Why is

I(x)=dP/dx*1/A

x is the wavelenght lambda
I is the intensity (I=P/A)
P is the power
A is the area

Thanks in advance

I don't understand the equation. Unit wise, the left side does not equal the right. Power is intensity/area, and dP/dl= intensity/volume, or energy density.

So the left side does not equal the right side.