A guided tracheal intubation system using an autonomous modulated light source, outputting modulated illumination at a constant level, and externally applied to the subject's larynx region. An optical imaging system receives a video stream from within the subject's throat, including modulated illumination from the subject's trachea. A display control system performs signal processing on the modulated content of the images, and outputs frames of those images in which the intensity level of illumination from the trachea can be controlled without any need to change the illumination output from the modulated light source. The light source has no connection with the rest of the system, and need contain no more than a battery, a power supply circuit and a light source. It can therefore be of low cost and can be made disposable, such as in the form of an adhesive patch applied to the subject's neck.