Power spectral density (PSD) is the measure of a signal's power content versus frequency. PSD gives designers an idea of how the power of a signal is distributed over frequency.

The concept of Power Spectral Density (PSD) is extremely beneficial when specifying optics of various types, with PSD proving to be a useful metric in laser fusion and other extreme application spaces ...

Defined as the amount of power used by lighting per unit of building area, lighting power density is typically measured in watts/square meter.