Objective When the optical systems are used for detecting and imaging targets, stray light generated by strong light sources such as the sun can incident on the image plane, so the imaging quality of the optical system is affected by the noise. In severe cases, it may even cover the energy of the imaging beam. The suppression ability of stray light in optical system is one of the important indicators of optical system performance. The annular folded aperture optical system is made of single optical element to achieve high image quality and system integration. Because the multi-reflective surfaces are arranged on the same substrate material surface, the image quality of the annular folded aperture optical system is affected seriously by the stray light. The method of stray light suppression is proposed for the annular folded aperture imaging system.

Methods The stray light analysis is applied at different off-axis angles, and the characteristic angle can be selected. When the beam with the smallest off-axis angle is found and suppressed, the influence of stray light can be eliminated. The system without stray light suppression can be subjected to stray light analysis, the characteristic angle of stray light is obtained through point source transmittance data analysis, and then the stray light path at this angle is analyzed (Fig.6). When there are obvious bright spots on the image and the point source transmittance value is smaller than one order of magnitude for the field of view of the system, the characteristic angle can be determined. According to the analyzed characteristic angle, the expression of the light shield length can be obtained.

Results and Discussions The focal length of the annular folded aperture optical system used for the stray light analysis is 60 mm, the diameter is 70 mm, the thickness is 18 mm, the ratio of total system length to focal length is 0.3, and the obscuration ratio is 0.74. A simulation model for the optical system is established and stray light analysis is performed to obtain the point source transmittance results. The off-axis angle of stray light analysis is 5° to 85°, and the number of rays traced at each angle is 2 billion. The annular folded aperture optical system without stray light suppression has severe influence, and the point source transmittance is 10⁻² to 10⁻³. Based on the analysis of stray light in this system, the characteristic off-axis angle at the critical state is 14.9°, and the length of the external light shield is 47.57 mm. When the annular folded aperture optical system with a light shield, it can be seen that the concentrated light spots are eliminated, and the entire energy is evenly distributed on the image plane, so stray light is effectively suppressed (Fig.13).

Conclusions The method of stray light suppression is proposed for the annular folded aperture imaging system. The characteristic angle of stray light can be obtained through point source transmittance data analysis, and the expression of the light shield length is derived. The point source transmittance curve of the annular folded aperture imaging system optimized by stray light suppression has fewer extreme points. It can be seen from the curve that the point source transmittance value of stray light has decreased by 2 orders of magnitude when the off-axis angle is less than 15°. When the off-axis angle is greater than 15°, the point source transmittance value of stray light has decreased by 2 to 4 orders of magnitude. The results show that, the stray light can be suppressed by the designed light shield effectively, and it is possible to achieve high image quality for single chip element.