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Inverse Square Law For Light Calculator

Work out inverse square law for light instantly with clear inputs, formula shown and shareable results.

About Inverse Square Law For Light Calculator

Light from a point source spreads over a sphere, so illuminance falls with the square of distance. Moving from 1 m to 3 m cuts 800 lux to 89 lux - a ninefold drop, or about 3.17 photographic stops. The product E.d^2 is constant and equals the source intensity in candela.

Formulas

Inverse square law

E2 = E1 x (d1 / d2)^2

Source intensity

I = E x d^2

Worked example

With the default values below, Inverse Square Law For Light Calculator returns Illuminance At The Second Distance of 88.889. Change any input above and every figure here is recalculated in your browser.

Inputs used

Illuminance at the first distanceminimum 0 lx
800 lx
First distanceminimum 0 m
1 m
Second distanceminimum 0 m
3 m

Results

Illuminance At The Second Distance
88.889 lx
Ratio To The Original
0.1111
Implied Source Intensity
800.0 cd
Change In Photographic Stops
3.170

Step by step

  1. 1Values used
  2. 2Inverse square law
  3. 3Source intensity
  4. 4Illuminance At The Second Distance= 88.889 lx
  5. 5Ratio To The Original= 0.1111
  6. 6Implied Source Intensity= 800.0 cd
  7. 7Change In Photographic Stops= 3.170

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Does this apply to a softbox or a fluorescent tube?

Only beyond a few times its largest dimension. Close in, an extended source falls off much more gently, which is why big softboxes are forgiving.

Why does moving a light slightly matter so much up close?

Because the fall-off is quadratic. Moving from 0.5 m to 1 m loses three quarters of the light; from 5 m to 5.5 m loses under a fifth.

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