Lighting
Daylight Factor Calculator
This page handles the arithmetic for daylight factor while leaving room to review units, limits, and source conditions.
Set up the Daylight Factor case
Small unit changes can move daylight factor; confirm scale before using the result.
What daylight factor means here
This worksheet is for calculate daylight factor from indoor illuminance and outdoor illuminance. The answer is easier to trust when the entered values describe the same equipment, period, and operating condition.
Daylight Factor changes with mounting height, beam shape, surface reflectance, and driver limits.
If two cases differ in indoor illuminance, save them as separate runs. A blended entry can hide the reason the daylight factor changed.
Outdoor illuminance and the other entries
On Daylight Factor, confirm decimal placement and unit scale before using daylight factor; small prefixes can move an electrical result by orders of magnitude.
- Indoor illuminance. Example value: 300 lx. Enter indoor illuminance in lx.
- Outdoor illuminance. Example value: 12000 lx. Enter outdoor illuminance in lx.
A separate Foot-Candles Lux run is cleaner when that result becomes the real design question.
The Daylight Factor equation
The Daylight factor value and any supporting metrics belong to this same Daylight Factor case, not to a nearby run with different inputs.
Read the Daylight Factor equation from the units on this page. A familiar symbol can shift meaning between DC, AC, energy, component, or billing work.
A clean Daylight Factor result confirms the arithmetic for daylight factor; it does not prove the equipment rating or source data is the right one.
Sample Daylight Factor run
Using the sample values, Indoor illuminance = 300 lx; Outdoor illuminance = 12000 lx, the primary result is 2.50%.
This Daylight Factor one-input test is a scale check, showing direction and sensitivity without turning the changed case into a recommendation.
If the changed indoor illuminance comes from a real measurement, keep the original Daylight Factor run so the source of the difference remains clear.
Turning daylight factor into a decision
Use daylight factor as a checkpoint. If it drives a decision, compare a conservative Daylight Factor case before accepting the number.
Daylight Factor changes with mounting height, beam shape, surface reflectance, and driver limits.
Changing indoor illuminance while leaving outdoor illuminance fixed is a useful way to see which assumption controls the answer.
What not to assume from Daylight Factor
Both readings should be simultaneous under comparable sky conditions.
- Fixture spacing
- Surface reflectance
- Dimming compatibility
When daylight factor is close to a rating, use the stricter review path instead of relying on the extra decimals shown by Daylight Factor.
Notes to save with Daylight Factor
Keep fixture model, mounting height, distribution angle, and target plane with the Daylight Factor result.
| Input set | Indoor illuminance, and Outdoor illuminance |
|---|---|
| Result | Daylight factor |
| Condition note | Both readings should be simultaneous under comparable sky conditions. |
Common Daylight Factor questions
Which value should be rounded first?
Keep indoor illuminance at source precision during Daylight Factor; round daylight factor only where presentation or purchasing requires it.
What should be recorded with the answer?
Save the Daylight Factor inputs, unit basis, operating case, and result note so daylight factor can be reproduced later.
Why might a real measurement disagree?
A field reading for Daylight Factor can include losses, tolerance, temperature effects, waveform distortion, or instrument limits that the worksheet does not model.