# Research Progress of Visual Inspection Technology of Steel Products--A Review
## 3 The Hardware Composition of the Inspection System  
### Camera  
An industrial camera is at the core of the system hardware. The frame rate (the rate at which the camera collects and transmits images) and resolution are two important parameters of the camera. The frame rate must be greater than the detection speed; 10 fps is usually sufficient to meet industrial requirements. The required resolution depends on the size of the features relative to the overall image. For example, suppose the surface scratch of an object is detected, the size of the object to be photographed is \(a\times b\) mm, and the detection accuracy is 0.01 mm. Then, the minimum resolution formula of the camera can be determined as:  
\[(a/0.01)\times(b/0.01)=Resolution \tag{1}\]  
Industrial cameras are divided into two types according to the differences of the image sensors: charge coupled devices (CCD) and complementary metal oxide semiconductor devices (CMOS) [PERSON] [51]. The differences are as follows.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Steel Type** & **Type of Defect** & **References** \\ \hline Billet & Cracks, scratches & [17; 18; 19; 20; 21] \\ \hline \multirow{4}{*}{Hot strip} & Cracks, longitudinal scratches, & \\  & transverse scratches, scales, & \\  & delamination, roll marks, & [16; 22; 23; 24; 25; 26; 27; 28; 29; 30] \\  & pit defects, seams, inclusions & \\ \hline \multirow{4}{*}{Cold strip} & Bruises, slags, inclusions, seams, oxide & \\  & scales, cracks, holes, feather roll marks, & [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43] \\ \cline{2-3}  & Late marks & \\ \hline \multirow{2}{*}{Stainless steel} & Slag inclusions, scratches in pickling, & [44] \\  & scratches & [45] \\ \hline Rod/bar & Cracks, scratches & [46; 47; 48; 49] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Common defect types of different steel products.  
1. Different imaging processes: A CCD utilizes a small number of output nodes to output data uniformly, thereby ensuring good signal consistency. By contrast, each pixel in a CMOS chip has its own signal amplifier, with charge conversion done separately, so output signal consistency is poorer and more greatly affected by signal noise than with a CCD. However, a significant advantage of CMOS is low power consumption.
2. Different integration: The CCD manufacturing process is complex, and the output of a CCD consists only of an analog electrical signal, which requires a decoder, analog converter, and image signal processor. As a result, a CCD has low integration. A CMOS, on the other hand, can collect signals with an analog-to-digital converter on a chip with high integration and low cost. With the advancement of CMOS imaging technology, CMOS will have greater applications in the future.
3. Different image output speed: A CCD adopts photosensitive outputs sequentially, relatively slowly. With a CMOS, each charge element has its own switch controller, and the readout speed is very fast. Most high-speed cameras with a frame rate greater than 500 fps use CMOS.
4. Different noise levels: CCD technology is mature, and the imaging quality is superior to that of CMOS. CMOS has a higher degree of integration, a closer spacing distance, and more interference.  
As part of so-called Industry 4.0, factories around the world are developing automation and intelligence, in which smart sensors play an important role. The smart camera Lee [52] has the functions of processor, memory, communication interface, operating system, etc., which can process a large amount of data in advance and assist subsequent automatic detection and judgment. [PERSON] et al. [53] noted an ultra-high-speed silicon image sensor. The test chip of this image sensor realizes a temporal resolution of 10 ns. For a silicon image sensor, the limit is 11.1 fps. Considering the theoretical derivation, this high-speed image sensor can reach a frame rate near the theoretical limit.