Light intensity vs Irradiance

[El Flea]

What is the relationship between light intensity and irradiance? How do I convert one to the other?

[mgb_phys]

Strictly speaking irradiance is power/area and intensity is power/solid angle.
Typically irradiance is at the target and intensity is from the source.
But intensity is often used when you should use irradiance - so often they are the same thing,

[El Flea]

Thanks!
Isn't W(m^-2) the unit for light intensity?
So, if I have, let's say, 1W(m^-2)(nm^-1) spectral irradiance, in the 400nm wavelength range, can I say that I have 1x(10^-9)W(m^-2) light intensity over the same area?

[mgb_phys]

Isn't W(m^-2) the unit for light intensity?
Nope that the unit of irradiance. As I said - intensity is often losely used when you should say irradiance.


So, if I have, let's say, 1W(m^-2)(nm^-1) spectral irradiance, in the 400nm wavelength range, can I say that I have 1x(10^-9)W(m^-2) light intensity over the same area?
It depends on the bandwidth, if you have 1W(m^-2)(nm^-1) then you would have 100W/m^2 over a 100nm bandwidth