Room temperature pseudodielectric function spectra ε(ω) ≤ ε₁ (ω) + iε₂(ω) of the ordered defects compounds Cu₂In₄Se₇, CuGa ₃Se₅ and CuGa₅Se₈ have been measured by spectroscopic ellipsometry. The values of refractive index n and extinction coefficient k are given. The structures observed in ε(ω) spectra have been analysed using different methods, including fitting the numerically differentiated experimental spectrum (second derivative) to analytical line shapes. As a result, the energies corresponding to the fundamental gap (E ₀) and higher critical points have been determined. A linear correlation of the fundamental gap values with Ga/Cu atomic ratio contents in CuGa_xSe_y samples is deduced.