Lighting
Daylight Factor Calculator
The purpose of Daylight Factor is to turn indoor illuminance and outdoor illuminance into daylight factor. Keep the source condition with the saved result so later comparisons remain meaningful.
Enter values for Daylight factor
Use measurements from one operating condition.
How the result is calculated
For this worksheet, the governing relationship is DF = indoor ÷ outdoor × 100. The formula draws on Indoor illuminance and Outdoor illuminance.
Using the loaded examples gives 2.50%. Your saved case should identify where each entry came from.
Calculate daylight factor from indoor illuminance and outdoor illuminance. When uniformity ratio affects the next decision, use Illumination Uniformity Calculator.
Before entering values
Record reflectance, maintenance condition, beam definition, and measurement location. A source note beside each entry makes the result reproducible.
A denominator entry cannot be zero. A prefix error can move the result by several orders of magnitude.
- Indoor illuminance
- Default example: 300 lx. Enter indoor illuminance in lx.
- Outdoor illuminance
- Default example: 12000 lx. Enter outdoor illuminance in lx.
Checks that catch bad inputs
Values recorded at different operating points should not share one case.
Recheck decimal placement and unit conversion when the answer looks unusual.
What the formula leaves out
Both readings should be simultaneous under comparable sky conditions.
The equation does not include daylight variation, control behavior, depreciation, and spatial uniformity. Document which effects were checked elsewhere in the analysis.
Keep lumens, candela, lux, and foot-candles on the correct geometric basis.
Sensitivity check
Change Indoor illuminance from 300 lx to 360 lx and leave all other entries untouched. The displayed value changes from 2.50% to 3.00%.
Do not average the two cases if either could define an operating limit.
Questions about Daylight Factor
What should I verify beyond the displayed inputs?
Both readings should be simultaneous under comparable sky conditions. A broader review should include daylight variation, control behavior, depreciation, and spatial uniformity.
Why might a second instrument give another answer?
Differences can come from driver efficiency, optical loss, surface reflectance, and beam-edge definition, measurement uncertainty, or values taken under different conditions.
What is the cleanest way to test another scenario?
Change one uncertain entry at a time and preserve the baseline. Use photometric data for the actual fixture, optic, mounting height, and task plane.
Does Daylight Factor Calculator choose a final rating?
The calculator does not approve equipment or supply an unexplained margin. Compare against illuminance, uniformity, driver loading, voltage loss, energy use, and maintained output.
Why does Daylight factor show several decimals?
Retain the raw result for comparison and create a separately rounded reporting value if needed.