# Research Progress of Visual Inspection Technology of Steel Products--A Review
## 3 The Hardware Composition of the Inspection System
### Lighting Method Selection  
In addition to the influence of the light source and CCD sensor on detection effects, the lighting mode has a greater impact on the detection effect of steel products. Usually, lighting can be divided into light and dark lighting. The bright field lighting method is shown in Figure 5a. In this mode, the light source and the CCD are on the same side of the strip. The light emitted by the light source enters the camera after being reflected by the detection target. The reflection angle \(\beta\) is equal to the incident angle \(\alpha\), and the line between the CCD sensor and the image of the light source must be on the same line as the reflected light. The reflected light is evenly distributed on each area of the CCD sensor when there is no defect on the surface. However, reflected light at the defect position will change when a defect exists, and the illuminance entering the CCD sensor will be weakened. Therefore, the reflection of light in the defect area is altered for three-dimensional defects, and the illuminance of the defect position into the CCD sensor is less than the background light entering the CCD sensor; that is to say, the defect image is darker than the background image. As for two-dimensional defects, the reflection of the defect does not change, but two-dimensional defects are usually of different shades. The light in the darker region absorbs more light; therefore, the gray value of the defect image is higher than the background image when the defect color is lighter than the surface color of the strip. Conversely, the grayscale value of the defect image is lower than the background image when the color of defects is darker than the color of non-detects. Therefore, the use of bright field lighting can not only detect two-dimensional (2D) defects, it can also detect three-dimensional (3D) defects. However, it is worth noting that the results will be  
Figure 5: Lighting methods. significantly affected if a large fluctuation of the steel strip leads to exceeding the range of the reflection angle. Overall, the bright field method is more appropriate for detecting a type of defect that reflects and absorbs light, especially dark targets with a bright background, such as scales, oxide skins, pits, water marks, etc. In the dark field lighting method (Figure 5b), the light source and the CCD sensor are also on the same side of the strip. In this case, the reflection angle \(\beta\) is not equal to the incident angle \(\alpha\), and the line between the CCD sensor and the image of the light source is not on the same line as the reflected light; therefore, it is difficult for light to enter the CCD sensor. Only when three-dimensional defects exist on the strip surface will the defect change the reflective nature of the light into a diffuse reflection. Then, the camera is able to collect some diffuse light, and the light from the defect position will be stronger than from areas without a defect. In addition, the light source itself has a collection effect of high-intensity light. Even if the incident angle is changed, it has little effect on the illumination of reflected light on CCD sensors. As a result, the CCD can still effectively detect a surface defect when the surface of the strip steel generates vibration. On the whole, the dark field lighting method is more suitable for the type of defects that can emit diffuse reflected light on the surface of a bright steel plate, and, in particular, bright targets with dark backgrounds, such as skins, pits, and indentations. There is a certain degree of tolerance to the vibration of the detection point. The bright and dark double field lighting mode (Figure 5c) addresses the problems of detection of two-dimensional defects in dark field lighting and vibration at the detection point. However, this method has a high requirement for a high-intensity concentrated light effect of the light source; that is to say, the part of the dark field detection that is included will not be ideal if the concentrated light effect is not strong.