Use a backlight when the decision depends on silhouette, edge, hole, gap, or outer contour. Use coaxial lighting when the camera benefits from controlled on-axis reflection from a flat surface or planar mark. The correct route depends on the feature being measured, not on a light name alone.
Use product, process, facility, and inspection images to check whether the page claim is tied to a visible engineering condition.
Selection follows the observable condition, the optical setup, and the production constraint.
Backlighting is often used for the initial comparison for outer profile, hole, edge, gap, and silhouette contrast.
Silhouette size, edge tolerance, FOV, object-to-light distance, telecentric-lens requirements, and mechanical clearance.
Surface flatness, optical aperture, FOV, working distance, material reflection, and camera alignment.
Usually not as the primary method. Backlight is intended for silhouette and edge information; surface scratches often need directional, low-angle, coaxial, diffuse, or polarized tests.
Not when the measurement depends on a clean silhouette or edge. Coaxial light may help planar face features, but it does not create the same object-versus-background outline as a backlight.
