A Brief Discussion on Common Illumination Techniques for Metallurgical Microscopes
① Direct Illumination: This illumination mode is generally adopted for specimens requiring high-contrast lighting. The light shines directly onto the object and delivers high brightness. Nevertheless, it has a drawback: strong reflections may occur when used on highly reflective surfaces.
② Darkfield Illumination: Darkfield microscopy is mainly used to observe specimens featuring structures or refractive-index variations, such as diatoms, radiolarians and other regularly structured single cells, as well as linear structures within cells like flagella and fibres. In darkfield illumination, light strikes the specimen surface at a predefined angle. It is suitable for surfaces without inherent colour contrast, where little signal would be obtained under ordinary lighting. Both observation modes are commonly equipped on metallurgical microscopes.
③ Backlight Illumination: This illumination technique is widely used for dimensional measurement and orientation recognition. Light is projected from the rear side of the specimen. It provides even lighting, and the side contour of the object can be captured by the camera.
④ Diffuse Illumination: Diffused light resembles natural ambient light. The light has no concentrated focal direction; it is soft and non-glaring, ideal for highly reflective objects.
⑤ Coaxial Illumination: Coaxial illumination generates a uniform planar light source propagating vertically. A 45° semi-transparent mirror redirects the light beam perpendicularly downwards onto the specimen surface. This light source is especially useful for inspecting flat, highly reflective workpieces.
