This paper presents an optical system for taking shadow photographs of bubbles in a high-pressure evaporator tube. With this system and interpretation principles, not only the size and velocity of a bubble can be calculated from its shadow images on photos, but also its longitudinal position on the optical axis may be estimated from the size of a "bright spot" in its shadow image. A simple relation based on geometrical optics is applied. Performance of the new system and validity of the interpretation principles were examined through three series of calibration tests. Relative deviations of estimations to actual values of sizes and longitudinal positions of bubbles are within 1 and 5% respectively. Application at high-pressure was successful. Due to the general nature of the system and the interpretation principles, results may be extended to more dispersed two-phase flow situations.