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The paper describes efforts to establish traceable measurements of radiance temperature on laser-induced heated metal surfaces on the NIST Additive Manufacturing Metrology Testbed (AMMT). Knowledge of radiance temperature with a well understood uncertainty budget is a necessary initial step towards an ultimate project goal of traceable emittance and true surface temperature across the heat affected zone, which is a key objective in additive manufacturing research, and the subject of another paper at this conference. Reliable measurements of radiance temperature with an imaging system require (1) calibration of its responsivity at select radiance levels, (2) establishing a calibration equation that interpolates between these levels, (3) dealing with finite spectral bandpass and spatial non-uniformity of the sensor responsivity, and (4) ability for compensate effects of imperfect optical imaging and readout electronics on spatial distribution of the target. The developed system includes an integrating sphere-based calibration source, a pyrometer for its calibration against external blackbody, and an imaging system co-axially aligned with the heating laser, each of which using identical narrow band filters. This paper describes the evaluation of an 850 nm band, with additional wavebands planned for the future. This paper presents experimental results, description of measurement equation and processing algorithm, as well as a framework for establishing an uncertainty budget, including current estimates and future performance goals. |
