A radiation imaging apparatus comprising: an optical imaging device configured to image real world structures; a position and orientation detector configured to determine an instantaneous position and orientation of the radiation imaging apparatus in six degrees of freedom relative to the real world structures as a fixed frame of reference; a radiation detector configured to detect ionising radiation; a radiation source determination section configured to determine a distribution of radiation sources based on data of the ionising radiation detected by the radiation detector combined with data of the position and orientation of the radiation imaging apparatus relative to the real world structures; and an image generator configured to generate data for a combined image comprising an optical image of the real world structures imaged by the optical imaging device and an overlay image representing the distribution of radiation sources based on data from the radiation source determination section, wherein the position and orientation detector comprises a range sensor configured to measure range data of distances from the radiation imaging apparatus to real world structures in at least two dimensions; the radiation imaging apparatus comprises a processing unit configured to align the measured range data with reference range data of the real world structures so as to determine an instantaneous position of the radiation imaging apparatus in at least two dimensions relative to the real world structures as a fixed frame of reference; and the processing unit is configured to calculate a translation and rotation that best aligns the measured range data with previously measured range data so as to measure a change in position and/or orientation of the radiation imaging apparatus over time. an optical imaging device configured to image real world structures; a position and orientation detector configured to determine an instantaneous position and orientation of the radiation imaging apparatus in six degrees of freedom relative to the real world structures as a fixed frame of reference; a radiation detector configured to detect ionising radiation; a radiation source determination section configured to determine a distribution of radiation sources based on data of the ionising radiation detected by the radiation detector combined with data of the position and orientation of the radiation imaging apparatus relative to the real world structures; and an image generator configured to generate data for a combined image comprising an optical image of the real world structures imaged by the optical imaging device and an overlay image representing the distribution of radiation sources based on data from the radiation source determination section, wherein the position and orientation detector comprises a range sensor configured to measure range data of distances from the radiation imaging apparatus to real world structures in at least two dimensions; the radiation imaging apparatus comprises a processing unit configured to align the measured range data with reference range data of the real world structures so as to determine an instantaneous position of the radiation imaging apparatus in at least two dimensions relative to the real world structures as a fixed frame of reference; and the processing unit is configured to calculate a translation and rotation that best aligns the measured range data with previously measured range data so as to measure a change in position and/or orientation of the radiation imaging apparatus over time.