This study estimates the phase state of water using an evaluation index, which is based on the absorbed light intensity ratio at two wavelengths. Experimental results showed that the evaluation index is a linear function of the ratio of ice. The results indicate that the index could be used to quantitatively evaluate the degree of water freezing. In this study, we apply the detection method to the two-dimensional distribution of the phase state of water using a near-infrared camera with new image-capturing optics, where two wavelength images of the same scene can be independently formed on one image element. For each pixel of the image, a value of the index is calculated from the output values of corresponding pixels of two wavelength images. Thus, a two-dimensional distribution of index values is obtained. The method is proven by experiment to be effective for two-dimensionally monitoring the water-freezing process with supercooling.
