Ghost imaging systems use down-conversion sources that produce twin output beams of position-correlated photons to produce an image of an object using photons that did not interact with the object. One of these beams illuminates the object and is detected by a single pixel detector while the image information is recovered from the second, spatially correlated, beam. We utilize this technique to obtain images of objects probed with 1.5μm photons whilst developing the image using a highly efficient, low-noise, photon-counting camera detecting the correlated photons at 460nm. The efficient transfer of the image information from infrared illumination to visible detection wavelengths and the ability to count single-photons allows the acquisition of an image while illuminating the object with an optical power density of only 100 pJ cm<sup>-2</sup> s<sup>-1</sup>. We apply image reconstruction techniques based on compressive sensing to reconstruct our images from data sets containing far fewer photons than conventionally required. This wavelength-transforming ghost imaging technique has potential for the imaging of light-sensitive specimens or where covert operation is desired.
Two microlens arrays that are separated by the sum of their focal lengths form arrays of micro-telescopes. Parallel light rays that pass through corresponding lenses remain parallel, but the direction of the transmitted light rays is different. This remains true if corresponding lenses do not share an optical axis (i.e. if the two microlens arrays are shifted with respect to each other). The arrays described above are examples of generalized confocal lenslet arrays, and the light-ray-direction change in these devices is well understood [Oxburgh et al., Opt. Commun. 313, 119 (2014)]. Here we show that such micro-telescope arrays change light-ray direction like the interface between spaces with different metrics. To physicists, the concept of metrics is perhaps most familiar from General Relativity (where it is applied to spacetime, not only space, like it is here) and Transformation Optics [Pendry et al., Science 312, 1780 (2006)], where different materials are treated like spaces with different optical metrics. We illustrate the similarities between micro-telescope arrays and metric interfaces with raytracing simulations. Our results suggest the possibility of realising transformation-optics devices with micro-telescope arrays, which we investigate elsewhere.