Method for converting at least one image of a first spectrum into an image of a second spectrum, comprising: —recording at least one first-spectrum reference image (NRI) of at least one reference scene (RS) with a first-spectrum recording apparatus (1), the first-spectrum reference image (NRI) comprising first image portions with corresponding first-spectrum sensor reference data (b1, b2); —providing corresponding second-spectrum reference information (RGB); —providing at least one set of reference data (T1, T2) from at least part of the first-spectrum sensor reference data (b1, b2) relating to the reference scene (RS), and at least a corresponding part of the second-spectrum information (RGB) relating to that reference scene (RS); —providing at least one target first-spectrum image (NTI) or target second-spectrum image; and —converting each target first-spectrum image (NTI) or target second-spectrum image into a respective second-spectrum image (CI) or first-spectrum image, respectively, using the set of reference data (T1, T2).