evaluation method in Indian climate.
Softwares used to calculate daylight autonomy
Daylight modelling is an effective way of predicting the daylight performance of the building. Ecotect provides an excellent tool for the same. The modelling can be done in Ecotect while lighting analysis can be carried out in radiance or daysim. Radiance can provide a time by time calculation of daylight, whereas daysim provides an annual matrix in the form of daylight autonomy. The upper and lower levels of illumination can be set in daysim for usable daylight.
Light, life and architecture 