1. An X-ray imaging apparatus comprising: an X-ray generator configured to generate and emit X-rays; an X-ray detector configured to detect the emitted X-rays and acquire X-ray data; and a controller configured to convert measurement data acquired by detecting X-rays by the X-ray detector and reference data acquired via simulation into X-ray characteristic coordinates as coordinates on a predefined space and to estimate a response characteristic function of the X-ray detector from a relationship between the converted reference data and the converted measurement data.