In this paper the last results obtained in the field of design and realization of planar infrared microsensors are presented. These sensors can be considered as a pattern of cells electrically serialized. For each cell, the incident radiation is converted into heat transfer by way of alternatively absorbent and reflecting areas. The center of each area is crossed with a thin microthermocouple whose hot and cold junctions are submitted to the superficial thermal field. By using micromachining, cell dimensions can be shrinked and 5 X 5 cm2 microsensors have been manufactured with more than 3000 cells. KaptonTM is used as substrate and a liquid resin polyimide intended to constitute the infrared absorbing layer. To determine the intrinsic absorption spectrum of this resin, processing a membrane of some cm2 was needed. In this case, the spectral transmittance of this membrane was measured with an infrared spectrometer (Perkin - Elmer) and absorptivity can be mathematically deduced. The sensitivity represented by the ratio between the voltage delivered by the sensor and the absorptive heat flux is calculated by the way of a monodimensional analytical model dependent on a parameter representing the penetration depth of heat in the monolithic substrate. This parameter is computed from 2D finite elements modeling and takes into account the geometrical characteristics of each basic cell constituting the sensor. Finally, by multiplying the absorption spectrum with the sensitivity, the curves of sensors spectral sensitivities in the range 5 - 20 μm can be deduced