Definition of "a" missing in Fresnel conductor equations?

[LukasRuppert]

Hi,

I was looking at the equations for Fresnel reflectance between a dielectric and a conductor in Eq. 8.3, but cannot find the definition for the term a in the text, which would be needed to implement them.
Only a^2+b^2 is given, but a also appears on its own, so I'm wondering if its definition should be added there, e.g.

$a = \sqrt{\frac{1}{2}\left(\sqrt{(\eta^2-k^2-\sin^2\theta)^2+4\eta^2k^2}+\eta^2-k^2-\sin^2\theta\right)}$

(as in Shirley's Thesis, Eq. 2.6, assuming eta_i = 1)

Thanks for your great work and best regards,
Lukas