Chromatic modulation as an on-line plasma monitoring technique

Abstract

A novel fibre-optic sensor has been designed to function as a real-time plasma diagnostic tool. Plasma composition and discharge power have been monitored by recording the chromaticity of the glow discharge. The chromatic sensor operates in a manner analogous to the human eye and consists of three photodiodes with overlapping spectral responses. When used in conjunction, the three photodiodes are capable of measuring the hue and saturation of a plasma emission spectrum, in addition to the intensity measured by traditional photodiode sensors. Hue and saturation can be mapped onto a chromatic plane, each point of which corresponds to a set of spectra. Although many spectra can be mapped onto a single point on the chromatic plane, the degree of degeneracy is less than that of intensity sensors where spectra map into points restricted to a line. Chromatic sensing is shown to retain the cost and sampling rate advantages of intensity sensors whilst demonstrating an ability to detect spectral shifts, more often associated with spectrometry.