Who this tool is for
The Daylight Calculator answers the question every architect, designer, and homeowner asks before laying out a space: how much natural light will this room actually get?Pick a city, set the room and windows, see workplane illuminance, sun position, and WELL L03 compliance in real time.
Architects & designers
Schematic-stage daylight study without firing up Rhino + Climate Studio. See the impact of window size, orientation, building rotation, and seasonal sun angle on interior illuminance — and which surfaces stay in the dead zone.
WELL APs & LEED consultants
Get a fast read on daylight mel-EDI per CIE S 026:2018 — the metric WELL v2 L03 references — for any room in any city. Hand the configuration off to CircadianLab to add electric supplement and produce a documented compliance report.
Healthcare & wellness designers
Verify that patient rooms, clinical spaces, and care facility common areas receive enough morning sun and adequate mel-EDI for circadian benefit — especially critical at high latitudes and in north-facing exposures.
Homeowners & renovators
Will my basement office be bright enough? Will this north-facing apartment feel gloomy? Should I add a skylight? Set up your space, drag the time slider, watch the sun move across the floor across the year.
How it works
Three computations chain together to produce the daylight result for any room configuration:
- Solar position — given your latitude, longitude, and the local date and time, the sun's exact altitude (above the horizon) and azimuth (compass bearing) are computed with the simplified NREL Solar Position Algorithm, accurate to ±0.01° from 1995–2050.
- Exterior illuminance — the chosen sky condition (clear, intermediate, or overcast) determines how much of the sun's energy reaches the ground as direct beam (DNI) and how much as diffuse sky light (DHI), via the Perez 1990 model parameterized for that sky type.
- Interior daylight via split-flux method — for each workplane point (we sample on a 24×24 grid), the tool computes (a) whether direct sun reaches the point through any window aperture, (b) the diffuse-sky view factor through each window, and (c) an internally-reflected average term using the BRE / IES TM-25 split-flux formula. Sum: photopic illuminance in lux.
- Melanopic EDI — for each point, the effective CCT (sun + sky weighted) determines mel-DER per the CIE S 026:2018 calibration; mel-EDI = lux × mel-DER. WELL L03 thresholds (150 / 200 / 275 lx) are checked automatically.
The whole pipeline runs in under 20 milliseconds per update — fast enough to animate the time slider through a full day in real time. For radiosity-grade detail with multi-bounce inter-reflections and electric supplement, CircadianLab picks up where this preview leaves off.
Frequently asked questions
How does the daylight calculator work?
You define a room (dimensions, surface reflectance), add windows (size, orientation, glazing), and pick a location and time. The tool computes the sun position via NREL's Solar Position Algorithm, exterior illuminance via Perez sky models, and interior illuminance via the BRE / IES TM-25 split-flux daylight method. Results update in your browser in real time.
What's the difference between this and CircadianLab?
The Daylight Calculator is a fast preview — sub-20 ms per recompute — designed for "what-if" exploration: try different room sizes, window orientations, times of year, and locations. CircadianLab is a full-radiosity simulator that adds inter-reflections, electric fixture supplementation, glare, and detailed WELL compliance documentation. Click "Open in CircadianLab" to hand the configuration off for the deeper simulation.
How accurate is the daylight estimate?
The fast-preview engine is calibrated against the BRE Worked Example reference rooms (within ~10–15% relative error for typical configurations). Solar position is accurate to ±0.01° via the NREL SPA. For LEED EQc Daylight or WELL L06 documentation requiring sDA / ASE annual simulation, use a dedicated tool like Climate Studio or DIVA for spec-stage credit submission.
Can it tell me if my room is bright enough for circadian benefit?
Yes. The tool reports daylight melanopic-EDI per CIE S 026:2018, the metric WELL v2 Feature L03 references. Three thresholds (150 / 200 / 275 lx mel-EDI) are visualized so you can see what fraction of the room reaches each WELL tier from daylight alone — and what fraction needs supplemental electric lighting.
What do I do if my building is at a non-cardinal angle?
Use the Building rotation slider to set how many degrees the building is rotated clockwise from true north. The room geometry stays clean on screen (axis-aligned), but sun calculations use your actual real-world orientation. Most architects on a site survey can read this directly from the project drawings.
Can I share results or save a configuration?
The "Open in CircadianLab" button generates a URL that encodes the entire configuration (room dimensions, windows, location, date, time, sky condition). Bookmark or share that URL to come back to the same scene later, or to send it to a colleague.
What sky models does the tool support?
Three CIE-aligned sky conditions: clear (low diffuse fraction, sun-dominated), intermediate (mixed clouds), and overcast (no direct sun, fully diffuse — equivalent to CIE Standard Overcast Sky). For research-grade annual climate simulation with TMY weather files, that's a future tool we're planning (Daylight Autonomy Calculator).