Comparative study of aperture-induced aberrations in reflective and transmissive optical spectrometers using ZEMAX

This study presents a design-level comparison of aperture-induced imaging degradation in reflective and transmissive optical spectrometers using ZEMAX OpticStudio. Two representative grating-based spectrometer configurations were modeled and evaluated under identical input-aperture, wavelength, detector, and grating-order conditions while varying the entrance pinhole diameter from 5 to 100 µm. The optical response was assessed using complementary image-quality metrics, including geometric, Huygens, and diffraction modulation transfer functions, line spread function, edge spread function, FFT-derived spread functions, and detector-plane irradiance. The results show that increasing the input pinhole size systematically reduces modulation transfer, broadens the line and edge responses, and redistributes optical energy over a larger detector region, indicating a transition from an optics-limited response at small apertures to a source-size-limited response at larger apertures. Within the specific optical prescriptions considered here, the reflective configuration preserved modulation, edge sharpness, and detector-plane energy confinement more effectively than the simple singlet-lens transmissive configuration. The study establishes a direct design connection between entrance-aperture size, spatial-frequency response, spread-function broadening, and detector-plane energy confinement, providing practical guidance for compact spectrometer designs where resolution, throughput, and aberration tolerance must be balanced.