LED light sources used in machine vision can be divided into two broad categories: front-illuminated LED light sources and back-illuminated LED light sources.
1) Frontal illumination LED light source is used to detect surface features of objects;
2) Backlight LED light source is used to detect the contour of an object or the purity of a transparent object.
The front illumination LED light source can be divided into a ring light source, a strip light source, a coaxial light source and a square light source according to different structures. At present, the ring light source is used most, including direct illumination ring light source, diffuse reflection ring light source, Dome light source and the like.
1) Direct illumination of the ring light source, suitable for detection of non-reflective objects;
2) diffuse reflection ring light source, suitable for the detection of reflective objects;
3) The Dome light source is also a kind of diffuse reflection source, but it is reflected multiple times through the inner wall of the hemisphere type, which can completely eliminate the shadow. Mainly used to detect spherical or curved objects.
According to the different illumination angles, direct illumination of the ring light source can be divided into: vertical illumination ring light source, angled ring light source, low angle ring light source and horizontal illumination ring light source. It can be simply understood that the angle between the optical axis of each LED and the ring lamp housing is 0°, 20°, 60° and 90° in sequence (the specific model may vary slightly).
Different lighting angles are available for different inspection requirements. Vertical illumination and angled illumination are bright-field illumination, that is, most of the reflection on the surface of the object to be measured can enter the camera, so the background is white, such as the detection of prominent features on the surface of the object. Low-angle illumination and horizontal illumination are dark-field illumination, that is, most of the reflection on the surface of the measured object does not enter the camera, so the background is black, and only the reflection of the unevenness of the object enters the camera, such as the detection of scratches on the metal surface, background It is black and the scratch is white. At the same time, the difference between vertical illumination and angled illumination is that the former one is farther away and the latter is closer. The same is true for low-angle ring illumination and horizontal illumination ring illumination. At the same time, the diffuse reflection ring also has direct and low angle points.
Low angle diffuse reflective strip light for two applications. One is wide-width detection, two or four bars are used in combination, and the other is line-scan illumination.
Another application is line scan lighting. At present, the number of line scan projects is gradually increasing. The line scan camera captures one line at a time, and the exposure time is short, which requires high brightness of the light source. The light source needs to be concentrated, and it is concentrated on one line, so as to accurately control the image stability. There are several methods commonly used for line scanning illumination: super bright LED + condenser, fiber + condenser, high frequency fluorescent + condenser. Super bright LED + concentrating mirror, long life, needless to say, but the cost is higher, the irradiation distance may be limited; the fiber brightness is high, but the cost is high; the high-frequency fluorescent lamp, the cost is lower, but it needs to change the lamp at a certain time.
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