Video imaging techniques were used to measure concentration fluctuations in the wake of a stack using a highspeed video camera. From the experiments, the following results were obtained. The image signals had linear relationships with gas concentrations for mean values in the gray range > 1.5. The turbulence intensity of image signals also had good relationships with that of gas concentrations in the range of I^^- > 0.04. The video image spectra were found to coincide well with those measured by a high-response hydrocarbon analyzer if the noise of the image signals were removed from the images using the numerical lowpass filter. Under this camera condition, the zone capable to take video images was limited in the region of I^^- > 0.04.
