Regions of the blackbody photosphere probed by different experiments, COBE FIRAS with sensitivity μ = 10^-5, Pixie with sensitivity μ = 10^-8 and a hypothetical experiment with sensitivity of μ = 10^-10. Shaded regions below the curves are allowed and inaccessible to these experiments. We also plot the ratio of final μ distortion today to the initial μ distortion as a function of energy injection redshift. We define the blackbody surface as the energy injection redshift such that μ^final/μ^initial = 1/e, z_bb = 1.98 × 10⁶.