 assuming radio waves travel at 300 000 000 metres per second, this means our wavelengths of interest scale from 100 kilometres (very long wave) to 0.0001 metre (300 GHz). Optical wavelengths of (human) visible interest scale from 300 to 700 nanometres but let's say we are also interested in ultraviolet (below 300 nanometres) and infrared (above 700 nanometres. Let's be imaginative and say we are interested in the 100 to 1000 nanometre band. Assuming light waves also travel at 300 000 000 metres per second (of which more later), this means a frequency of interest from 0.000 001 metres to 0.000 000 1 metres or 3000 to 0 000 THz (a nanometre is one billionth of a metre). 10.2.1 Image Noise and Linear and Nonlinear Distortions Having collected our radiant energy in a lens we subject it to a number of distortion processes partly through the lens itself (optical aberrations) and then via the sensor and colour-filter array. The sensor array converts the photons into electrons (an obliging property of silicon) but in the process introduces noise. The combination of lens imperfections and sensor noise introduce ambiguity into the image pipe that needs to be dealt with by image-processing algorithms. 10.2.2 The Human Visual System Interestingly this is not dissimilar to the human visual system where we sample the real optical analogue world with two parallel sampling grids consisting of cones and rods. Cones are centred around the middle of the retina and work in medium to bright light and are colour sensitive. Rods are concentrated in an area outside the cones. There are fewer of them. They are optimised for low-light conditions but are not colour sensitive. This is why we see in black and white at night. If you look at a distant dim object (a star at night) it will be easier to see if you shift your gaze off centre. A sensor array emulates the functionality of the cones in the human visual system. It has to deliver similar or better dynamic range, similar or better colour accuity and perceptually similar resolution. Not