It may be easier to detect a fallen person in the IR, but it isn't obvious to me. At any price point a much lower noise and higher pixel count image is available in the visible and NIR wavelengths. Also, it has been my experience that people are at least as sensitive about infrared images from a privacy standpoint as they are about visible images. You MAY be able to get around this issue at any wavelength if your signal processing never makes an image externally available, just simple messages such as occupied, occupant on floor, vacant and so on. This would possibly also require that any communication between the sensor and signal processor be inaccessible or incomprehensible.