In a previous work, we succeeded in connecting normal LWIR apparent emissivity to the spectral one of an aluminium nitride ceramic plate. We showed a good agreement with the assumption of spectral bandwidth of the used IR system. Our aim in this paper is to justify the considered spectral band [7.5µm, 12µm]. Hence we have developed an analyzer of IR system. The analyzer proceeds by comparing thermosignals with integrated blackbody radiance and adapts spectral bandwidth in order to minimize the dispersion from linearity of the characteristic thermosignals / integrated radiance over a temperature range of the IR system. The capacities of the analyzer are tested for 6 commercial cameras. Each of these systems exhibits a similar formatting process implemented during the thermogram recording. The effective spectral bandwidth exhibits plausible values. It varies significantly from one model to the other and the residual non-linearity is connected to the NETD of the IR system. Applied to the same system which served to characterize the apparent emissivity, the analyzer permits to quantify the effective spectral band. We obtain an excellent agreement between the classical model of apparent emissivity and measurement, both in terms of accuracy and in terms of temperature dependence. The absolute error is 0.005 for emissivity and the temperature coefficient is less than 6 10-5 °C-1 within the temperature range [40°C, 130°C].