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Resolution can also be defined by the number of millions of pixels (MegaPixels) visible on a display. The table on the previous page shows the resolution of popular image formats. Scene brightness is defined in lumens. A lumen is the basic unit of visible light emitted from a source. A camera sees the light reflected from scene images (not the incident light falling on it). A camera’s light gathering ability is measured in Lux. One Lux equals one lumen of illumination per square metre. Illumination can be measured using a “photometer”. The chart below shows the range of operation of different types of image sensor. Whilst most camera tubes operate over the range of frequencies covered by visible light, most also have an extended bandwidth covering the infra red (IR) region as well. Suitable infra red illumination can therefore be used to provide CCTV images whilst the scene looks almost totally dark to the human eye, or in foggy situations. Some indoor cameras have built-in IR illumination. CCTV can also be used to detect infra red heat sources. Black-and-white (monochrome) cameras can produce images in darker conditions than colour cameras. Some colour cameras automatically switch to monochrome when the scene illumination falls below a certain level. The transfer characteristic between scene brightness and visibility to the human eye can be altered to make the scene more visible, by, for example, stretching 44 CAI Feedback Spring 2018