TDI line scan technology, or Time Delay Integration line scan imaging, is a specialized industrial camera method that captures high-quality images of moving objects at very high speeds. Unlike standard line scan cameras, TDI sensors integrate multiple exposures of the same moving target to significantly improve signal-to-noise ratio, making them ideal for low-light or fast-moving inspection environments. This technology is critical for applications requiring both speed and image clarity, such as web inspection, flat panel display testing, and semiconductor wafer examination.

1、TDI line scan camera

A TDI line scan camera is a specialized imaging device that uses Time Delay Integration to capture images of moving objects with exceptional sensitivity and speed. Unlike conventional area scan cameras that capture a full frame at once, TDI cameras operate by scanning a single line of pixels across a moving target. The key innovation is that the sensor accumulates charge from multiple exposures of the same point on the object as it moves, effectively multiplying the signal without increasing noise. This allows TDI line scan cameras to operate in low-light conditions or at extremely high line rates, often exceeding 100 kHz. They are commonly used in industrial environments where high throughput is required, such as in the inspection of printed circuit boards, solar panels, and continuous web materials. The camera typically includes a high-speed data interface like Camera Link or CoaXPress to handle the massive data throughput. Choosing the right TDI line scan camera depends on factors like pixel size, number of stages, and maximum line rate. For example, a 128-stage TDI camera can provide up to 21 dB improvement in signal-to-noise ratio compared to a standard line scan camera. These cameras are also available in color and monochrome versions, with color models often using Bayer filters or prism-based technology. The robustness of TDI cameras makes them suitable for harsh industrial conditions, including vibration, temperature fluctuations, and dust. Manufacturers like Teledyne DALSA, Hamamatsu, and Basler offer a range of TDI line scan cameras tailored for different applications. The integration of TDI technology into line scan cameras has revolutionized high-speed industrial inspection, enabling detection of sub-micron defects that would be invisible with standard cameras.

2、TDI line scan principle

The principle of TDI line scan imaging is based on the concept of time delay integration, where the sensor accumulates charge from multiple exposures of the same moving object. As the object moves under the camera, the charge packets in the sensor are shifted synchronously with the motion. This means that each pixel on the sensor sees the same point on the object multiple times, and the accumulated charge is read out as a single, stronger signal. The core advantage is that the signal increases linearly with the number of TDI stages, while the noise only increases as the square root of the number of stages. This results in a significant improvement in signal-to-noise ratio, often by 10 to 20 dB or more, depending on the number of stages used. The TDI principle relies on precise synchronization between the sensor readout and the object motion, usually achieved through an encoder or a timing generator. The sensor itself is a CCD or CMOS device with multiple rows of pixels, where each row corresponds to a TDI stage. As the object moves, the charge is transferred from one row to the next, adding the signal from each exposure. The number of stages typically ranges from 16 to 256, with higher stages providing better sensitivity but requiring more precise motion control. The TDI principle is especially effective for imaging fast-moving objects because it allows longer effective exposure times without motion blur. This is critical for applications like printing inspection, where the web moves at speeds of several meters per second. The principle also enables imaging in low-light conditions, such as in X-ray inspection or fluorescence imaging. Understanding the TDI line scan principle is essential for designing inspection systems that require both high speed and high sensitivity. The technology has been refined over decades and is now a standard tool in industrial machine vision, particularly for continuous processes where traditional area scan cameras would fail due to motion blur or insufficient light.

3、TDI line scan vs conventional line scan

Comparing TDI line scan versus conventional line scan technology reveals key differences in sensitivity, speed, and application suitability. Conventional line scan cameras capture a single line of pixels at a time, relying on the object's motion to build a 2D image. They require sufficient light to achieve a good signal-to-noise ratio, which can be challenging at high line rates. In contrast, TDI line scan cameras integrate multiple exposures, allowing them to operate with less light or at higher speeds. For example, a conventional line scan camera running at 100 kHz line rate might require intense illumination, while a TDI camera with 64 stages can achieve the same signal with 64 times less light per exposure. This makes TDI cameras ideal for applications where illumination is limited or where heat from bright lights could damage the product. Another difference is in image quality: TDI cameras typically produce images with lower noise and higher dynamic range, making them better for detecting subtle defects. However, conventional line scan cameras are simpler and less expensive, making them suitable for applications where light is abundant and speeds are moderate. TDI cameras also require more precise synchronization because the charge transfer must match the object motion exactly. Any mismatch can cause image blurring or loss of signal. Conventional line scan cameras are more forgiving in this regard. In terms of resolution, both technologies can achieve high pixel counts, but TDI cameras often have larger pixel sizes to accommodate charge storage, which can limit spatial resolution. The choice between TDI and conventional line scan depends on the specific inspection requirements: for high-speed, low-light, or high-sensitivity applications, TDI is superior; for cost-sensitive or simpler applications, conventional line scan may be sufficient. Industries such as PCB inspection, flat panel display manufacturing, and solar cell inspection often prefer TDI for its superior performance, while packaging inspection and barcode reading may use conventional line scan.

4、TDI line scan applications

TDI line scan applications span a wide range of industrial sectors where high-speed, high-sensitivity imaging is essential. One of the most common applications is web inspection, where continuous materials like paper, film, metal foil, or textiles are inspected for defects such as holes, streaks, or contamination. TDI cameras can detect these defects at line speeds exceeding 1000 meters per minute, which is impossible with area scan cameras. Another major application is in the electronics industry for inspecting printed circuit boards (PCBs), semiconductor wafers, and flat panel displays. TDI cameras can identify micro-cracks, solder defects, and pattern misalignment with high precision. In the solar industry, TDI line scan is used for inspecting photovoltaic cells for cracks, electrode defects, and surface irregularities during manufacturing. The technology is also critical in the printing industry for quality control of newspapers, labels, and packaging materials, where even minor print defects can lead to costly waste. In medical device manufacturing, TDI cameras inspect catheters, syringes, and other components for dimensional accuracy and surface defects. Additionally, TDI line scan is used in the food industry for inspecting packaged products, such as checking seal integrity and foreign object detection. The automotive sector uses TDI cameras for inspecting painted surfaces, welds, and assembly components on production lines. In the textile industry, TDI technology monitors fabric quality for weaving defects and color consistency. Another niche application is in the inspection of banknotes and security documents, where high sensitivity is required to detect counterfeit features. The versatility of TDI line scan makes it a preferred choice for any application requiring continuous, high-speed inspection with high defect detection rates. As manufacturing processes become faster and quality standards stricter, the demand for TDI line scan applications continues to grow, driving innovation in sensor design and data processing.

5、TDI line scan sensor technology

TDI line scan sensor technology is at the heart of high-performance industrial imaging systems. The sensor is typically a charge-coupled device (CCD) or CMOS device with multiple rows of photodiodes arranged in a linear array. Each row represents a TDI stage, and the number of stages can range from 16 to 256 or more. The sensor operates by transferring charge from one row to the next in synchronization with the object motion, accumulating signal from each exposure. The key parameters of a TDI sensor include pixel size, number of stages, quantum efficiency, and charge capacity. Pixel sizes typically range from 5 to 14 micrometers, with larger pixels offering better sensitivity but lower resolution. The quantum efficiency of modern TDI sensors can exceed 90% in the visible spectrum, making them highly sensitive. The charge capacity determines the maximum signal level before saturation, which is important for high-dynamic-range applications. TDI sensors also feature advanced noise reduction techniques, such as correlated double sampling and dark current subtraction, to further improve image quality. Recent advancements in CMOS TDI technology have enabled higher speeds and lower power consumption compared to traditional CCD-based sensors. Some sensors now offer up to 256 stages and line rates exceeding 200 kHz. The sensor technology also includes anti-blooming structures to prevent charge overflow in bright areas, ensuring accurate imaging even in high-contrast scenes. Color TDI sensors use Bayer filters or prism-based color separation to capture color information while maintaining the TDI benefits. The integration of TDI sensors with high-speed data interfaces like CoaXPress or GigE Vision allows real-time data transmission for immediate defect detection. The reliability of TDI sensors is critical in industrial environments, where they must withstand temperature variations, vibrations, and continuous operation. Manufacturers continually improve sensor technology to meet the demands of emerging applications, such as deep UV inspection for semiconductor lithography and near-infrared imaging for material sorting. Understanding TDI line scan sensor technology is essential for selecting the right camera for a specific inspection task.

6、TDI line scan for web inspection

TDI line scan for web inspection is a critical application in industries that produce continuous materials such as paper, plastic film, metal foil, textiles, and nonwovens. The web moves at high speeds, often exceeding 500 meters per minute, and must be inspected for defects like holes, streaks, wrinkles, contamination, and coating irregularities. TDI technology provides the sensitivity needed to detect these defects under low-light conditions, which is common in web inspection because bright illumination can cause glare or heat damage. The high signal-to-noise ratio of TDI cameras allows for detection of sub-millimeter defects even at high line rates. In web inspection systems, the TDI camera is typically mounted across the web width, and the image is captured line by line as the web moves. The camera must be synchronized with the web speed using an encoder to ensure accurate image reconstruction. Modern TDI cameras can achieve line rates up to 200 kHz, enabling inspection at web speeds of 1000 meters per minute or more. The system often includes specialized lighting, such as LED arrays or laser line generators, to enhance defect contrast. Software algorithms process the image data in real time to identify and classify defects, triggering alarms or marking the web for later removal. TDI line scan for web inspection is particularly valuable in the production of lithium-ion battery separators, where even microscopic defects can cause battery failure. In the paper industry, TDI cameras detect holes, dirt spots, and coating defects that affect print quality. For plastic film, the technology identifies gel particles, die lines, and thickness variations. In textile inspection, TDI cameras detect broken threads, weaving errors, and color inconsistencies. The use of TDI line scan in web inspection reduces waste, improves product quality, and increases production efficiency. As web speeds continue to increase and quality standards become more stringent, TDI technology remains the preferred solution for high-performance web inspection.

7、TDI line scan resolution and speed

TDI line scan resolution and speed are two critical performance parameters that must be balanced for optimal inspection results. Resolution refers to the smallest feature that can be detected, typically measured in micrometers per pixel. Speed refers to the line rate, or the number of lines captured per second, which determines how fast the object can move. TDI cameras can achieve high resolution with pixel sizes as small as 5 micrometers, allowing detection of defects down to a few micrometers. However, smaller pixels reduce the amount of light captured per pixel, which can be compensated by the TDI integration effect. The number of TDI stages also affects speed: higher stages provide better sensitivity but require slower line rates to allow charge transfer. For example, a 128-stage TDI camera might have a maximum line rate of 100 kHz, while a 32-stage camera could reach 200 kHz. The trade-off between resolution and speed is managed by selecting the appropriate pixel size and number of stages for the application. In high-speed web inspection, line rates of 100 to 200 kHz are common, with pixel resolutions of 10 to 20 micrometers. For applications requiring very high resolution, such as semiconductor inspection, line rates may be lower, around 10 to 50 kHz, but with pixel sizes of 5 micrometers or less. The sensor's data throughput, measured in megapixels per second, is another important factor. Modern TDI cameras can generate data rates exceeding 10 gigapixels per second, requiring high-speed interfaces like CoaXPress or Camera Link HS. The resolution and speed also depend on the optics used, including the lens and the working distance. Proper illumination is essential to achieve the full potential of TDI resolution and speed. Understanding the relationship between TDI line scan resolution and speed helps engineers design inspection systems that meet both throughput and defect detection requirements. As sensor technology advances, the trend is toward higher resolution at higher speeds, enabling even more demanding applications in the future.

In this article, we have explored seven highly relevant aspects of TDI line scan technology, including the camera itself, its operating principle, comparison with conventional line scan, key applications, sensor technology, web inspection use, and the balance between resolution and speed. Together, these topics provide a comprehensive understanding of why TDI line scan is a critical tool for high-speed industrial inspection. From detecting microscopic defects in semiconductor wafers to ensuring the quality of continuous web materials, TDI technology offers unmatched sensitivity and speed. Whether you are evaluating TDI line scan for your own manufacturing process or seeking to deepen your technical knowledge, the insights shared here will guide you in leveraging this powerful imaging solution to enhance quality control, reduce waste, and improve production efficiency.

TDI line scan technology represents a significant advancement in industrial imaging, enabling inspection tasks that were previously impossible due to speed or lighting constraints. By integrating multiple exposures of moving objects, TDI cameras achieve exceptional signal-to-noise ratios, allowing defect detection at line rates exceeding 100 kHz. This makes them indispensable in industries such as electronics, solar, printing, textiles, and packaging. The seven key topics covered in this article—from camera specifications to application areas—demonstrate the versatility and power of TDI line scan. As manufacturing continues to evolve toward higher speeds and stricter quality standards, TDI technology will remain at the forefront of machine vision innovation. Understanding these concepts is essential for anyone involved in industrial inspection system design or procurement. We hope this article has provided valuable insights and encourages further exploration of TDI line scan solutions for your specific needs.