Thermally driven indoor air turbulence in a large-scale optical metrology environment: quantitative characterization...

Turbulence-induced refractive-index fluctuations limit large-aperture optical metrology, yet their response to routine air-handling unit (AHU) operation remains insufficiently quantified. We characterized thermally driven indoor turbulence in a large-scale optical metrology facility using the INdoor TurbulENce SEnsors (INTENSE) method over a 1.5 m path selected to match the present meter-class mirror testing configuration. Six-hour measurements were performed while the AHU maintained 21.5-22.9 °C and generated quasi-periodic cooling and recovery cycles. Local extrema of the temperature time series identified the cooling and recovery phases, and the Fried parameter (r₀) response classified three optical turbulence regimes using r₀ boundary values of 0.6 m and 0.9 m. At 635 nm, r₀ ranged from 0.13 m to 1.83 m, with a mean of 1.09 m. During the active-cooling regime, r₀ decreased, and path-averaged equivalent C_n² increased, indicating enhanced refractive-index fluctuations. Using the regime boundaries, equivalent optical seeing in the active-cooling regime was approximately 50% degraded relative to the steady regime. The temperature change rate (dT/dt) correlated strongly with r₀ and C_n², showing that turbulence strength was linked to cooling transients. Temporal PSD analysis showed Kolmogorov-like behavior mainly in the active-cooling regime. These results provide a quantitative basis for selecting optically stable measurement periods.