How to Choose a Bi Telecentric Lens?

Choosing a bi telecentric lens begins with one practical question: what must remain accurate?

In dimensional inspection, a small part may sit slightly closer to the camera. A conventional lens changes its apparent size. A bi telecentric lens helps preserve consistent magnification from both object and image sides. That stability can protect measurements of holes, edges, pins, and molded profiles.

It is not magic.

Optical design expert Dr. David Shafer has described telecentric imaging as “a measurement tool with trade-offs.” That reminder matters. A telecentric lens can improve geometric reliability, yet it may require more space, controlled lighting, and careful alignment. A larger lens diameter may also raise cost.

Experienced engineers usually compare object-space telecentricity, image-space telecentricity, or true bi-telecentric performance. They check the required magnification, sensor format, working distance, depth of field, and distortion specification. A 0.5× lens may suit one inspection station, but fail when the camera sensor changes. Tiny errors become visible.

Look closely.

This guide, “How to Choose a Bi Telecentric Lens?”, follows the decisions made on a real inspection bench. It considers measurement tolerance, part movement, illumination, mounting clearance, and calibration practice. It also questions a common assumption: maximum telecentricity is not always the best purchase. Sometimes, a simpler lens delivers enough accuracy with easier integration. The correct choice depends on the application, not the product label alone.

Define the Measurement Goals and Object-Space Requirements

Choosing a bi telecentric lens starts with the measurement goal, not the camera. Decide whether you need diameter, edge position, height, or surface-defect data. Each goal demands different contrast and resolution. In 2023, 541,302 industrial robots were installed worldwide, according to the International Federation of Robotics. Automated inspection is expanding, but speed cannot replace traceable accuracy.

Define the object-space requirements with measured values. Record the part’s field of view, working distance, largest height variation, and smallest feature. A telecentric lens should cover the full object without forcing the part near the image edge. Check depth of field carefully. A thick metal component may look sharp on one face and soft on another. Low distortion matters when dimensions are calculated from pixels. VDI/VDE 2634 recommends calibrated artefacts for testing optical measurement systems. Use one during validation, not only during installation.

Tips: Measure the actual part, not a simplified drawing. Leave clearance for lighting and fixturing. Test the lens with production materials, because reflective surfaces can expose weaknesses. A common mistake is choosing magnification first. Reconsider it if the sensor cannot resolve the smallest tolerance. The correct setup may require a larger field of view and better lighting, not more magnification. Even experienced engineers sometimes underestimate vibration, thermal drift, or imperfect alignment. Record those uncertainties before approving the measurement design.

Match Magnification, Working Distance, and Field of View

How to Choose a Bi Telecentric Lens?

A bi telecentric lens keeps both object and image-side rays nearly parallel. This helps maintain stable measurements when part height changes. Start by matching magnification to your camera sensor and required detail. A 0.5× lens may show a wide area, while a 1× lens reveals smaller features with greater pixel coverage. Calculate the field of view before purchasing. For example, a 12 mm sensor width at 0.5× can inspect roughly a 24 mm object width.

Working distance matters just as much. It must leave space for lighting, fixtures, and moving parts. A 100 mm working distance may suit a shallow inspection station, but a deeper enclosure could need more clearance. Do not trust a catalog value alone. Mechanical tolerances, protective windows, and mounting errors can reduce usable space. I have seen a theoretically correct setup fail because the light could not reach the inspected edge.

Tips: Measure the largest object dimension, not the average one. Check the lens image circle against the sensor. Test several height positions using a calibrated target. Record edge sharpness and measurement drift. Small errors matter. Also, avoid choosing magnification only for maximum detail. Excessive magnification can shrink the field of view and force unnecessary repositioning. Recheck the required working distance after installing the actual fixture, because the final setup often differs from the drawing.

Evaluate Sensor Coverage, Resolution, and Telecentricity

How to Choose a Bi Telecentric Lens?

Evaluate sensor coverage, resolution, and telecentricity before comparing specifications. The lens image circle must cover the sensor diagonal with a small margin. A 2/3-inch sensor, for example, needs more than a nominal 2/3-inch image circle. Check the actual active area, not only the sensor label. This prevents dark corners and wasted pixels.

Resolution should match the smallest feature you need to measure. If a 0.1 millimeter edge must be inspected, calculate the required pixel size first. A useful target is several pixels across that feature. Higher magnification is not always better. It can reduce the field of view and make alignment less forgiving. Working distance also matters, especially around clamps, conveyors, or uneven parts. Leave practical space for lighting and maintenance.

Telecentricity controls measurement stability when object height changes. In a bi telecentric lens, both object-space and image-space chief rays remain nearly parallel. This helps preserve magnification and reduces perspective errors. Review the stated telecentric angle, distortion, and depth of field under real lighting. Test a calibrated target at several heights. I once trusted a catalog value and ignored mounting tilt; the measurements still drifted. That mistake was avoidable. Even a precise lens cannot correct poor fixturing, vibration, or an unsuitable sensor. Verify the complete optical setup with production samples, not only a clean laboratory target.

Check Lighting Compatibility and Mechanical Integration

How to Choose a Bi Telecentric Lens?

A bi-telecentric lens is effective only when its lighting and mechanics match the inspection system. Test the complete setup, not the lens alone. For flat metal parts, diffuse backlighting can reveal edges without creating strong surface glare. Glossy plastic may need controlled coaxial illumination and a polarizing filter. Check wavelength compatibility too. A lens designed for visible light may lose contrast under near-infrared lighting. Measure the required field of view, working distance, and depth of field under actual illumination. Do not trust catalog contrast alone.

I have seen a sharp image become unusable after moving a light only a few millimeters. Mechanical integration deserves equal attention. Confirm the sensor format, flange distance, mounting thread, and clearance around the housing. The lens should fit the camera without forcing adapters into a vibrating assembly. Check whether the light, lens, and inspected part share one rigid reference frame. Small alignment errors can create uneven brightness or hide edge defects. Leave room for cables and adjustment tools. This detail is easy to miss.

Before approval, inspect sample parts at production speed and with realistic tolerances. Record exposure settings and rejected images for later review. If results depend on perfect manual alignment, the design probably needs another iteration. That is not failure. It is useful evidence.

Compare Distortion, Depth of Field, and Total System Cost

Choosing a bi telecentric lens requires more than checking magnification. In production trials, distortion often decides measurement reliability. A 0.1% error can shift a 50 mm part by 0.05 mm. That matters when tolerances are tight. Bi telecentric designs reduce perspective changes when parts move slightly toward or away from the camera. Still, the lens cannot correct poor lighting, vibration, or an unstable fixture. I once focused heavily on distortion and underestimated the mounting structure. That was a costly lesson.

Tips: Measure the full field, not only the center. Use a calibrated target with known dimensions. Record readings at several positions. Compare edge error, repeatability, and image contrast. Do not trust one sample.

Depth of field needs equal attention. A larger aperture may improve brightness, but it can reduce usable focus range. A smaller aperture usually increases depth of field, although diffraction may soften fine edges. Test real parts with their actual surface finish. A polished metal edge behaves differently from a matte plastic edge. This is where specifications can mislead. The listed value may not match your inspection threshold or lighting setup.

Total system cost includes more than the lens. Add the camera, coaxial or backlight illumination, mounting hardware, calibration tools, software, and installation time. A cheaper lens may require extra lighting and complex correction. A higher-priced option may reduce rejected parts and operator adjustments. Calculate cost per inspected unit. Include maintenance and future part changes. The best choice is rarely the lowest invoice.