What is disclosed is a method for vehicle occupancy detection using a single band infrared imaging system. First, an infrared image of a vehicle intended to be processed for human occupancy detection is captured using a single band infrared camera set to a predefined wavelength band. A candidate sub-image is identified within the captured image. A cumulative histogram is formed using the reflectance values of each pixel in the candidate region. A threshold reflectance value is then determined from the cumulative histogram using a pre-defined cumulative occurrence fraction value which corresponds to a value equivalent to an average sized human face. Embodiments for setting the threshold reflectance value are disclosed. Thereafter, human occupants can be distinguished in the image from the vehicle's interior by comparing pixel reflectances in the sub-image against the threshold reflectance value.