5.1 Light Exposure (L01) & Visual Lighting Design (L02)

5.1 Light Exposure (L01) & Visual Lighting Design (L02)

Core Exam Takeaway: Light has 2 preconditions (L01, L02) and 7 optimizations (L03–L09), and carries 9 scored exam questions. L01 Light Exposure is unusual in offering four alternative options, one of which is simply "earn a point in L03." L02 Visual Lighting Design is about illuminance for visual acuity, and its predetermined-level option hinges on an age condition.

The two jobs light has to do

WELL splits lighting into two physiologically distinct problems, and the feature structure follows the split:

1. Visual function — enough light, well distributed, with good color rendering and no flicker or glare, so people can see. This is L02, L04, L07, L08.
2. Circadian entrainment — light of the right spectrum and intensity, at the right time of day, reaching the eye on the vertical plane, so the body clock aligns with the day–night cycle. This is L01 and L03, and it is why circadian metrics are measured at eye level rather than on the desk.

L05, L06 and L09 then supply the daylight and the personal control that serve both jobs at once.

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Feature L01: Light Exposure — Precondition

Intent: provide indoor light exposure through daylight and electric light strategies. The inclusion of electric light in the intent is what makes Option 4 possible.

For all spaces except dwelling units — four options

Option 1: Daylight simulation. Demonstrate by computer simulation one of:

• (a) Regularly occupied spaces achieve average sDA₂₀₀,₄₀% for > 30% of regularly occupied floor area (per IES LM-83-12), or target illuminance 19 fc (200 lux) for > 30% of individual unit area throughout 50% of daylit hours of the year (per Annex A of CEN 17037:2018).
• (b) Common spaces with unassigned seating for at least 15% of regular occupants at any given time achieve average sDA₃₀₀,₅₀% for > 75% of floor area, or the CEN equivalent — 28 fc (300 lux) for > 30% of individual unit area and average 9 fc (100 lux) for > 95% of individual unit area throughout 50% of daylit hours.

Option 2: Interior layout. One of:

• (a) At least 30% of the regularly occupied area is within a 20 ft (6 m) horizontal distance of envelope glazing on each floor and/or in each individual unit.
• (b) Common spaces have unassigned seating accommodating at least 15% of regular occupants at any given time, and at least 70% of all seating in those spaces is within a 16 ft (5 m) horizontal distance of envelope glazing.

Option 3: Building design. One of:

• (a) Envelope glazing area is no less than 7% of the floor area for each floor level or individual unit.
• (b) The floor plate is no more than 65 ft (20 m) between opposite walls that each have transparent envelope glazing, and there are no opaque obstructions higher than 3.2 ft (1 m) within a 20 ft (6 m) horizontal distance of the transparent envelope glazing.

Option 4: Circadian lighting design. The project achieves at least one point in Feature L03: Circadian Lighting Design.

For dwelling units — three options

• Option 1: Daylight simulation — average sDA₂₀₀,₄₀% for > 30% of regularly occupied floor area, or the CEN equivalent of 19 fc (200 lux) for > 30% of individual unit area throughout 50% of daylit hours.
• Option 2: Façade design — envelope glazing no less than 7% of the regularly occupied floor area for each dwelling unit.
• Option 3: Circadian lighting design — at least one point in L03.

+-----------------------------------------------------------------------------+
|                 L01: FOUR WAYS TO SATISFY ONE PRECONDITION                  |
|                                                                             |
|   OPTION 1  SIMULATE       sDA targets per IES LM-83-12, or CEN 17037        |
|   OPTION 2  LAY OUT        >=30% of area within 20 ft (6 m) of glazing        |
|                            OR 70% of common-space seating within 16 ft (5 m) |
|   OPTION 3  BUILD          glazing >= 7% of floor area                        |
|                            OR floor plate <= 65 ft (20 m) between glazed      |
|                               walls, no opaque obstruction > 3.2 ft (1 m)     |
|                               within 20 ft (6 m) of glazing                   |
|   OPTION 4  ELECTRIFY      earn >= 1 point in L03 Circadian Lighting Design   |
|                                                                             |
|   -> A deep-plan basement office with no daylight at all can still meet      |
|      this PRECONDITION, via Option 4.                                       |
+-----------------------------------------------------------------------------+

[!IMPORTANT] Option 4 is the reason no project is disqualified by its floor plate. A windowless space cannot pass Options 1–3, but tuned electric lighting delivering L03's circadian light levels satisfies the precondition outright. For WELL Core, projects pursuing Option 4 must have access to at least 10% of leased spaces for testing.

[!TIP] Default surface reflectances. Where finishes are not finalized, WELL Core daylight simulations may assume ceilings 80%, walls 50%, floors 20%. Also, for WELL Core, the entire floorplate except circulation areas in non-leased spaces is considered regularly occupied — which makes the simulation harder, not easier.

Reading sDA notation

sDA₃₀₀,₅₀% means spatial Daylight Autonomy: the percentage of floor area receiving at least 300 lux for at least 50% of occupied hours annually, using annual climate-based simulation with operable shading modelled. The subscripts are the illuminance target and the time fraction. So sDA₂₀₀,₄₀% is a materially easier target than sDA₃₀₀,₅₀% on both axes — which is exactly why L01 uses the former for regularly occupied space and the latter for high-value common space.

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Feature L02: Visual Lighting Design — Precondition

Intent: provide visual comfort and enhance visual acuity for all users through electric lighting — explicitly "for regular users of all age groups, as required for the tasks performed."

L02 Part 1 — Provide Visual Acuity

For all spaces except dwelling units.

Option 1: Visual lighting design.

• (a) All indoor and outdoor spaces, including transition areas, comply with the illuminance thresholds in one of: IES Lighting Handbook 10th Edition; EN 12464-1:2011; ISO 8995-1:2002(E) (CIE S 008/E:2001); GB50034-2013; or the CIBSE SLL Code for Lighting.
• (b) The illuminance thresholds take into consideration the tasks and the age groups of the occupants.

Option 2: Predetermined light levels. Both:

• (a) More than 50% of the occupants are under the age of 65.
• (b) At least 90% of the project area comprises these space types and meets these thresholds:

For dwelling units — Option 1: Promote Visual Acuity.

• (a) Lighting installed in kitchens and bathrooms complies with the illuminance thresholds in the IES Lighting Handbook 10th Edition, ISO 8995-1, GB50034-2013 or CIBSE SLL Code (note: EN 12464-1 is not on the dwelling-unit list).
• (b) For spaces where lighting is not installed, tenants are provided with illuminance thresholds for common tasks and specifications, quantity and location of light fixtures required to meet those levels based on a sample layout.

Multifamily note: Bronze or Silver achievable without dwelling-unit testing; Gold and Platinum are not. WELL Core guidance: non-leased spaces; an additional point for also meeting requirements in leased spaces, which requires access to at least 10% of leased space for testing.

[!IMPORTANT] The age condition gates Option 2. Visual acuity declines with age — an older eye needs substantially more light for the same task performance — so WELL only permits the simplified fixed-level table where more than half the occupants are under 65. A senior living project must use Option 1 and design to a reference guideline that accounts for age. This is a favourite exam scenario.

[!TIP] Note the measurement plane differences in the Option 2 table: offices, classrooms, dining, lounges and restrooms are measured at the task surface, while lobbies, atria, transitions and storage are measured at floor level. Getting the plane wrong is as much an error as getting the number wrong.

Why the Light concept measures three different planes

Circadian response depends on light entering the eye, not light landing on paper — so L03 deliberately abandons the horizontal plane that L02 uses.