Irradiance ($E$) is a measure of the flux area density at a single point, whereas it seems that $\frac{\Phi}{A}$ is the average flux over an entire surface. Is saying $E = \frac{\Phi}{A}$ here just a slight abuse of notation or are we actually saying something about irradiance?
kmcrane
@dvernet: You're assuming the surface here is a really simple one: just a little flat piece. :-)
Irradiance ($E$) is a measure of the flux area density at a single point, whereas it seems that $\frac{\Phi}{A}$ is the average flux over an entire surface. Is saying $E = \frac{\Phi}{A}$ here just a slight abuse of notation or are we actually saying something about irradiance?
@dvernet: You're assuming the surface here is a really simple one: just a little flat piece. :-)