# Homework Help: Energy of a signal

1. Aug 31, 2007

### n0_3sc

I have a graph of "dBm vs Wavelength".
I want the total Power in a certain area of the graph.

Do I convert the graph to:
"Power (Watts) vs Wavelength" then just Integrate within the required wavelength region?
ie.
$$\int_{\lambda_1}^{\lambda_2} Power d\lambda = Total Power$$

2. Sep 2, 2007

### Irid

The way you described, you'll get some quantity with dimensions "power x length", which is not "power". If you had a graph of "Power per unit wavelength vs Wavelength", then you could integrate and obtain total power in the region of your choice. For example, see Planck's law in Wikipedia or somewhere else.

3. Sep 2, 2007

### Gokul43201

Staff Emeritus
You have not sufficiently described the quantity on the y-axis. Besides, are you sure it's not in units of dBm/Hz or somesuch? It is meaningless to speak of power at a given wavelength or frequency.

4. Sep 2, 2007

### n0_3sc

The y-axis is a power measurement. It is in decibels with respect to a milliwatt (dBm).
Yes it is meaningless to talk of power per nm...

I realised now that it just needs to be some form of energy unit because it will be used as a ratio with other quantity's of the same units thus the units end up cancelling.