Circular Dichroism Spectroscopy: Units

Definition
The CD analogue of the Beer–Lambert Law is

CD = ΔA(absorbance units) = A_ℓ  – Ar 
      = Δεcℓ                                                          (1)

However, most CD spectropolarimeters, although they measure differential absorbance (at least at differential absorbance signals below ∼0.1) (Nordén et al. 2010; Rodger and Nordén 1997), largely for historical reasons, give a CD spectrum in units of ellipticity, θ, in millidegrees, versus λ, rather than ΔA versus λ. The ellipticity, θ, is obtained from the ratio of the minor and major axes of the ellipse traced out by the electric field vector of the elliptically polarized light when it emerges from the chiral sample onto which linearly polarized light was incident (Rodger and Nordén 1997; Barron 2004; Nordén et al. 2010). The conversion between ΔA and millidegrees is (Barron 2004; Nordén et al. 2010)

$$\begin{array}{ll}{\rm CD} &= \Delta A{\text{(absorbance units)}} = {A_{\ell }} - {A_r} \\ & =...