This paper describes a calibrated broadband emitter for the millimeter-wave through terahertz frequency regime, called the aqueous blackbody calibration source. Due to its extremely high absorption, liquid water is chosen as the emitter on the basis of reciprocity. The water is constrained to a specific shape (an optical trap geometry) in an expanded polystyrene (EPS) container and maintained at a selected, uniform temperature. Uncertainty in the selected radiometric temperature due to the undesirable reflectance present at a water interface is minimized by the trap geometry, ensuring that radiation incident on the entrance aperture encounters a pair of s and a pair of p reflections at $45°$ . For water reflectance $R_w$ of 40% at $45°$ in the the W-band, this implies a theoretical effective aperture emissivity of $(1-R_{ws}^2{R}_{wp}^2)>98.8\%$ . From the the W-band to $450\mathrm{GHz}$ , the maximum radiometric temperature uncertainty is $\pm 0.40\mathrm{K}$ , independent of water temperature. Uncertainty from $450\mathrm{GHz}$ to $1\mathrm{THz}$ is increased due to EPS scattering and absorption, resulting in a maximum uncertainty of $-3\mathrm{K}$ at $1\mathrm{THz}$ .

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