Short throw interactive projection systems are widely used in education, training, commerce, and entertainment in
recent years. Different interactive techniques have been developed. And among them, the infrared location technique is
one of the attracting methods, because of the advantages such as independent of whiteboard and low cost, and so on.
However, the main defect is that the infrared pen point is easy to be blocked by user's hand.
In this paper, we introduced our recent progress on indirect measurement of the pen point used in a short throw
interactive projection system. Two infrared LEDs are fixed along the pen's body near the tail end. By separately
measuring the position of the two LEDs, the location of the front end pen point can be calculated. Such placement can
effectively avoid the LEDs from being blocked by user's hand. The mathematical model of this measurement scheme is
given separately in the situations of infinite focal length and short focal length of the camera. Errors are analyzed by both
analytical and numerical method. We used our position sensitive detector (PSD) based location system to test the effect
of this method.