Objective With the expanding of imaging distances in mobile phone camera modules, traditional Modulation Transfer Function (MTF) testing methods involving frequent repositioning of the chart have become inefficient and error-prone. To address the need for high-precision MTF measurement, this paper presents the optical design of a variable virtual object distance collimator. By adjusting the position of the chart within the collimator’s object focal plane, the system enables continuous simulation of object distances ranging from 150 mm to infinity. The collimator features a 16 mm exit pupil diameter and a 75 mm working distance, meeting the requirements of most imaging test scenarios. The system was optimized using ZEMAX across multiple wavelengths and object distances, achieving diffraction-limited performance throughout. A Monte Carlo tolerance analysis was conducted to verify manufacturability. Following fabrication, the system was evaluated using the Trioptics ImageMaster Universal MTF measurement instrument and a resolution chart. Experimental results confirm that the system maintains excellent imaging quality across all virtual object distances. This work provides a practical solution for MTF measurement under variable object distances and offers a reference design for future optical metrology applications.
Methods This paper proposes a method for simulating object distances through a variable virtual object distance collimator to enable imaging performance testing at different object distances. In this approach, a resolution chart serves as the simulated object and is driven along the optical axis by a motorized stage, allowing precise adjustment of the distance between the chart and the collimating lens group. This variation directly controls the location of the virtual image formed by the collimator. Based on a Gaussian optical system model, the quantitative relationship between the object distance and image distance is derived, establishing a mapping between the position of the chart and the corresponding virtual image. This enables the system to simulate object distances ranging from infinity to short distances, overcoming the limitations of traditional systems with fixed object distances and significantly improving testing efficiency and consistency.
Results and Discussions The optical system was optimized using the ZEMAX optical design software. The wavelength range was set within the visible spectrum (0.486-0.656 μm). The optimization process was considered complete when the MTF met the design criteria across all predefined virtual object distances. The collimator system comprises seven lens elements, all fabricated from optical glasses sourced from CDGM (Fig.4). All lens surfaces are standard spherical. The designed system achieves a focal length of 144 mm and a working distance of 75 mm. The MTF curves demonstrate that under various simulated object distances, the system exhibits stable performance across both on-axis and off-axis field positions, with values consistently approaching the diffraction limit (Fig.5). The RMS wavefront error distributions under different simulation conditions further indicate that the wavefront quality is well-controlled (Fig.6). To verify the consistency between object and image positions, a set of data points was selected in ZEMAX and plotted alongside the theoretical curve derived from Gaussian optics. The results show that all measured points align closely with the theoretical curve, confirming the high degree of agreement between the actual optical design and theoretical predictions (Fig.8). Monte Carlo tolerance analysis reveals that the system possesses excellent manufacturability (Fig.9). After fabrication, the lens was evaluated using an MTF measurement system and resolution target imaging. Experimental results show that the system maintains high spatial resolution performance post-manufacturing, validating the effectiveness of the proposed optical design.
Conclusions In response to the practical demands of high-precision MTF measurements, this study presents the design and implementation of a collimator system capable of simulating variable virtual object distances. The proposed system enables continuous adjustment of simulated virtual object distances ranging from 150 mm to infinity, with an exit pupil diameter of 16 mm and a working distance of 75 mm, thereby meeting the requirements of most optical testing scenarios. Simulation and optimization results indicate that the system maintains diffraction-limited image quality across multiple wavelengths and object distances, demonstrating excellent optical performance. Tolerance analysis confirms that the system possesses excellent manufacturability. Experimental validation, including MTF measurements and resolution target imaging, reveals that the assembled and aligned system retains high imaging quality, in agreement with the design expectations. These results support the proposed collimator as a practical and effective solution for variable virtual object distance simulation in MTF testing applications. Future work will focus on further optimizing the optical configuration, expanding the range of simulated virtual object distances, and increasing the exit pupil diameter to accommodate a broader set of testing scenarios.
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