Daylight Metrics

Daylight is merely the visible part of the radiant energy that enters through windows. Furthermore, the bulk of the daylight energy that enters a space is converted into thermal energy after just a few reflections. Many office buildings in moderate climates now have air conditioning largely due to the high internal gains. In warmer climates cooling may be needed for large parts of the year.

Most of the software used today are based on radiance but have different interface. While there are a number of options available for daylight simulation, there are certain criteria that you should ask yourself before choosing the simulation software for daylighting. Following are few daylight metrics used to describe or measure daylight in a building:

1. Illuminance

Iluminance is the most widely used photo metric quantity to describe the light in spaces. It is defined as the total luminous flux incident on a surface and measured in lumen per unit area :

2. Daylight Autonomy (DA)

Daylight Autonomy (DA) was the first of a string of annual daylight metrics, now commonly referred to as ‘dynamic daylight metrics’. It is represented as a percentage of annual daytime hours that a given point in a space is above a specified illumination level.
Unit: %

Fig. Daylight autonomy on online simulation platform- www.andrewmarsh.com

3. Daylight Factor (DF)

Daylight Factor is a ratio that represents the amount of illumination available indoors relative to the illumination present outdoors at the same time under overcast skies. Daylight Factor is typically calculated by dividing the horizontal work plane illumination indoors by the horizontal illumination on the roof of the building being tested and then multiplying by 100. For example, if there were 20,000 lux available outdoors and 400 lux available at any given point indoors, then the DF for that point would be calculated as follows DF = 400/20,000 *100 or DF=2.

Daylight Factor is to be used under overcast sky conditions only. Daylight factor is the most common metric used when studying physical models to test daylighting designs in ‘overcast sky simulators’.
Unit: %

Fig. Daylight Factor on online simulation platform- www.andrewmarsh.com

4. Useful Daylight Illuminance (UDI)

Useful Daylight Illuminance (UDI) is a modification of Daylight Autonomy. This metric bins hourly time values based upon three illumination ranges, 0-100 lux, 100-2000 lux, and over 2000 lux. It provides full credit only to values between 100 lux and 2,000 lux suggesting that horizontal illumination values outside of this range are not useful.

The graphical percent values represent the percentage of the floor area that that meets the UDI criteria at least 50% of the time.
Unit: %

Fig. Useful daylight on online simulation platform- www.andrewmarsh.com

5. Annual Daylight Exposure (ASE)

With higher levels of daylight sufficiency comes the potential for glare and solar heat gain. That’s where Annual Sunlight Exposure (ASE) steps in. Meant to complement sDA, ASE is intended to help designers limit excessive sunlight in a space.

Annual Sun Exposure (ASE) describes how much of space receives too much direct sunlight, which can cause visual discomfort (glare) or increase cooling loads. Specifically, ASE measures the percentage of floor area that receives at least 1000 lux for at least 250 occupied hours per year

Unit: lx-hours/year

Fig. Annual Sun exposure on online simulation platform- www.andrewmarsh.com