Ratio of fundamental noise to instrumental noise on a grid of amplitude and phase perturbations for the shortest-wavelength bin at λ_b = 4.1 µm with width ∆λ_b = 0.2 µm. The perturbations are assumed to be pink noise with the specified reference rms at 10 µm, and the phase perturbation is converted to an OPD. The contours indicate where the fundamental noise dominates the systematic noise by a factor of one, five, or ten, respectively, for the λ_b = 4.1 µm case (in red) and for comparison the λ_b = 6.1 µm (in orange) and λ_b = 10.1 µm (in cyan) cases. The crosses indicate the reference levels specified by Table A.1.