Various embodiments of the present disclosure are directed towards an image sensor, and a method for forming the image sensor, in which an inter-pixel trench isolation structure is defined by a low-transmission layer. In some embodiments, the image sensor comprises an array of pixels and the inter-pixel trench isolation structure. The array of pixels is on a substrate, and the pixels of the array comprise individual photodetectors in the substrate. The inter-pixel trench isolation structure is in the substrate. Further, the inter-pixel trench isolation structure extends along boundaries of the pixels, and individually surrounds the photodetectors, to separate the photodetectors from each other. The inter-pixel trench isolation structure is defined by a low-transmission layer with low transmission for incident radiation, such that the inter-pixel trench isolation structure has low transmission for incident radiation. The low-transmission layer may, for example, be or comprise metal and/or some other suitable material(s).