As the key equipment for reproducing real solar radiation characteristics in a laboratory setting, the solar simulator plays a vital role in many fields such as meteorology, aerospace, and agriculture. Among the optical components of the solar simulator, the uniform optical device is crucial for the homogenization of the Gaussian distribution radiation flux, which directly affects the irradiation uniformity and overall performance of the solar simulator. The optical integrator based on the Kohler illumination principle is favored by many researchers due to its high energy efficiency and large-area uniform irradiation. However, the traditional planar microlens array optical integrator faces challenges such as a small effective radiation area and insufficient irradiation uniformity when applied to multi-source solar simulators. To address these issues, a design scheme for a curved microlens array optical integrator is proposed to improve both the radiation uniformity and effective radiation area of multi-source solar simulators.

In this paper, based on matrix optics, we deeply analyze the beam transmission path of the optical integrator with a curved microlens array in the homogenizing process. We derive the quantitative relationship between the beam distribution on the irradiation surface and the parameters of the homogenizing system. Using diffraction theory, we construct the mathematical model of the aperture of the curved microlens array and the complex amplitude distribution on the irradiation surface. The influence of the sub-lens aperture and projection distance on irradiation uniformity is analyzed qualitatively. Based on the above theory, the optical integrator with a curved microlens array is designed and simulated. Firstly, considering the spot size of the second focal surface of the ellipsoidal condenser and the simplified integrator structure, we determine the channel number, sub-lens aperture, and relative aperture of the optical integrator. Secondly, according to the geometric light system of the field mirror microlens array and the projection mirror microlens array, the focal length and curvature radius of the front surface of each sub-lens in the field mirror microlens array are calculated by the classified design of the front surface of each sub-lens. At the same time, the feasibility of machining the optical integrator is analyzed using glass precision molding technology. Finally, the planar microlens array and curved microlens array optical integrators are modeled and imported into LightTools for Monte Carlo ray tracing. We compare the optical integrator with the traditional planar microlens array optical integrator and analyze the light smoothing performance of the curved microlens array optical integrator.

The simulation results show that, compared with the planar microlens array optical integrator, the curved microlens array optical integrator can increase the receiving area of the incident light with its curved structure, reduce the influence of stray light between adjacent channels, and thus improve the beam uniformity of the irradiation surface (Figs. 10 and 11). The total irradiation area of the spot on the irradiation surface is 172.5 mm×172.5 mm. The effective irradiation area of the optical integrator with the curved microlens array is 122.5 mm×122.5 mm, and the effective irradiation area of the optical integrator with the planar microlens array is 117.5 mm×117.5 mm. They account for 50.43% and 46.40% of the total irradiated area, respectively. The irradiance of the curved microlens array optical integrator in the effective irradiation area is 1314.47 W/m², with a minimum value of 1278.29 W/m², and the irradiation uniformity reaches 98.60%. The maximum irradiance of the planar microlens array optical integrator is 1655.10 W/m², the minimum is 1564.43 W/m², and the irradiation uniformity is 97.18% (Fig. 12). Compared with the planar microlens array optical integrator, the edge coordinate points of the curved microlens array optical integrator are shifted by 5 mm, the proportion of the effective irradiation area in the total irradiation surface is increased by 4.03 percentage points, and the irradiation uniformity is increased by 1.42 percentage points. Although the irradiance of the curved microlens array optical integrator has decreased, its performance is still close to the standard requirement of a solar constant.

We propose a design method for a surface microlens array optical integrator. When applied to a multi-source solar simulator, this optical integrator can effectively increase the beam receiving area of the optical integrator without changing the sub-lens aperture, reduce the exit beam aperture angle of the field mirror, and decrease the impact of stray light within the adjacent sub-lens apertures. Simulation results show that, while meeting the requirement for a solar constant index, the surface microlens array optical integrator performs excellently in terms of effective irradiation area and irradiation uniformity. Compared to the planar microlens array optical integrator, the effective irradiation area as a percentage of the total irradiation area has increased by 4.03 percentage points, which indicates that, within the same optical system, the surface microlens array optical integrator can cover a larger working area. At the same time, the irradiation uniformity within the effective irradiation area has also improved by 1.42 percentage points, thus achieving a leap from Class B to Class A solar simulator irradiation standards. The studied surface microlens array optical integrator provides a feasible method for designing light homogenizing devices in multi-source solar simulators. Although we have conducted a detailed study on the performance of surface microlens arrays through simulation, there are inevitably some errors, such as differences in optical characteristics between ideal light sources and actual light sources, and differences between the optical parameters of materials used in simulations and actual values due to measurement errors or environmental conditions (such as temperature and humidity), which may affect the accuracy of the simulation results to a certain extent. In the future, further optimization of the simulation model will be carried out through experimental verification to more accurately evaluate the performance of the surface microlens array optical integrator.