The main difficulty in extracting T NP from NP emission spectra is the strong wavelength dependence of the optical system sensitivity, resulting from residual chromatic aberration, grating efficiencies, camera quantum efficiencies, etc. To obtain intensity-corrected spectra, it is necessary to calibrate the system sensitivity under conditions as close as possible to those pertaining to NP spectroscopy, i.e., using an emission source with size similar to the oscillation volume of an NP in the trap (≤ 150 μm), and with the system assembled and under vacuum, as it is during experiments. As a calibration emitter, we fabricated a type C thermocouple with a bead diameter of ~180 μm, which can be inserted into the trap center using a precision XYZ manipulator and then heated to temperatures up to 2300 K using the CO 2 laser. The laser focal diameter is ~1 mm, and a mirror is used to retroreflect the transmitted beam so that the thermocouple bead is heated from both sides to obtain a uniform temperature. The type C alloys are mostly tungsten, and its emissivity has been well studied over a broad wavelength and temperature range. Thus, the thermocouple provides an appropriately sized calibration emitter with well-known intensity vs. wavelength that can be positioned precisely in the trap center under vacuum using a pair of CCD alignment cameras that view the trap from axial and radial directions. After correction for system sensitivity, the NP emission spectra are smooth and blackbody-like, and T NP can be extracted by fitting them to a thermal emission model, as shown in Fig. . Many additional details of the calibration process, the thermal emission model, sensitivity testing, and the resulting uncertainties in T NP are discussed elsewhere. The overall uncertainty in T NP is estimated to be ~6 %. Fig. shows typical emission spectra for a single hafnium NP laser-heated and stabilized at four different temperatures. Note that as expected for thermal emission, increasing T NP resulted in spectra shifting to the blue, and apart from the 2400 K spectrum, increasing in intensity. The reason that the 2400 K intensity was not higher is that reaching this temperature required heating using a combination of a 532 nm continuous wave laser (~1 W, ~300 μm beam waist) and a duty-factor-modulated CO 2 laser (10 W maximum power, modulated at 20 kHz, ~1 mm beam waist). The CO 2 laser produces substantial radiation pressure on the NP modulated at 20 kHz, which can drive NP motion that reduces the fraction of the time the NP is in the central ~100 μm volume for which the spectroscopy collection efficiency is optimized. As noted, after passing through the trap center the CO 2 laser is retro-reflected and refocused back through the trap center, canceling the radiation pressure, but incompletely due to imperfect optics. If the CO 2 laser is not retro-reflected, it can easily drive NPs completely out of the trap. Spectral data are acquired over the entire wavelength range between 400 and 1600 nm, but the spectra are only fit (and plotted) for 600 nm < λ < 970 nm and 1100 nm < λ < 1570 nm because the raw intensities outside these ranges are too low for reliable intensity correction. The gap around 1000 nm results from low grating efficiencies for the two spectrographs, the 1570 nm cutoff reflects the sharp decrease in InGaAs quantum efficiency when cooled to -80 °C, and the short λ limit results from a 550 nm long-pass filter used to block scattered 532 nm photons.