Ortholumen is a light pen based tabletop interaction system that can employ all the pen?s spatial degrees of freedom (DOF). The pen?s light is projected from above onto a horizontal translucent screen and tracked by a webcam sitting underneath, facing upwards; system output is projected back onto the same screen. The elliptic light spot cast by the pen informs the system of pen position, orientation, and direction. While this adds up to six DOFs, we have used up to four at a time. In order to better separate input and output light we employ polarizing filters on the webcam and on the projector lens. Two applications, painting and map navigation, are presented. Ortholumen can be expanded to track multiple pens of the same or different colors. This would enable bi-manual input, collaboration, and placed pens as external memory. Visible light, as opposed to infrared or radio, may be perceived more directly by users. Ortholumen employs only low-cost parts, making the system affordable to home users.