An optical filter having a laterally variable transmission wavelength that is monotonically variable along a length dimension of the optical filter within a wavelength range, the optical filter comprising: a bandpass filter comprising a stack of alternating first and second layers (211, 212) comprising first and second materials, respectively, and having laterally varying thicknesses for providing the laterally variable transmission wavelength; and a first blocking filter comprising a stack of alternating third and fourth layers (213, 214), comprising third and fourth materials, respectively, and having laterally varying thicknesses coordinated with the laterally varying thicknesses of the first and second layers, for blocking wavelengths within the wavelength range that are larger or smaller than the laterally variable transmission wavelength; and a second blocking filter, wherein the bandpass filter is between the first blocking filter and the second blocking filter, characterised in that the bandpass filter further comprises at least one fifth layer comprising the fourth material, whereby a blocking wavelength region of the bandpass filter is broadened, and a thickness of the bandpass filter is reduced, wherein the at least one fifth layer is located in the valley of the standing optical wave at the transmission wavelength inside the bandpass filter, wherein the fourth material comprises a semiconductor material, wherein the first, second, and fourth materials are different materials, and wherein a refractive index of the first material is smaller than a refractive index of the second material, and wherein the refractive index of the second material is smaller than a refractive index of the fourth material. a bandpass filter comprising a stack of alternating first and second layers (211, 212) comprising first and second materials, respectively, and having laterally varying thicknesses for providing the laterally variable transmission wavelength; and a first blocking filter comprising a stack of alternating third and fourth layers (213, 214), comprising third and fourth materials, respectively, and having laterally varying thicknesses coordinated with the laterally varying thicknesses of the first and second layers, for blocking wavelengths within the wavelength range that are larger or smaller than the laterally variable transmission wavelength; and a second blocking filter, wherein the bandpass filter is between the first blocking filter and the second blocking filter, characterised in that the bandpass filter further comprises at least one fifth layer comprising the fourth material, whereby a blocking wavelength region of the bandpass filter is broadened, and a thickness of the bandpass filter is reduced, wherein the at least one fifth layer is located in the valley of the standing optical wave at the transmission wavelength inside the bandpass filter, wherein the fourth material comprises a semiconductor material, wherein the first, second, and fourth materials are different materials, and wherein a refractive index of the first material is smaller than a refractive index of the second material, and wherein the refractive index of the second material is smaller than a refractive index of the fourth material.