Prior art techniques for converting the wavelength of electromagnetic radiation by non-linear interactions such as sum frequency generation SFG produces blurred images when applied to polychromatic images. The invention provides an improved arrangement (15) for up-converting incoming electromagnetic radiation with dissimilar first wavelengths (λ,) distributed in a first wavelength interval (Δλ,) into up-converted electromagnetic radiation comprising corresponding dissimilar up-converted wavelengths (λ3), smaller than the first wavelengths (Δ1). The arrangement applies a nonlinear crystal (5) and a laser (16) for providing a laser beam (10) of second wavelength, different from the first wavelengths (λ1) inside the nonlinear crystal, and a Fresnel zone plate (17) for focusing or converging the incoming radiation inside the nonlinear crystal in spatial overlap with the laser beam. The up-converted radiation is produced inside the non-linear crystal by sum frequency generation between the incoming radiation and the laser beam. The arrangement (15) can comprise an image forming lens arrangement (20) for forming an object image (2), a second zone plate (18) for forming an up-converted image (3) and an image sensor (19) on which the up-converted image (3) is formed.