This report describes the possibility of quantitative analysis of Schlieren photographs as an internal combustion engine diagnostic.Using a recently developed photographic analysis system, it was attempted to analyze Schlieren photographs. Results showed simple integration calculations produced significant distortion in the analyzed results. To eliminate the distortion, some correction techniques were developed in this research. Accuracy of the analyzed results were evaluated roughly with uncertainty analysis. The results showed that this analysis technique can be one of the approximate diagnostics for the measurement of fuel vapor and density distribution in internal combustion engine research.SCHLIEREN photography is one of the most popular visualization techniques in the research of internal combustion engines. Although the photographs have density information in themselves, they are used mainly for the purpose of qualitative visualization. The objective of this research is to develop a quantitative analysis of Schlieren photographs for internal combustion engine research.One important parameter in engine research, for example, is the air entrainment in fuel jet vapor. Its measurement has previously been possible by gas sampling, laser holography, or laser Raman spectroscopy. The gas sampling method provides only point data, and the laser methods are very sensitive to vibration or optical settings. With this present Schlieren photographic analysis technique, it will be possible to measure instantaneously the whole space of the density field with a very simple optical system which is commonly used in engine research. Lakshminarayan and Dent(1)* analyzed interferometric photographs of jet sprays impinging on a wall. A major part of their (analysis method involved the counting of fringes which are sometimes difficult to distinguish in photographs. Nishida et al.(2)(3) analyzed Schlieren photographs reconstructed from laser holograms of jet vapor. However, they did not refer to the accuracy of the results.This report describes the background of the quantitative analysis and shows some analyzed results. The analysis was made for a CO2 jet injected into air. The Schlieren photographs were analyzed by an image processor and a personal computer.The results showed that the elimination of integrated error due to non-uniform background level was one of the most important parameters to be considered. In this study, a correction method is shown to determine the non-uniform background level and also to improve the accuracy of the results. This report also describes analyzed results for transient free jets and wall-impinging jets, which were compared with experimental results.