Energy, Power and Radiation Flux

Energy

The radiant energy $Q$ associated with radiation depends both on the number of photons and the amount of energy carried by each individual photon: \begin{align} Q := \sum_i^\text{photons} Q_i = \sum_i h \cdot \nu_i \end{align} When considering radiation of only a single wavelength $\lambda$ (monochromatic radiation), all photons carry the same amount of energy and the total radiation energy can be thought of as a measure of the photon number.

In most other branches of physics the letter $E$ would be more common for energy but in the area of radiometry the irradiance is denoted by $E$ as well and eventually won over energy.^[1]

Power/Radiation Flux

In many situations it is more interesting to consider the rate at which energy is transferred per time $t$. The corresponding quantity is called power \begin{align} P := \frac{\D{Q}}{\D{t}} \end{align} and has dimensions of energy per time ( [P] = J s^-1 = W ).

When speaking about radiant power that is emitted by, passing through or incident on a particular surface, the term flux $\phi$ (for $\phi$lux) is perhaps more common.

However, some people use flux interchangeably with what I call flux density.^[2] When I am unsure about what is meant in a particular context, I usually try to figure out the units that correspond to the quantity in question in order to understand what is meant. ^[3][4]