Comparison between the analytic form of the infrared background measured by FIRAS (Fixsen et al. 1998) and the modified FIRB models produced by incrementing by 30% the luminosity (crosses) or the number of sources (boxes) in the redshift range z = 4 − 5. The solid and dashed lines represent the most accurate estimation and the error at 1-σ in the FIRB spectrum obtained with FIRAS; the four boxes and crosses are the fluxes of the modified FIRB model at 143, 217, 353, and 450 GHz.