In this study, to help basic researches on nondestructive measurement of synthetic and natural polymer materials consisting mainly of carbon, hydrogen, and oxygen, I confirmed the linearity between density and brightness values of carbon materials as reference materials for dual-energy X-ray absorption spectroscopy, and then determined the equation to derive the carbon equivalent density from the brightness values of various materials. From the X-ray beam dependence of the carbon equivalent density of various materials, I examined whether it is possible to determine the mass ratio of oxygen in the materials. It was shown that the mass ratio of oxygen in various plastic materials has a linear relationship with the carbon equivalent density ratio and that this principle can be applied to the determination of the mass ratio of oxygen in various plastic materials, but it was found that it is difficult to determine the mass ratio of oxygen in wood in the same way. The reason for this was thought to be a problem with the assumption that wood is composed of three elements: carbon, hydrogen, and oxygen.

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