As the critical dimensions of integrated circuits continue to grow smaller, overlay error is becoming increasingly important. Overlay error is mainly determined by the telecentricity of the exposure system in a photolithography tool. Existing telecentricity-measuring methods—which are photoresist-based—are complex and cannot obtain the telecentricity in real time. Moreover, the obtained result is influenced by the photoresist performance. We propose a light-cone-central-line method that measures telecentricity without the need of photoresist. Compared with photoresist-based methods, the light-cone-central-line method can be used in real time, accelerating the setup and tuning of the exposure system and thus significantly reducing the complexities and expenses of the existing measurement methods. The proposed method was applied in a lithography tool with a 90-nm resolution, and the results for five field positions (the center position and four corner-field positions) are presented. This method is found to provide sufficient measurement repeatability, and it satisfies the telecentricity measurement requirements of the tool.