Ratio of fundamental noise to instrumental noise on a grid of the detector environment temperature and the detector dark current for the shortest-wavelength bin at λ_b = 4.1 µm with width ∆λ_b = 0.2 µm. The temperature induces blackbody thermal emission that is propagated directly into the detector. The dark current is assumed to be constant with temperature. The instrument is configured according to the reference state in Table A.1. The contours indicate where the fundamental noise dominates the systematic noise by a factor of one or five. The cross indicates the reference levels of Τ = 11 Κ and I_D = 10⁻⁴ e⁻ px⁻¹ s⁻¹ used in Fig. A.1.