(d) Erosion (damage) on the wings of an aircraft, due to the mechanical influence of the wind and rain, leads to an increase of roughness which can cause a reduction in an aircraft's efficiency. Therefore the company also wishes to detect the amount of surface roughness at various points on the aircraft, by measuring surface irregularity with a camera of high resolution (e.g. of less than 10 microns). Explain why photometric stereo would also be useful here and say how you would expect the field-of-view of the roughness measurement system to differ from that of the area inspection system. The company have confirmed they intend to employ photometric stereo to measure the irregularity on the front of the wings but are concerned about depth-of-field issues. Explain what they mean by this and outline how depth-of-field may be influenced by the sensor resolution and the lens aperture. [8 marks)