LED light sources used in machine vision can be divided into two main categories: front-lighting LED light sources and back-lighting LED light sources.
1) Front-lit LED light source is used to detect surface features of objects;
2) Backlit LED light source is used to detect the outline of an object or the purity of a transparent object.
LED light sources for front lighting can be categorized into ring light sources, strip light sources, coaxial light sources, and square light sources based on their structure. Currently, ring light sources are the most widely used, including direct-illumination ring light sources, diffuse-reflection ring light sources, and dome light sources.
1) Direct illumination of the ring light source is suitable for detecting non-reflective objects;
2) Diffuse reflection ring light source, suitable for detecting reflective objects;
3) The dome light source is also a type of diffuse reflection light source, but it completely eliminates shadows through multiple reflections from the inner wall of the hemispherical shape. It is mainly used to detect spherical or curved objects.
Depending on the illumination angle, direct-illumination ring light sources can be categorized as: vertically illuminating ring light sources, angled ring light sources, low-angle ring light sources, and horizontally illuminating ring light sources. Simply put, the angle between the optical axis of each LED and the ring light housing is 0°, 20°, 60°, and 90° respectively (the specific angle may vary slightly depending on the model).
Different illumination angles are suitable for different detection requirements. Vertical illumination and angled illumination are bright-field illumination, meaning most of the reflected light from the surface of the object being measured can enter the camera, so the background appears white, such as for detecting prominent features on the object's surface. Low-angle illumination and horizontal illumination are dark-field illumination, meaning most of the reflected light from the surface of the object being measured cannot enter the camera, so the background appears black, and only the reflections from uneven areas of the object enter the camera, such as for detecting scratches on a metal surface, where the background is black and the scratch appears white. The difference between vertical illumination and angled illumination is that the former has a longer illumination distance, while the latter has a shorter distance. The same applies to low-angle ring illumination and horizontal ring illumination. Furthermore, diffuse ring illumination can be either direct or low-angle.
Low-angle diffuse reflection strip lights have two applications. One is wide-area detection, where two or four strips are used in combination, and the other is line scan illumination.
Another application is line scan illumination. Line scan projects are increasingly common. Line scan cameras capture only one line at a time with short exposure times, requiring high light source brightness. The light source needs to be a focused type, concentrating the light onto a single line to accurately control image stability. Common methods for line scan illumination include: ultra-high brightness LED + condenser lens, fiber optic + condenser lens, and high-frequency fluorescent lamp + condenser lens. Ultra-high brightness LED + condenser lens offers long lifespan but is more expensive, and its illumination distance may be somewhat limited; fiber optics offer high brightness but are also costly; high-frequency fluorescent lamps are cheaper but require periodic lamp replacement.
