Question

The main wing spar of an aircraft is a built up I-beam, with cap extrusions buck- riveted to the spar web. A visual inspection of the spar with a field of view of 250 mm x 250 mm is desired using a Charge Coupled Device (CCD) with 500 x 500 pixels, each of which is 10 μ n x 10 μm in size 4. Pre-drilled Rib Station Spar Cap Extrusion Spar Web Solid Rivets Flanged Lightening Hole Eigure6: Wing Spar of an aircraft Draw a schematic of the visual inspection system specifying the layout of the system, the lighting used and any other precautions that need to be taken to ensure a good quality image (a) (5 marks) (b) Determine the focal length of the lens which we need to image the desired field of view on to the CCD if the distance from the lens to the CCD is 20 mm. How far is the object from the lens? (5 marks)
Please help with question b.

Answer 1

Given Field of view: FOV- 250x250mm Size of CCD 500x10 u m-5mmx b) focal length of lens? obiect distance-? Image distance (distance of lens from CCD)-20 mnm Solution The magnification of the lens in terms of FOV and camera size is given a:s Magnification- Cälmera size_5 0.02 250 Also we know the relation between magnification, image distance and object distance as Magnification- - image distance Obiect dis tan ce Thus, object distance-image distance / magnification --20/0.02 =-1000mm-1m towards the left side of lens Now, focal length of a lens can be determined if we know the image distance and object distance: 0.51 f O 1000 20 f1.96mnm