Daylight Factor

Daylight Factor (DF) is a metric used in lighting design to measure the availability of natural light in an indoor environment. It is expressed as a percentage of the outdoor illuminance received indoors under overcast sky conditions. This means that a Daylight Factor of 5% indicates that the indoor illuminance is 5% of the outdoor illuminance under an overcast sky. Daylight Factor does not consider direct sunlight; therefore, it is independent of location, orientation, time, and date.

The simplicity of the Daylight Factor is one of its key advantages. It's easy to understand and calculate, making it a valuable tool in the early stages of design. Since it does not require information about the location or orientation of the space, it can be used as a general indicator of daylight potential in a design, regardless of where it will be built.

The main disadvantage of the Daylight Factor is its lack of comprehensiveness. It assumes an overcast sky and doesn't consider the sun's position or variations in sunlight intensity and colour throughout the day or year. This lack of detail can lead to an inaccurate representation of the actual daylight conditions in the space. Additionally, the Daylight Factor doesn't account for the effects of solar shading devices or changes in the reflectance of surrounding buildings or landscapes.

Spatial Daylight Autonomy

Spatial Daylight Autonomy (sDA) is a dynamic daylight metric that provides a temporal and spatial analysis of daylight in a space. It is expressed as the percentage of an analysis area (like a desk or a room) that achieves a certain illuminance level (typically 300 lux) for a certain percentage of the annual occupied hours. For example, a sDA300/50% of 75% means that 75% of the analysis area achieves at least 300 lux for at least 50% of the annual occupied hours.

Spatial Daylight Autonomy offers a more comprehensive understanding of daylight availability throughout the year. Unlike the Daylight Factor, it considers the sun's path, variations in sky conditions, and the effect of window shading devices. This makes it a more accurate and detailed measure of daylighting performance, especially in spaces with complex shading devices or climates with significant sunlight variations throughout the year.

The main drawback of sDA is its complexity. Calculating it can be computationally intensive and requires detailed inputs such as the location, building orientation, and window specifications. This makes it less suitable for quick evaluations or early stages of design when such detailed information may not be available.

Annual Sunlight Exposure

Annual Sunlight Exposure (ASE) is a dynamic daylight metric that quantifies the amount of direct sunlight received in a space. It is expressed as the percentage of an analysis area that receives more than a certain amount of direct sunlight (usually 1000 lux) for a certain number of hours per year (usually 250 hours). An ASE1000,250h of 25% means that 25% of the analysis area receives more than 1000 lux of direct sunlight for more than 250 hours per year.

The ASE helps to identify areas that receive too much direct sunlight, which could cause glare or overheating issues. This can be particularly useful in spaces with large windows or in climates with high solar radiation. It considers variations in sun path and sky conditions throughout the year, making it a more accurate measure of sunlight exposure than the Daylight Factor.

ASE does not measure diffuse daylight, which is a significant part of total daylight in many climates. This can lead to underestimating daylight availability in spaces with limited direct sunlight but abundant diffuse daylight. Like the sDA, calculating the ASE can be computationally intensive and requires detailed inputs.

Useful Daylight Illuminance

Useful Daylight Illuminance (UDI) is a dynamic daylight metric that assesses the usefulness of daylight in a space. It divides the year into periods of under-lit (below 100 lux), satisfactorily lit (between 100 and 2000 lux), and over-lit (above 2000 lux) conditions. It then calculates the percentage of the time each condition occurs. A UDI100-2000 of 60% means that the space is satisfactorily lit (between 100 and 2000 lux) for 60% of the time.

The UDI considers both under-lit and over-lit conditions, providing a balanced measure of daylight availability. Unlike other measures, it provides information about the quality of light, not just the quantity. This makes it a valuable tool for designing spaces where good visual comfort and productivity are important.

Defining what is "useful" can be subjective and vary among occupants and tasks. This makes the UDI less straightforward to use than other measures. Like the sDA and ASE, calculating the UDI can be computationally intensive and requires detailed inputs.

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