Colouring the near infrared
Current digital camera sensors are inherently sensitive to the near-infrared part of the spectrum. To prevent the near-IR contamination of images, an IR blocking filter (hot mirror) is placed in front of the sensor. In this work, we start by replacing the camera's hot mirror by a piece of clear glass, thus making the camera sensitive to both visible and near-IR light. Using a pair of lens-mounted filters, we explore the differences in operating the camera to take visible and near-IR images of a given scene. Our aim is to enhance the visible images using near-IR information. To do so, we first discuss the physical causes of differences between visible and near-IR natural images, and remark that these causes are not correlated with a particular colour, but with atmospheric conditions and surface characteristics. We then investigate image enhancement by considering the near-IR channel as either colour, luminance, or frequency counterpart to the visible image and conclude that using information from two different colour encodings, depending on the image content, produces vivid, contrasted images that are pleasing to the observers.