A spectroscopic observation of emitted radiation from spray droplets, to measure the radiation mostly covering the visible region but extending to both near ultraviolet and infrared, was made along the plasma/spray flow from just the downsteam of a gun exit to a position where substrates are usually placed. The results revealed that although plasma radiation from some molecular and atomic lines are superimposed on a continuum background radiation at a position close to the gun exit, these lines almost completely disappear at a distance of more than 50 mm from the gun exit. In addition, the continuum radiation is well represented by the Planck’s formula with the same fitted temperature of 2,750 ± 50 K at all positions. Finally, the wavelength dependencies of the surface emissivity of the droplets, ε(λ), lower the fitted temperatures by about 300 and 500 K, as one adopts the wavelength dependencies of ε(λ) ∝ λ−1 and λ−2, respectively.

For achieving satisfactory performance of coating layers obtained using the thermal spray technique, pre-treatments are carried out by roughening the smooth surfaces of metals to more than a few tens of μm.  Although a surface roughness tester is reliable for quantitatively measuring the surface roughness, it requires expertise and a long measuring time.  In the present article, a simple optical method is presented which the authors hope will replace the above surface roughness tester. The proposed method relies on first projecting a narrow beam of light from a light-emitting diode obliquely over the roughened surface, and then on measuring the distortion of the light as observed perpendicular to the surface.  Proof-of-principle experiments were performed first against a surface roughness comparator, and then against actual blasted surfaces using both the optical method and the surface roughness tester, yielding the two methods to be in good agreement.