A sensor comprising: a structure through which fluid and/or objects can move and in which photons can emanate from the fluid and/or objects as they move, with emanating photons having energies within a respective range of photon energies; and, the structure having a set of one or more channel portions defined therein, the sensor further comprising: on the structure, an IC that includes a photosensor array (100; 150; 418) arranged to photosense objects travelling within a channel portion of the one or more channel portions; the array including a set of cells and the set of cells including: two or more subsets of cells, each subset of cells including one or more subrange photosensing cells and each subset being associated with a respective subrange of the range of photon energies; each subrange photosensing cell being capable of photosensing emanating photons within the respective subrange of its subset; the respective subranges of at least two of the subsets of cells being different from each other wherein the subsets of cells are arranged extending along the channel portion of the one or more channel portions; characterised in that the array includes, nearby to at least one of the subrange photosensing cells in one of the subsets of cells, a reference photosensing cell capable of photosensing emanating photons throughout the range of photon energies, to produce a reference for the nearby subrange photosensing cell. a structure through which fluid and/or objects can move and in which photons can emanate from the fluid and/or objects as they move, with emanating photons having energies within a respective range of photon energies; and, the structure having a set of one or more channel portions defined therein, the sensor further comprising: on the structure, an IC that includes a photosensor array (100; 150; 418) arranged to photosense objects travelling within a channel portion of the one or more channel portions; the array including a set of cells and the set of cells including: two or more subsets of cells, each subset of cells including one or more subrange photosensing cells and each subset being associated with a respective subrange of the range of photon energies; each subrange photosensing cell being capable of photosensing emanating photons within the respective subrange of its subset; the respective subranges of at least two of the subsets of cells being different from each other wherein the subsets of cells are arranged extending along the channel portion of the one or more channel portions; characterised in that the array includes, nearby to at least one of the subrange photosensing cells in one of the subsets of cells, a reference photosensing cell capable of photosensing emanating photons throughout the range of photon energies, to produce a reference for the nearby subrange photosensing cell. on the structure, an IC that includes a photosensor array (100; 150; 418) arranged to photosense objects travelling within a channel portion of the one or more channel portions; the array including a set of cells and the set of cells including: two or more subsets of cells, each subset of cells including one or more subrange photosensing cells and each subset being associated with a respective subrange of the range of photon energies; each subrange photosensing cell being capable of photosensing emanating photons within the respective subrange of its subset; the respective subranges of at least two of the subsets of cells being different from each other wherein the subsets of cells are arranged extending along the channel portion of the one or more channel portions; characterised in that the array includes, nearby to at least one of the subrange photosensing cells in one of the subsets of cells, a reference photosensing cell capable of photosensing emanating photons throughout the range of photon energies, to produce a reference for the nearby subrange photosensing cell. two or more subsets of cells, each subset of cells including one or more subrange photosensing cells and each subset being associated with a respective subrange of the range of photon energies; each subrange photosensing cell being capable of photosensing emanating photons within the respective subrange of its subset; the respective subranges of at least two of the subsets of cells being different from each other wherein the subsets of cells are arranged extending along the channel portion of the one or more channel portions; characterised in that the array includes, nearby to at least one of the subrange photosensing cells in one of the subsets of cells, a reference photosensing cell capable of photosensing emanating photons throughout the range of photon energies, to produce a reference for the nearby subrange photosensing cell.