A three-colour quantitative schlieren system

A colour schlieren system was designed to make instantaneous measurements of the changing methane distribution in a stratified methane-air mixture in a 10 cm wide plastic-walled container. The usual light source was replaced by a grid made of strips of coloured gelatine and the returning light beam was interrupted by a monochrome grid slightly displaced from the focus of the colour grid. The monochrome grid selected the colour to be transmitted, and an image composed of horizontal coloured bands was formed. Displacement of the bands indicated a change in refractive index gradient in the methane-air mixture. Methods of calibration by both calculation and experiment are described.