| Required object coverage | Field of view (FOV) | Approximately 5 × 5 mm to 200 × 150 mm | Select a lens whose calibrated object field is slightly larger than the target part, while maintaining sufficient pixel density. | An undersized FOV clips the part; an excessively large FOV reduces the number of pixels available for each measured feature. |
| Required measurement resolution | Magnification and sensor pixel size | Magnification: 0.1× to 2.0×; pixel size: commonly 2.74–5.5 µm | Estimate object-space pixel size using: sensor pixel size ÷ optical magnification. Aim for at least 2–3 pixels across the smallest feature to be detected. | Higher magnification improves sampling but decreases FOV and may require a larger working distance or higher-precision mechanics. |
| Dimensional accuracy | Distortion | Typically below 0.1%; precision models may be specified below 0.05% | Use a low-distortion bi-telecentric lens when measuring lengths, diameters, pitch, or edge positions across the entire FOV. | Low distortion minimizes position errors between the center and edge of the image and reduces dependence on software correction. |
| Stable size measurement at different heights | Telecentricity error | Commonly less than 0.1°; precision applications may require less than 0.05° | Choose bi-telecentric optics when object height variation, positioning error, or dimensional accuracy is critical. | Both object-space and image-space telecentricity help keep measured dimensions stable when the part or sensor moves slightly along the optical axis. |
| Available installation space | Working distance (WD) | Approximately 40–250 mm, depending on FOV and optical design | Define the distance from the front reference surface of the lens to the measured object, and allow additional space for lighting and fixturing. | Insufficient WD can cause mechanical interference, shadowing, or inadequate room for coaxial or backlighting components. |
| Depth variation of the part | Depth of field (DOF) | Approximately ±1 mm to ±15 mm, depending on aperture, magnification, and acceptable blur | Confirm the required sharpness range at the intended aperture instead of relying only on the nominal DOF specification. | A larger DOF accommodates uneven parts but may require more illumination because stopping down reduces transmitted light. |
| Camera compatibility | Image circle and sensor format | Common sensor formats: 1/1.8 in, 2/3 in, 1 in, 1.1 in, and 4/3 in | Ensure the lens image circle is equal to or larger than the camera sensor diagonal and verify the specified mount interface. | A mismatched image circle can produce corner shading, reduced resolution, or unusable areas at the edge of the sensor. |
| Lighting and contrast | Aperture and illumination method | Typical aperture range: approximately f/4 to f/16; illumination may be backlight, coaxial, or telecentric | Use backlighting for silhouette measurements, coaxial lighting for flat reflective surfaces, and diffuse lighting for irregular or textured parts. | Correct lighting has a direct effect on edge contrast, threshold stability, repeatability, and the reliability of automated measurement. |
| Surface and material inspection | Spectral range and coating | Visible spectrum commonly covers approximately 400–700 nm; near-infrared versions may extend beyond 700 nm | Match the lens transmission range and optical coating to the camera wavelength and illumination source. | Spectral mismatch can reduce brightness, contrast, and sharpness, especially when using monochromatic or near-infrared illumination. |
| Production-line reliability | Mechanical stability and environmental conditions | Typical operating range: approximately 0–50 °C; vibration-resistant mounting is recommended | Check temperature stability, locking mechanisms, mounting rigidity, and the required protection level for dust or moisture. | Thermal expansion, vibration, and loose focus or aperture controls can cause drift and reduce measurement repeatability. |
| Calibration and acceptance testing | Calibration target and verification method | Use a traceable grid, stage micrometer, certified gauge, or reference artifact | Request test data for magnification, distortion, telecentricity, resolution, and uniformity under the intended camera and lighting conditions. | Testing the complete optical system is more meaningful than evaluating the lens alone because camera, lighting, focus, and calibration affect final accuracy. |
