... SPEX

http://www.sron.nl/divisions/hea/spex/version2.0/release/index.html

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...U

U is defined as $${\frac{\phi({\rm H})}{n_{\rm e}~c}}$$ where the radiation field is expressed by $${\phi({\rm H})=\frac{k}{4\pi~r^2_0}\int^{\nu2}_{\nu1}}$$  $${\frac{\pi~F_{\nu}}{h\nu}{\rm d}\nu}$$ with $\nu1=1$ Ryd and $\nu 2=\infty$ and the relation between the normalisation costant k, and the bolometric luminosity of the source is expressed by $${L=k\int^{\nu2}_{\nu1}\pi F_{\nu}{\rm d}\nu}$$.

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