The type of imprinter used in modern SLR cameras is of the type described in patent #4,001,846 This imprinter is fixed to the back of the camera on the film backing plate. It uses a small LED display for illumination and optics to focus the light onto a specific part of the film. The LED display is exposed on the negative at the same time the picture is taken.
The imprinter superimposes the date and other information on the image and this can be distracting. Digital cameras can often encode all the information in the image file itself. The Exif format is the most commonly used format.