Dual UV narrowband filter-based differential detection for long-range extraction of fuel combustion signals

The hydroxyl (OH⁺) radical, with its distinctive 308 nm emission, provides a unique fingerprint for hydrocarbon combustion monitoring. Exploiting the low ultraviolet sky background, OH⁺-based detection achieves high accuracy without cooling but suffers from narrowband-filter blue shifts caused by light-convergence angles, degrading uniformity and reliability. We propose a differential correction method using a coordinated filter pair-308 nm target and 307 nm reference-engineered to share identical blue shifts. Images from both channels are acquired, and the background (I_off) is subtracted from the target (I_on). Because broadband background signals (mainly sky scattering) remain nearly equal after blue shift, subtraction cancels them while fully retaining the narrowband OH⁺ emission present only in the 308 nm channel. Experiments show that this approach cleanly extracts a 1 nm bandwidth 308 nm signal under ideal conditions and markedly improves the signal-to-noise ratio (SNR). The resulting dual-channel synchronized imaging and differential system provides a reliable, engineering-ready solution to mitigate filter blue shift and enhance UV spectral detection accuracy in dynamic scenarios.