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Based on your clarification, you can do what you need. First you calculate the vector in space. If you don't have the distortion matrix, you can ignore that part.

Mat tvec = _tvec.getMat();
Mat rvec = _rvec.getMat();
Mat camera_matrix = _cameraMatrix.getMat();
const Mat distortion_matrix = _distortionMatrix.getMat();

std::vector< Point2f > pts_in, pts_out;
pts_in.push_back( _pt );
undistortPoints( pts_in, pts_out, camera_matrix, distortion_matrix, noArray(), camera_matrix );

Mat los( 3, 1,CV_64F );< double >( 0 ) = pts_out[0].x;< double >( 1 ) = pts_out[0].y;< double >( 2 ) = 1;

if ( camera_matrix.type() != CV_64F )
    camera_matrix.convertTo( camera_matrix, CV_64F );
if ( rvec.type() != CV_64F )
    rvec.convertTo( rvec, CV_64F );
if ( tvec.type() != CV_64F )
    tvec.convertTo( tvec, CV_64F );

los = camera_matrix.inv() * los;

This gives you a vector pointing out from the center of the camera. You need to rotate it by the orientation of the camera, and use the position of the camera as the origin of the vector. If you calibrated your camera using OpenCV, use this function as a guide: HERE. Otherwise you'll need to use the information you have (presumably from the robot arm). From there it's just basic geometry.