Why do we measure light with the Decibel?

The brightness of light. We measure optical power in Decibels.

A decibel is a logarithmic ratio between two values, this doesn’t tell you double of what? is a dimensionless-unit, meaning it does not express an actual physical measurement on its own

Light, like sound, follows the inverse square law. And the signal is inversely proportional to the distance squared.

A signal travels distance X and loses half of its intensity. The signal travels another distance X and loses another half.

 After 2X only 25% remains, after 3X only 12.5% remains. Using a logarithmic scale simplifies the calculations.

Can calculate 3dB change is approximately half/double the original signal.

In the example above, there is a 3dB loss per distance X. At distance 2X there is 6dB of loss, at distance 3X it is 9dB.

Another easy one: +3dB is double -3dB is half

To express an absolute value, we need a reference, “dBm”. a decibel relative to 1 milliwatt (mW) of power.