A visibility transmissometer based on light intensity monitoring

Transmissometers provide high-accuracy visibility measurements, but existing systems often suffer from limited range and significant uncertainty. We developed a new transmissometer using a white light emitting diode that produces a 0.6-mrad parallel beam, with a 110 mm aperture lens ensuring full-energy collection. To minimize errors from light-source fluctuations, we introduced a sampling method based on the ratio of peripheral spot intensity to central beam intensity. In a 300-h stability test, a 0.28% change in source output resulted in only ∼0.014% variation in this ratio, effectively suppressing source-induced error. Calibration with neutral-density filters produced a calibration error of ∼0.016%, and multi-filter tests confirmed highly linear photoelectric conversion. By extending the optical path to 60 m using high-reflectance mirrors, the system achieved a maximum measurable visibility of 50 km with a relative standard uncertainty of 2.87%. Comparative tests in a visibility simulation chamber showed that the new transmissometer exhibits significantly lower uncertainty than three reference forward scattering meters. These results demonstrate that the proposed system offers a precise and reliable solution for high-accuracy visibility monitoring.