When self-scanned photodiode array image sensors are applied in object measurements, the outer sizes of the object are included in the video signal of image sensors. The identification and measurement of the object ,and the enhancement of the performance is depended on the symbol extraction of the video output signals. Based on the working principles of self-scanned photodiode array image sensors, the mathematical models of image sensor's video signals are established when the moving object is captured. The comparative method and the measurement error are researched. The results of this paper provide a theoretic basis for the research of video signals processing.