# Spectral irradiance unit conversion (Jansky)

I'm a bit confused between two units for spectral irradiance, specifically $$W/sr/m^2/nm$$ and $$\mu Jy/"^2$$, which is analogous to $$W/sr/m^2/Hz$$.

It seems to me that I need some sort of spectral bandwidth, but I don't get which one to use in the case of hyperspectral data.

• Spectral quantities can be expressed in terms of either frequency or wavelength. Take your pick. en.wikipedia.org/wiki/Radiance – G. Smith Aug 14 at 22:03
• The relationship is $L_\nu\,d\nu=L_\lambda\,d\lambda$. – G. Smith Aug 14 at 22:08
• Sure, but what number is that? Do I directly use $\nu$ and $\lambda$ or do I need to integrate somehow? – asimoneau Aug 14 at 22:13
• $d\nu/d\lambda$ or $d\lambda/d\nu$ is not a number. They are functions of $\nu$ or $\lambda$ which follow from $\lambda\nu=c$. So there is not a single conversion factor between the units. The factor depends on the frequency or the wavelength. – G. Smith Aug 14 at 22:21
• Alright, I think I'm getting it. So the factor $d\nu/d\lambda$ is about equal to $c/\lambda^2$ ? – asimoneau Aug 14 at 22:26