# how can I do back-projection?

I calibrated my single camera using Camera Calibration Toolbox for Matlab(http://www.vision.caltech.edu/bouguetj/calib_doc/), and already have the intrinsic and extrinsic parameters.

Now given a pixel of one image, how can i do back-projection from 2D pixel to 3D ray? That is, how can i calculate the equation of ray connecting the camera center and the pixel point on the image sensor plane? And how can i know the equation of physical image sensor plane in world reference frame?

thx very much.