Accurate calibration of intrinsic camera parameters by observing parallel light pairs

This study describes a method of estimating the intrinsic parameters of a perspective camera. In previous calibration methods for perspective cameras, the intrinsic and extrinsic parameters are estimated simultaneously during calibration. Thus, the intrinsic parameters depend on the estimation of the extrinsic parameters, which is inconsistent with the fact that intrinsic parameters are independent of extrinsic ones. Moreover, in a situation where the extrinsic parameters are not used, only the intrinsic parameters need to be estimated. In this case, an intrinsic parameter, such as focal length, is not sufficiently robust to combat the image processing noise, that is absorbed by both parameter types, during calibration. We therefore propose a new method that allows the estimation of intrinsic parameters without estimating the extrinsic parameters. In order to calibrate the intrinsic parameters, the proposed method observes parallel light pairs that are projected on different points. This is accomplished by applying the constraint that the relative angle of two parallel rays is constant irrespective of where the rays are projected. This method focuses only on intrinsic parameters and the calibrations are sufficiently robust as demonstrated in this study. Moreover, our method can visualize the error of the calibrated result and the degeneracy of the input data.