The main purpose of combining a telecentric lens (such as a Digifar lens) in a single-camera DIC system is to address the magnification variation error caused by out-of-plane displacement and improve measurement accuracy, especially when measuring strain and displacement in a plane.
Out-of-plane displacement causes a change in magnification
Characteristics of ordinary lenses: Ordinary lenses (especially finite conjugate lenses) exhibit perspective distortion. The size (magnification) of an object on the image plane changes with its distance from the lens (position in the Z direction). The closer the object is to the lens, the larger the image appears; the farther away the object is from the lens, the smaller the image appears (near objects appear larger, far objects appear smaller).
DIC Measurement Principle: DIC calculates in-plane displacement (U, V) and strain by tracking the sub-pixel displacement of the speckle pattern on the sample surface in an image sequence. Its algorithm assumes that the only change between images is the in-plane deformation and displacement of the speckle pattern.
Out-of-plane displacement interference: In actual experiments, the specimen will inevitably undergo out-of-plane displacement (W, moving along the Z direction of the optical axis) under load.
Error generation: When the sample undergoes out-of-plane displacement (W), the magnification (M) of the ordinary lens image changes accordingly. The DIC algorithm may mistakenly interpret this as expansion or contraction of the speckle pattern within the image plane, thus incorrectly calculating in-plane strain and displacement. This is one of the most significant sources of systematic error in a single-camera ordinary lens DIC system.
The core advantage of telecentric lenses: constant magnification.
The principle of object-centric telecentricity: The key design feature of a telecentric lens (usually object-centric) is that its principal ray is parallel to the optical axis in object space. This means that the lens entrance pupil is located at infinity.
Eliminating perspective distortion: Since the entrance pupil is at infinity, only light rays parallel to (or very close to parallel to) the optical axis can enter the lens and form an image. This results in:
Constant magnification: As an object moves along the optical axis (Z direction) within the depth of field of the lens, its image size on the image plane remains constant. Whether the object is closer to or farther from the lens (within the depth of field), its size in the image is the same.
Eliminate perspective distortion: The shape of an object will not be distorted by its position in the field of view (distance from the optical axis). A square grid will remain a square at the edge of the image, not a trapezoid.
The key role of telecentric lenses in single-camera DIC
Eliminating strain errors caused by out-of-plane displacement: This is the most important function. Since the magnification of the telecentric lens does not change with the working distance (Z), the out-of-plane displacement (W) of the specimen will not cause overall scaling of the speckle pattern in the image. The speckle displacement and deformation tracked by the DIC algorithm can more realistically reflect the true in-plane deformation of the specimen surface.
Why is this especially important in a single-camera system?
A single-camera DIC system can only provide one viewpoint. It cannot directly sense or measure out-of-plane displacement (W), nor can it calculate the impact of out-of-plane displacement on image magnification and distortion through geometric relationships. Using a telecentric lens is the most direct and effective optical means to overcome this limitation.
In a single-camera DIC system, out-of-plane displacement (W) causes a change in the magnification of a conventional lens, leading to incorrect in-plane displacement and strain calculations by the DIC algorithm. The core characteristic of telecentric lenses (such as Digifar lenses)—constant magnification resulting from their telecentric object-side design—directly eliminates this major source of error.
This ensures that movement of the sample along the optical axis does not alter its dimensions in the image, guaranteeing that DIC measurement results more accurately reflect the true in-plane deformation of the sample surface. For applications primarily focused on high-precision in-plane measurements, combining a telecentric lens is a key technical solution for improving the accuracy and reliability of single-camera DIC systems.
