Metrological framework and modulation-frequency optimization for robust near-field electromagnetic characterization via pulsed light fluorescence thermography

Non-invasive and accurate mapping of near-field electromagnetic (EM) distributions radiated by antennas is increasingly critical not only for communication system reliability but also for diagnosing latent defects in advanced radiofrequency (RF) components. Fluorescence thermography, based on the photothermal response of Rhodamine B films, enables high-resolution, contactless visualization of EM fields, preserving the structural and electromagnetic integrity of the radiating system. This study investigates the role of LED modulation frequency—a pivotal parameter for signal demodulation—in enhancing signal-to-noise ratio (SNR), suppressing environmental interferences, and minimizing hybrid frame contamination. By experimentally evaluating LED modulation between 0.1 Hz and 1.5 Hz, as well as continuous illumination conditions, we demonstrate that the optimal operating range lies between 0.1 and 1.0 Hz, where SNR values consistently exceed 25 dB, and hybrid frames—images partially recorded during LED state transitions—remain below 6%. These results were obtained under controlled and perturbed illumination scenarios, including the addition of a 60 W halogen lamp to simulate ambient lighting, confirming that modulated excitation provides robust demodulation against parasitic light. In contrast, continuous or high-frequency modulation above 1.0 Hz leads to degraded image quality, with SNR dropping to ∼ 15 dB and hybrid frame rates exceeding 9%, thus compromising the fidelity of spatial field features. The LED current was varied from 0.25 A to 4.0 A, with no significant deviation from linear photothermal response or image quality, as confirmed by calibration curves and emission stability measurements. The photophysical compatibility between the green LED spectrum and the absorption peak of Rhodamine B, filtered through a high-pass optical system, ensures optimal excitation/emission separation. This study builds upon past prior studies by delivering the first quantitative link between modulation parameters and image reconstruction fidelity, establishing a practical modulation framework for fluorescence thermography that is, in principle, independent of the RF carrier frequency. In this work, the methodology is experimentally validated in the GHz range on a 1.75 GHz ultra-wideband antenna, extending previous microwave and millimeter-wave demonstrations of Rhodamine-B based thermofluorescence. Fluorescence thermography thus emerges as a precise, scalable, and environmentally robust method for near-field EM characterization, suitable for integration in advanced research and prospective industrial diagnostics. Beyond reporting performance, we explicitly position pulsed-light fluorescence thermography (PLFT) as a measurement methodology: we provide (i) a traceable, operational SNR definition with confidence intervals; (ii) a closed-form model for hybrid-frame probability as a function of exposure and modulation that yields a design rule for repeatability; (iii) a thermal-diffusion analysis that quantifies the fundamental SNR–resolution trade-off; and (iv) a GUM-style uncertainty budget propagating along E → P a b s → Δ T → Δ F . These elements formalize repeatability, robustness, and reproducibility as primary outcomes of frequency selection rather than as incidental side effects of imaging. • Fluorescence thermography maps EM near-fields using modulated optical excitation. • Optimal LED modulation (0.1–1.0 Hz) enhances SNR and suppresses hybrid frames. • Robust field imaging maintained under ambient light perturbations. • LED current varied from 0.25–4.0 A without loss of linearity or signal fidelity. • Comparative evaluation shows non-invasive, fast, and scalable field mapping.