For the challenging task of monitoring compression deformation in small, complex structural components, a single-camera DIC system equipped with a telecentric lens (such as a Digifar lens) offers a highly effective solution. The core solution lies in the unique optical characteristics of the telecentric lens:
How does DIC technology (in conjunction with a telecentric lens single-camera DIC system) overcome the challenges?
Eliminate perspective distortion/parallax:
The root of the problem: Traditional lenses exhibit perspective effects, making objects appear larger in the image the closer they are to the lens. When small structural components undergo out-of-plane displacement (Z-direction movement) during compression, or have complex height variations, this parallax causes a significant shift in the apparent position of feature points in the image, interfering with the accuracy of in-plane (X, Y) displacement and strain measurements.
Telecentric lens solution: Telecentric lenses employ a special optical design where the principal ray is parallel to the optical axis in object space. This means that the magnification of the object does not change with the distance between it and the lens (working distance). Even if the workpiece undergoes Z-axis movement during compression or its surface height fluctuates, its size in the image remains constant as long as it is within the lens's depth of field.
Solution effect: It fundamentally eliminates perspective distortion caused by displacement from the surface or structural height difference, ensuring that changes in image coordinates only reflect the true in-plane displacement, thus guaranteeing the accuracy and reliability of deformation measurement for small-sized complex structural components.
Constant magnification to counteract height difference:
The root of the problem: The surfaces of complex structural components are usually not flat, but contain features such as steps, holes, and protrusions, resulting in different distances (working distances) from different areas of the surface to the lens. With traditional lenses, this height difference leads to inconsistent magnification in different areas, causing distortion in the displacement and strain fields calculated by DIC.
Telecentric lens solution: The core advantage of a telecentric lens is that it maintains a constant magnification throughout the entire field of view, unaffected by changes in the object's position along the optical axis. As long as the structural features are within the depth of field, the imaging scale is strictly consistent across different height areas.
Solution effect: The telecentric lens ensures that displacement calculations are performed using a uniform scale across the entire measurement area, avoiding systematic errors introduced by local magnification variations, and making strain measurement results on complex geometric features more accurate and comparable.
Advantages of a single-camera DIC system:
Simplified system: Compared to dual-camera stereo DIC systems, single-camera systems have a simpler structure, lower cost, and faster calibration process (usually only a two-dimensional planar calibration plate is needed).
Avoiding the stereo matching challenge: For small, complex structures with sparse, repetitive, or low-contrast surface textures, stereo matching (finding the corresponding point of the same physical point in two camera images) in a dual-camera system can be very difficult and error-prone. A single-camera system completely avoids this problem.
Applicable scenarios: When the deformation of the test piece mainly occurs in the plane and the displacement from the surface is relatively small (within the depth of field of the telecentric lens), the single-camera telecentric DIC system is the optimal, most efficient, and most accurate choice. Compression tests typically meet this condition.
High-resolution imaging:
The root of the problem: small size means that high spatial resolution is needed to distinguish tiny features and deformations.
Solution: Telecentric lenses are typically designed for industrial inspection and can be paired with high-resolution cameras (e.g., 5 megapixels or higher). High-resolution images, combined with DIC's sub-pixel algorithm, can accurately capture displacement and strain in minute areas.
Solution effect: Enables precise monitoring of deformation of minute features (such as microelectronic solder joints, microbeams, and fine ribs).
Overcoming lighting and speckle challenges:
The root of the problem: It may be difficult to apply high-quality speckle patterns to the surface of small, complex structural components, and achieving uniform illumination is also more challenging.
Solution:
Telecentric lenses: less sensitive to out-of-focus conditions (within the depth of field) and have a relatively higher tolerance for uneven lighting.
DIC technology is not sensitive to the absolute grayscale values of speckle patterns, but mainly relies on the local grayscale distribution (gradient, correlation) of the pattern. By optimizing speckle fabrication (such as spraying, photolithography) and coaxial illumination techniques, sufficient texture for DIC analysis can be obtained on small-sized complex surfaces.
The combination of a single-camera DIC system and a telecentric lens, through its constant magnification, completely eliminates perspective distortion caused by structural height differences and out-of-plane displacement during compression, providing an ideal solution for high-precision and reliable in-plane displacement and strain field measurement of small-sized complex structural components.
