The inverse square law of light defines the relationship between the irradiance from a point source and distance. It states that the intensity per unit area varies in inverse proportion to the square of the distance. Distance is measured to the first luminating surface - the filament of a clear blulb, or the glass envelope of a frosted bulb.

*Example:* You measure 10.0 lm/m² from a light bulb at 1.0 meter. What will the flux density be at half the distance?

*Solution:*

E_{1}=(d_{1}/d_{2})²*E_{2}

E_{0.5 m}=(1.0/0.5)²*10.0 = **40 lm/m²**

**Please note**: This calculator should only be used if the light source approximates a point source and the original measurement to calculate from is in irradiance or illuminance units (i.e. W/cm², lux, etc..). The calculator itself does not need to work with optical or distance units but those units are assumed and in the case of the distance units, must be consistent from the original distance to the new measurement distance. Data may be entered into the calculator either in decimal format (i.e. "0.000341") or in scientific notation (i.e. "3.41e-04").

In **Step 1**, enter the original measurement distance in the d1 box.

Next in **Step 2**, enter the original measurement value taken at the original distance into the e1 box.

Finally in** Step 3**, enter the new measurement distance in the d2 box.

After that, click the **Calculate** button to calculate what the new measurement value (Output) will be at the new distance based on the Inverse Square Law.

Click the **Reset** button to reset the calculator.