An optical filter includes a first layer including a first notch filter arranged to attenuate electromagnetic radiation having a first wavelength λ1 incident normally thereupon. The optical filter includes a second layer including a second notch filter arranged to attenuate electromagnetic radiation having a second wavelength λ2 incident normally thereupon. The first wavelength λ1 and the second wavelength λ2 are different. The second layer is stacked upon the first layer. In use, the first notch filter attenuates the electromagnetic radiation having a predetermined wavelength λ incident thereupon at a first angle of incidence θ1 and the second notch filter attenuates the electromagnetic radiation having the predetermined wavelength λ incident thereupon at a second angle of incidence θ2, wherein the first angle of incidence θ1 and the second angle of incidence θ2 are different.