TE Enriched Sample (E)

Endoscopy 79 2.3 Image formation A coherent fibre bundle is used to transmit images. The relative positions of the fibres in the bundle are the same between the two ends (Fig. 2.33). When the light from an object is projected onto one end of the bundle through the objective lens, each fibre transmits the light incident on its end. As different amounts of light enter the fibres, the viewer end is illuminated to different extents. The viewer can then see the image reproduced through an eyepiece or a camera mounted to the end (Fig. 2.34). ( MC: DSE 2012 Q4.2, 2014 Q4.4, 2017 Q4.6, 2022 Q4.2) Teaching notes Ss should be able to describe how coherent fibre bundle forms image. Note that the incoherent fibre bundle is used to convey light from source to distal tip, and the coherent fibre bundle carries image back to the eye. Since the organs inside a human body are non-luminous, another fibre bundle is necessary to send light into the organ. An incoherent fibre bundle rather than a coherent one is usually used for illumination because it is cheaper. Teaching notes 1. A coherent fibre bundle is more expensive than an incoherent fibre bundle. In a coherent bundle, each fibre needs to be carefully positioned. Also, the endoscope may twist during use, so the design of coherent bundle has to cater such potential problems (avoiding distortion). 2. Remind Ss that a coherent bundle, in theory, can be used to transmit light for illumination; although, in practice, it is not used. (See DSE 2022 Q4.2) Fig. 2.34 How an image is transmitted by a coherent fibre bundle (the eyepiece is not shown) coherent fibre bundle objective lens (a convex lens) object Fig. 2.33 Difference between a coherent bundle and an incoherent bundle 19 17 18 15 14 16 13 1 12 2 3 4 5 6 7 9 8 10 11 19 17 18 15 14 16 13 1 12 2 3 4 5 6 7 8 9 10 11 4 17 13 1 7 14 2 6 15 12 16 11 10 5 18 3 8 19 9 coherent bundle incoherent bundle Teaching notes Remind Ss that the image is pixelized after transmission (see DSE 2012 Q4.2) Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.