In remote sensing simulation environments, solar simulators serve as essential laboratory instruments to replicate natural sunlight for indoor experiments. Their critical technical metrics include spectral match, irradiance non-uniformity, and temporal instability. According to the international standard IEC 60904-9, solar simulators are classified into three performance tiers: Class A, B, and C. Specifically, a Class A designation requires a spectral match within the range of 0.75–1.25, a non-uniformity of better than ±2%, and a temporal instability of better than ±2%. Remote sensing calibration workflows typically mandate Class A or higher customized specifications to fulfill the precision calibration demands of multispectral, hyperspectral, and thermal infrared sensors. Regarding light source architecture, solar simulators primarily utilize xenon arc lamps due to their spectral distribution showing high fidelity to the natural solar spectrum, particularly in the visible and near-infrared bands. To optimize spectral match, specialized optical filter systems are integrated to regulate the relative intensity of specific wavelength regions. To ensure high spatial uniformity, optical integrators—such as fly's-eye lens arrays or integrating rods—are critical components to achieve uniform large-area illumination. Collimated solar simulators replicate the parallel beam angle of solar rays, making them highly suitable for calibrating remote sensors with stringent directional alignment requirements. The HG-Solar-SC series of customized solar simulators, developed by Hagorun Technology Limited, can be configured with varied illumination areas, spectral coverage boundaries, and collimation angles based on user technical specifications, and has been successfully deployed across numerous remote sensing research institutes. Solar simulators are classified by their effective target area into small-area (≤50 mm × 50 mm), medium-area (≤300 mm × 300 mm), and large-scale array simulators. Distinct application scenarios dictate varied demands for exposure area, spectral bandwidth, and collimation attributes. Driven by the recent evolution of remote sensors toward large apertures and multi-channel configurations, contemporary requirements for increased output clear apertures and expanded spectral boundaries have accelerated the technological development of large-scale and modular tileable solar simulator arrays.

Prior to launch, remote sensing payloads must undergo rigorous laboratory calibration procedures to define the quantitative transfer function between detector digital counts and the absolute radiance at the entrance pupil. Operating as a standard reference source for radiometric calibration, the solar simulator—when integrated with diffuse reflectance plaques or integrating spheres—delivers highly uniform illumination with known reflectance parameters to multispectral cameras, hyperspectral imagers, and polarimetric sensors. The absolute calibration pipeline encompasses radiometric cross-calibration, relative spectral response profiling, and photo-response non-uniformity (PRNU) correction. During these characterizations, standard detectors or reference radiometers traceable to national metrology standards measure the absolute radiance values of the solar simulator within specific bands, ensuring the accurate calibration of the remote sensors. For hyperspectral imagers characterized by narrow spectral bandwidths (typically 5–10 nm) and high channel counts (reaching hundreds of bands), solar simulators must exhibit a smooth, continuous spectral distribution across the corresponding operating spectrum. By pairing the simulator with monochromators or tunable lasers, step-by-step Spectral Response Function (SRF) calibration can be performed. The solar simulator generates a uniform area source covering the entire field of view (FOV) of the hyperspectral imager, enabling an effective assessment of pixel-to-pixel response consistency across diverse wavelengths. In polarimetric remote sensor calibration, the solar simulator operates alongside polarizers or Polarization State Generators (PSG) to output a standard polarized field with known degrees of polarization (DoP) and polarization angles, which is required to resolve the polarization response matrix of the instrument. Taking the HG-Solar-SC series of customized solar simulators from Hagorun Technology Limited as an example, this product line allows the customization of optical filtering systems tailored to the sensor's spectral band configuration. This optimizes spectral match characteristics across designated intervals (e.g., visible, near-infrared, and short-wave infrared), satisfying broadband calibration requirements from ultraviolet to short-wave infrared regions. In radiometric calibration experiments, its stable irradiance output helps suppress random errors across multi-measurement sequences, improving calibration accuracy and reproducibility.

Beyond instrument payload calibration, solar simulators have fundamental applications in remote sensing scene simulation and inversion algorithm verification. By constructing controllable indoor reflectance scenes—comprising standard target boards representing varied land cover classes, geometric targets, and 3D terrain models—the solar simulator supplies stable illumination fields to replicate surface reflectance characteristics under varying solar zenith angles, azimuth angles, and atmospheric conditions. Image datasets acquired by remote sensors under these controlled configurations are used to validate the mathematical validity and precision of geometric registration, radiometric calibration, atmospheric correction, and target identification algorithms. In multi-angle remote sensing simulation experiments, the solar simulator operates as a fixed or rotatable illumination vector. In tandem with a 2D goniometer stage that modifies the observation geometry between the remote sensor and the target, researchers can extract Bidirectional Reflectance Distribution Function (BRDF) data for various targets. These empirical datasets provide foundational support for modeling surface BRDF parameters and evaluating the multi-angle observation capabilities of remote sensors. Within ground-based remote sensing validation site infrastructures, the solar simulator serves as an indoor-to-outdoor transfer standard light source, linking laboratory calibration to in-orbit telemetry and minimizing calibration uncertainty propagation across the transfer chain. As the demand for quantitative remote sensing precision escalates, the fidelity requirements for physical simulation environments have increased. Future solar simulators are advancing toward tunable spectra, programmable irradiance profiles, and continuously variable illumination angles. Emerging solar simulator architectures based on integrated LED arrays and xenon lamp hybrids are poised to deliver more agile spectral tuning capabilities, satisfying the customized simulation demands of specific surface feature spectral signatures.

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