Lighting
Inverse Square Lighting Calculator
Use Inverse Square Lighting when illuminance is the quantity you need. Enter luminous intensity and distance, then compare the answer with the equipment or operating limits that apply to your case.
Calculate Illuminance
Do not mix values recorded under different conditions.
Preparing the calculation
Keep lumens, candela, lux, and foot-candles on the correct geometric basis. Nameplate and meter values are not interchangeable unless they describe the same condition.
At least one field has a positive lower bound because it appears in a denominator. Use the displayed unit for every field and document conversions separately. Continue to Daylight Factor Calculator if daylight factor is also needed.
- Luminous intensity
- Default example: 1200 cd. Enter luminous intensity in cd.
- Distance
- Default example: 3 m. Enter distance in m.
Illuminance: calculation method
Here, Illuminance is obtained from E = I ÷ d². The participating entries are Luminous intensity and Distance.
With the current entries, the primary answer is 133.33 lx. Preserve the raw inputs when replacing this example.
Estimate on-axis illuminance from luminous intensity and distance. If you also need luminous intensity, continue with Candela Lumens Calculator.
Compare two operating cases
Change Luminous intensity from 1200 cd to 1440 cd with every other entry unchanged. The result changes from 133.33 lx to 160.00 lx.
The comparison demonstrates sensitivity, not a guaranteed field response.
Applying the result
Interpret Illuminance on the same basis used for the source values. Compare it with initial, maintained, and worst-case lighting conditions.
Store the measurement state and conversion notes with the answer. The next worksheet may be Foot-Candles Lux Calculator if illuminance is needed.
What the formula leaves out
The inverse-square relationship assumes a point source, normal incidence, and no reflections.
The equation does not include driver efficiency, optical loss, surface reflectance, and beam-edge definition. Use measurements, manufacturer data, or another calculation for effects that can change the decision.
Keep lumens, candela, lux, and foot-candles on the correct geometric basis. For the companion uniformity ratio calculation, open Illumination Uniformity Calculator.
Questions from practical use
What explains a gap between calculated and observed illuminance?
Differences can come from driver efficiency, optical loss, surface reflectance, and beam-edge definition, measurement uncertainty, or values taken under different conditions.
How should I compare two Inverse Square Lighting cases?
Keep unaffected entries fixed so the cause of the difference remains visible. Keep lumens, candela, lux, and foot-candles on the correct geometric basis.
Can I select equipment from this number alone?
The calculator does not approve equipment or supply an unexplained margin. Compare against initial, maintained, and worst-case lighting conditions.
Should intermediate values be rounded?
Keep full precision while transferring illuminance; round only at the final reporting or selection step.