Typically, a dual layer multi-spectral X-ray detector is capable of providing two points of spectral data about an imaged sample, because the front X-ray detector also acts to filter part of an incident X-ray spectrum before detection by a rear X-ray detector. A pre-filter can be placed in front of the front X-ray detector to enhance the spectral separation. However, the provision of a pre-filter implies that the intensity of the X-ray radiation must be increased to achieve the same signal to noise ratio. The present application concerns a multi-spectral X-ray detector with a front X-ray detector, a rear X-ray detector, and a structured spectral filter placed in-between them. The structured spectral filter has first and second regions configured to sample superpixels of the front X-ray detector, enabling three separate items of spectral information to be obtained per superpixel.