I would like to view the side profile of a 140μm diameter glass fiber under a microscope. I am having some difficulty resolving the fiber’s border, is there any special lighting technique to use?

There are multiple ways to achieve the effect you require; however, the most straightforward is utilizing Darkfield Illumination. Darkfield Illumination is ideal for imaging transparent or translucent objects. Light from the Darkfield illuminator, usually a special type of ring light guide, enters the object through the edge, rather than from above, enabling one to see the object’s profile. View Choosing the Correct Illumination for additional information on pros and cons of various illumination setups.