Optical surface defect detection is essential for ensuring the reliability and efficiency of optical systems. Traditional visual inspection methods, however, have become inadequate for modern production demands due to limitations in efficiency and precision. Therefore, automated detection equipment, with machine vision technology at its core, has developed rapidly. Among its components, the illumination source is critical, as an optimized lighting structure and method are key to improving image quality in defect detection. Currently, most defect detection systems use light emitting diode (LED) arrays in an annular configuration. While these light sources can be arranged for multi-angle illumination, their fixed angles limit adaptability to diverse optical element characteristics. In addition, the LED’s large dispersion angle can lead to reflected light interference, particularly problematic in curved surface detection. Therefore, we propose a multi-angle uniform illumination annular light source optimized for detecting defects on curved optical surfaces.

In designing an illumination source for detecting defects on curved optical surfaces, achieving uniformity in illumination and controlling the light’s divergence angle are primary considerations. After defining the desired illumination characteristics, an energy mapping relationship between the LED light source and the target surface is established. A plano-convex dimming lens is placed in front of the LED to achieve a narrow divergence angle and uniform illumination. The emitted light undergoes an initial adjustment upon passing through the inner surface of the lens and is further modulated by the outer surface of the lens to precisely focus on the target surface. Based on Snell’s law and energy conservation principles, a point-by-point algorithm for the lens profile bus is developed enabling the construction of a 3D model by fitting and rotating discrete profile points. To meet the requirements for defect detection on optical surfaces, unit light sources with aspheric dimming lenses are arranged in an annular pattern at equal azimuthal intervals, allowing superimposed illumination in centrally symmetric areas. A displacement mechanism enables Z-axis adjustments of the light source, while each unit light source’s incident angle is precisely controlled. This approach achieves multi-angle uniform illumination adaptable to various curved surfaces, effectively reducing reflected light interference and supporting effective dark-field illumination.

After importing the aspherical lens configuration into optical simulation software LightTools, we establish light source parameters and simulation conditions. Simulations verified the illumination effectiveness and the symmetric area superposition approach, confirming that design goals are met (Fig. 7). Effective illumination efficiency within the designated area is recorded at 79.96%. In addition, tests at various working distances reveal stable illumination without abrupt changes, with an average uniformity of 92.59% (Table 1). The divergence angle is controlled within ±7° (Fig. 8). Further analysis of the annular array light source’s performance across planar, concave, and convex surfaces demonstrates uniformity exceeding 90% in all cases (Fig. 9 and Fig. 11). A comparative analysis with a standard annular light source lacking a dimming lens (Fig. 12) shows that the new design significantly reduces reflected light interference. Finally, surface machining and assembly tolerances are analyzed (Fig. 13 and Fig. 14) by applying periodic error functions to the profile sampling points and adjusting the LED position. The results indicate that illumination distribution remains consistent despite machining and fitting errors.

By integrating optical dimming lenses with translational and angular adjustment mechanisms, this system achieves multi-angle uniform illumination, offering greater flexibility for inspecting optical elements with varying curvatures. The dimming lens is designed based on the camera’s field of view, narrowing the LED divergence angle to provide uniform illumination over the effective area. The annular light source ensures high uniformity across flat, concave, and convex surfaces through complementary center-symmetric illumination. Adjusting the incident angle within the beam’s effective range enables multi-angle illumination, effectively mitigating the reflective interference common to classical annular sources used on curved optical surfaces. Tolerance analysis shows that with a processing accuracy of 3 µm and mounting tolerance of 0.1 mm, the illumination distribution curve remains stable, confirming the design’s ease of processing and assembly.