A log-domain method for decoupling environmental interference from hardware response in multi-filter calibration of forward-scatter visibility sensors

Abstract. Accurate calibration of forward-scatter visibility sensors remains challenging because their response depends on instrument geometry and scattering conditions, while field calibration is additionally perturbed by background atmospheric extinction. Existing indoor multi-filter protocols can characterize sensor response under controlled conditions, but their transfer to outdoor operation is hindered by environmental contamination during reference measurements. Here, we propose a log-domain calibration framework that separates a hardware-dependent calibration factor, f_cal  , from an environment-dependent interference term, f_env , by comparing paired darkroom and outdoor regressions of measured versus reference meteorological optical range (MOR). In this formulation, the darkroom intercept represents the hardware baseline, whereas the outdoor intercept contains both hardware and environmental contributions. The framework was evaluated using two forward-scatter sensor families, the Vaisala PWD22 and Biral SWS-200, with neutral-density filter sets and an LT31 transmissometer used to define model-specific reference meteorological optical range (MOR) values and monitor outdoor background visibility.

The PWD22 exhibited near-unit darkroom slope and stable intercept transfer, allowing modest  f_env  contributions to be resolved under relatively clear outdoor conditions, whereas the SWS-200 was more sensitive to slope instability and filter-seating effects. In both cases, calibration improved bias more than scatter, so the framework is best viewed as a practical field-transfer method for suitable forward-scatter sensors that does not replace transmissometer-based traceability and remains conditional on sufficiently high background visibility and stable sensor slope.