TEN Key Daylight & Electric Light Metrics

By Liam Buckley on Thursday 30 May 2019

This short article summarizes ten key daylighting & electric lighting metrics that building designers are required to consider for various international building rating systems such as LEED, BREEAM & WELL; and also for various international building energy codes & standards such as ASHRAE 90.1, Title 24, etc.

1) ILLUMINANCE is a photometric term that quantifies light incident on a plane or a surface and can include contributions from electric light and daylight. The Illuminating Engineering Society (IES) recommends horizontal and vertical illuminance targets to ensure adequate illumination and safety for occupants of various ages. Illuminance is expressed in lux (lumens per square meter) or footcandles (lumens per square foot). 1 footcandle = 10.76391 lux.

Figure 01: Dynamic Daylight Illuminance (lux) – Greyscale & False-color

  • LEED v4.1 IEQ-DAYLIGHT (OPTION 2) requires illuminance calculations at 9am & 3pm on a clear-sky day at the equinox for each regularly occupied space. The four results are averages and are required to demonstrate illuminance levels are between 300 lux and 3,000 lux (28 ft.cd – 279 ft.cd).

Figure 02: LEED v4.1 IEQ Daylight Option 2 – Simulation: Illuminance Calculations

  • The WELL v2 Building Standard (L03 – Circadian Lighting Design) requires that a modeled illuminance value (lux) is multiplied by a melanopic ratio to calculate an Equivalent Melanopic Lux (EML). Melanopic ratios can be determined with this spreadsheet.

2) USEFUL DAYLIGHT ILLUMINANCE (UDI) is the annual occurrence of illuminances that is within a “useful” range for occupants.

Figure 03: Useful Daylight Illuminance (UDI)

Four UDI ranges are defined:

  1. Insufficient UDI (e.g. below 100 lux) represents lighting levels that are considered insufficient without electric lighting.
  2. Supplementary UDI (e.g. 100 lux – 500 lux) represents acceptable daylight levels to be integrated with electric lighting. 
  3. Autonomous UDI (e.g. 500 lux – 2,500 lux) represents acceptable daylight levels. Electric lighting would not be needed for the majority of the day. Achieving a high percentage of Autonomous UDI represents a predominantly daylit space during occupied periods and glare is controlled.  
  4. Exceedance UDI (e.g. above 2,500 lux) represents excessive amounts of daylight and a source of glare. This creates an expectation that blinds would be required.

3) DAYLIGHT FACTOR (DF) is the ratio of the illuminance at a point on a plane in a room due to the light received from a sky of assumed or known luminance distribution, to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct light is excluded from both values of illuminance, so a cloudy sky is modeled (e.g. CIE Overcast Sky). DF calculations provide the same results regardless of time of day (shown below) or orientation. DF is expressed as a percentage. 

Figure 04: Left: Dynamic Daylight Illuminance (lux) and Right: Static Daylight Factor (%)

  • 0-2% DF is inadequately light and so electric lighting is required
  • 2-5% DF is adequately light, but electric lighting may be required during some of the time
  • >5% DF is a well-lit space and electric lighting should not be required during daytime periods. Glare may be an issue.

4) UNIFORMITY is the ratio of minimum illuminance to average illuminance (EMIN/EAVE). For side-lit rooms, uniformity should be in the range of 0.3 – 0.4.  For top-lit spaces such as an atrium, a uniformity of 0.7 could be expected. In the example below, light-redirecting blinds are ‘throwing’ light further into the room while eliminating the excessive direct daylight under the window. A  Bidirectional Scattering Distribution Function (BSDF) is assigned to the window to represent the daylight redirecting blinds.

Figure 05: Uniformity variation with & without daylight-redirecting blinds

5) VERTICAL SKY COMPONENT (VSC) is a measure of the amount of sky visible from a center point of a window, though excludes direct light (i.e. uses a CIE Overcast Sky). A window that achieves 27% or more is considered to provide good levels of daylight. VSC is often viewed from the exterior of the model building and is particularly appropriate in a congested or urban environment.  Windows shown is green below pass the VSC threshold.

Figure 06: Vertical Sky Component

6) SPATIAL DAYLIGHT AUTONOMY (sDA) is the annual sufficiency of daylight levels in a space. sDA examines the percentage of an analysis area (e.g. working plane) that meets a minimum illuminance level (e.g. 300 lux) for a specified fraction of the operating hours per year (e.g. 50% of the operational hours of the year). In the example below, 20.8% of the working plane area exceeds the specified illuminance level of 300 lux for the specified amount of annual operational hours; i.e. 50% of the hours from 8am to 6pm. The sDA300lux/50% = 20.8% (7.2%+13.6%) is below a nominally acceptable threshold for sufficient daylight.

  • LEED v4.1 IEQ-DAYLIGHT (OPTION 1) defines the 40% as the minimum sDA to be awarded 1 point, 55% for 2 points and 75% for 3 points.  

Figure 07: Spatial Daylight Autonomy (sDA)

7) ANNUAL SUNLIGHT EXPOSURE (ASE) describes the annual potential for visual discomfort in a space. ASE is the percentage of an analysis working plane area that exceeds a specified illuminance level more than a specified number of hours. LEED v4 IEQ-Daylight Option 1 limits 10% of the working plane area to 1,000 lux for 250 hours per year. In the example below, 20.8% of the working plane area height exceeds the specified illuminance level of 1000 lux for more than 250 operational hours. ASE1000 lux/250 hours = 20.8%.

Figure 08: Annual Sunlight Exposure (ASE)

8) LUMINANCE measures light that is leaving a surface in a particular direction and considers the illuminance on the surface and the reflectance of the surface. Luminance is sometimes referred to as brightness and is measured in candelas/m2 (also known as nits) or candelas/ft2. (also known as foot-lambert). 1 cd/ft2 = 10.76391 cd/m2.

Figure 09: Dynamic Luminance & Daylight Glare Probability (DGP)

9) DAYLIGHT GLARE PROBABILITY (DGP) is a robust glare metric whereby glare sources are detected by contrast ratios with direct daylight considered, as are specular reflections. DGP is a newer Glare metric (2006) when compared against older glare metrics such as UGR, DGI, CGI & VCP.  

Figure 10: Glare & Daylight Glare Probability (DGP) from a fixed Occupant Position

DGP ranges are:

  • <0.35 = Imperceptible Glare
  • 0.35-0.4 = Perceptible Glare
  • 0.4-0.45 = Disturbing Glare
  • >0.45 = Intolerable Glare

10) ROOM CAVITY RATIO (RCR) is an important factor used in illuminance calculations and is the ratio of room dimensions used to quantify how light will interact with room surfaces.

Figure 11: Room Cavity Ratio (RCR)

RCR for Electric Lighting: The RCR is required to compare luminaire zonal lumen distribution because any given luminaire’s lumen values only represent the raw light emitted from a product, with no indication of light distribution in the actual application; i.e. in a room.  A luminaire’s Coefficient of Utilization (CU) provides such applied performance and is based on the RCR for any given space. Seemingly identical lighting fixtures in terms of lumens outputs and watts input, can vary by ~20% in actual lumens applied in the space once the RCR is known.

RCR for Daylighting: Processing the RCR is a common requirement in various building energy codes such as ASHRAE 90.1, IECC, Title 24 in California & NECB in Canada, just to name a few.  The relationship between a calculated RCR, glazing (vertical or horizontal) area and glazing locations helps to determine the primary side-lighted, secondary side-lighted and skylit zones in a room. Each daylight zone could see a corresponding photocell controls, auto-shutoff controls &/or daylight controls. Controls are applied to all or apportioned installed lighting power in associated daylight zones.  Some energy codes will determine the Lighting Power Density (LPD) of the baseline/standard building as a function of RCR and target illuminance values.

In summary, the range of daylight-related metrics can feel overwhelming and they will likely evolve as new building regulations become more energy-conscious and more considerate to the well-being of building occupants. To establish which metrics are appropriate for your building(s), the building owner & occupant should be made aware of the impacts of occupant visual comfort and the energy/cost impacts associated to complying with a building rating systems. 

See also: TEN Key Building Energy Metrics