 ability to focus an image on the sensor array with minimum distortion. Optional functions include a good depth of field, focal length, angle of view and optical zoom. Unfortunately, because of cost, size and weight constraints, most lenses used in camera phones are far from perfect. The unwanted effects that result include vignetting (not enough light getting to the edge of the sensor array causing edge/corner shading) and lateral chromatic aberration caused by rays of light being sent obliquely across the colour filter array. The effects can be cancelled out or reduced by using antivignetting/shading correction algorithms, antiblur algorithms and colour correction. 8.2.2 The Sensor Array A sensor array is an array of photosensitive cells, the discrete parts of which are described as pixels. The photosensitive cells are more or less identical to solar cells in that their job is User-Centric Software Photons Photons Photons Microlens Colour Filter Metal Opaque Layer Green Photodiode Silicon Figure 8.1 Microlens, colour filter and photodiode. Reproduced with permission of Aptina Inc. to turn incoming photons from the lens into countable electrons. The efficiency with which they accomplish this task is described as quantum efficiency (QE) measured in bits per lux per second. Bigger pixels collect more photons, but take up more space and cost more. More pixels on a sensor array increase resolution but take up more space and cost more. In the implementation pictured in Figure 8.1 a microlens is placed over each pixel. The photons pass through a colour filter and an opaque metal layer and are captured by a photodiode. A sensor array is either a charge coupled device (CCD) or CMOS device. In a charge coupled device, the charge is transported across the chip and read at the corner of the array. In a CMOS device, each pixel has either 3 or 4 transistors, which amplify and move the charge across a wired backplane. Because each pixel on a CMOS sensor has several transistors located next to it, some of the incoming photons 