Abstract: In view of the imperfections in equation for the threshold of detection sensitivity of image tubes used in optoelectronic imaging devices. The paper endeavors in the derivation of analytic expressions for the transfer function of signal to noise ratio of image tubes working under low light levels, theoretical calculation and practical tests have been done for an image tube. The calculation coincides basically with test results.The calculated curve of the threshold of detection sensitivity indicates that the threshold curve corresponds well with practical curve given by predecessors. Thus signal-to-noise ratio transfer function proves itself suitable in the low light level working area.