# Understanding the formula used by FDTD to calculate absorbed power per unit volume

I’m looking for a reference that shows a derivation of the formula for the absorbed power per unit volume as discussed in this Lumerical Knowledge Base entry. Anyone know of one? I’d like to understand this better.

The loss defined in KB is Ohmic loss and is true if current density is zero (which is the case in our optical simulaitons). You can take a look at this link, check Eqs. 1.9.6 and 1.14.5.

Please let me know if you have further questions.

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This is exactly what I was looking for. Thanks @bkhanaliloo!

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