About the Light Intensity Calculator
What is the Light Intensity Calculator?
Light, sound and radiation from a point source spread over a sphere, so their intensity falls with the square of distance. This inverse square law calculator works out the intensity at any distance.
Use the comparison mode to see how illuminance (lux) or radiation changes when you move closer or further away.
How to use the Light Intensity Calculator
- Calculation (sample: Point source – power and distance)
- Source Power (W) (sample: 100)
- Distance from the Source (m) (sample: 2)
- Known Intensity or Illuminance (W/m² or lux) (sample: 400)
- At Distance (m) (sample: 1)
- New Distance (m) (sample: 2)
- Press Calculate Intensity (results also refresh as you type). The result card shows the Intensity, along with At double the distance and Surface area of the sphere.
Formula used
I = P ÷ (4πr²); I₂ = I₁ × (d₁ ÷ d₂)²
- P = power of the source
- r = distance from the source
The law applies to point sources; long tube lights and lasers fall off more slowly.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Calculation | Point source – power and distance |
| Source Power (W) | 100 |
| Distance from the Source (m) | 2 |
| Known Intensity or Illuminance (W/m² or lux) | 400 |
| At Distance (m) | 1 |
| New Distance (m) | 2 |
| Result | Value |
|---|---|
| Intensity | 1.9894 W/m² |
| At double the distance | 0.4974 W/m² |
| Surface area of the sphere | 50.265 m² |
Tips for using the Light Intensity Calculator
- Moving a reading lamp twice as far away quarters the light on the page.
- Doubling distance from a radiation source cuts the dose rate to a quarter – a key safety principle.