Study on methods for deriving spatial distribution of electron temperature and density from RGB (red–green–blue) digital image in non-thermal equilibrium plasma

A method used to derive the spatial distributions of electron temperature and density in non-thermal equilibrium plasma, respectively, from the relative and absolute intensities of RGB (red–green–blue) JPEG (Joint Photographic Experts Group) images acquired by using a digital camera is investigated. Since the RGB values of the JPEG image obtained from the complementary metal–oxide–semiconductor sensor of commercial digital cameras are automatically corrected, the RGB values are calibrated by simultaneously observing lamps with known RGB values using the camera and a spectrometer calibrated for absolute sensitivity as well as determining the correlation between the RGB values and emissivity. The validity of the calibrated RGB values is confirmed by observing the blackbody radiation of a candle flame using the camera and the spectrometer. In the case of optically thin plasmas, excitation radiation with a stronger intensity than bremsstrahlung radiation is observed. Since this strong excitation radiation is reflected in the RGB values, the electron temperature is derived from the relative RGB values by relating the excitation temperature obtained by using the line-pair method to the electron temperature obtained from continuum spectral analysis using bremsstrahlung radiation. The average grayscale calculated from the RGB values reflects the density distribution of the object’s presence. The spatial distributions of electron temperature and density are obtained from the digital image using RGB relative values and the average grayscale, respectively.