Light Intensity Calculator
Calculate the irradiance of a point light source at any distance using the inverse-square law.
How it works
- 1Enter the radiant power P of the source in watts and the distance r from the source in metres.
- 2The calculator applies the inverse-square law I = P ÷ (4π·r²) to find the irradiance at that distance.
- 3Results are shown in W/m² and converted to mW/cm² for lab and photometry reference.
Use cases
- Estimating solar irradiance from a point-source model at different distances.
- Designing photography or cinematography lighting to hit a target exposure.
- Checking exposure levels when working with high-power laser or UV sources.
Frequently asked questions
What formula does this calculator use?
I = P ÷ (4π·r²), where I is irradiance in W/m², P is radiant power in watts, and r is the distance in metres. A 100 W isotropic source at 2 m gives I = 100 ÷ (4π × 4) ≈ 1.99 W/m².
What is the inverse-square law?
Irradiance is inversely proportional to the square of the distance. Double the distance → one quarter the intensity; halve it → quadruple. The same power spreads over a sphere whose area grows as r².
What does “isotropic point source” mean, and when does it break down?
An isotropic point source radiates equally in all directions from a single point. Real sources (LEDs, lamps, lasers) have finite size and directional patterns, so this is an approximation — most accurate when r is large relative to the source size.
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