This paper is about the collaborative use of a far-infrared spectrum human detector and a visible spectrum human detector; the idea is to make collaborate these two detectors of different nature to automatically adapt the human detection whatever the luminosity changes and whatever the infrared emission changes of the scene. Our collaborative approach of detection handles: 1) gradual luminosity changes due, for instance, to the passage from night to day (and vice-versa), 2) sudden luminosity changes due, for instance, to navigation in a forest (when going through a glade in a forest), 3) infrared emission saturation when the global temperature of the scene is very high and does not permit to distinguish human people in infrared. Our approach of detection permits to detect people 24 hours a day and regardless the weather conditions. Furthermore, the proposed approach is relatively fast: it is practically as fast as using one detector alone whereas two are used in the same time.