Non-uniformity correction of wide field of view imaging system

Developing a high-performance wide field of view (FOV) optical system requires attaining uniform illumination without compromising the image quality. However, achieving these two critical requirements concurrently remains a big engineering challenge. On one hand, increasing the FOV dramatically increases the distortions and geometric aberrations, resulting in the non-uniformity of image quality. On the other hand, the optical systems lose a certain amount of illumination with increased FOV. The drop in the irradiance is always in accordance with the cosine-fourth of the field angle on the image plane.

The monocentric cascade imaging system (MCIS), which divides the imaging task between the shared monocentric objective and a parallel array of the relay imagers, has been proposed to address the above issues. Although the MCIS can achieve uniform illumination and image quality across the entire FOV range, vignetting at the entrance lens and non-uniformity image quality are still problems in the individual relay imagers. These two issues affect the image stitching required for wide-FOV image acquisition, thereby contributing to poor performance in most FOV systems.

Generally, for the marginal FOV, the aperture stops lie in each relay imager and not at the center of the monocentric objective. Thus, the ray path in the monocentric objective is not in an axial symmetrical form. The non-uniformity-induced local aberrations created by the asymmetric ray paths also result in noisy overlapping regions. These non-uniformity issues were qualified and verified in optical imaging experiment prototypes. Nevertheless, non-uniformity correction of the MCIS is still underexplored in the literature.

In response to the growing demand for both excellent image quality and uniform illumination in wide FOV imaging systems, Soochow University researchers: Dr. Yiqun Ji, Dr. Chenxin Zeng, Dr. Fenli Tan, Dr. Anwei Feng and Dr. Jizhou Han proposed a wide-FOV MCIS. The relationship between the aperture stop position, vignetting and local aberrations in MCIS was discussed based on the aberration theory. Additionally, the feasibility of using aspheric surfaces to balance the local aberrations was studied and discussed. Their work is currently published in the journal, Optics Express.

In their approach, moving the aperture stop towards the object space resulted in uniform illumination, while introducing aspherical surfaces on proper surfaces proved effective in balancing the local aberrations. The authors evaluated the optical performance of the proposed aspheric MCIS across the entire FOV. Results showed that it achieved a relative illumination exceeding 97% and an instantaneous FOV of 0.0021° and a wide FOV of 116.4°. Additionally, the modulation transfer function was above 0.285 at a Nyquist frequency of 270 lp/mm.

In summary, Soochow University scientists reported the correction of non-uniformity of illumination and image quality for individual relay imagers in the wide VOF imaging system. The resulting aspheric MCIS exhibited a good image quality with uniform illumination. It was worth noting that good uniformity did not only improve the imaging resolution but was also conducive to image stitching for the imaging system. In a statement to Advances in Engineering, Dr. Yiqun Ji stated that their findings provided valuable theoretical insights and guidance that would contribute to further research and development as well as application of MCIS.