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In this research characteristics of standard commercial CCD and CMOS cameras are evaluated experimentally and compared. Special attention is paid to the operation of these devices in low light level condition, which is typical to many surveillance and consumer electronics applications. One emerging application utilizing inexpensive image sensors at variable illumination condition is the UMTS (Universal Mobile Telecommunications System), which will deliver wirelessly, for example, pictures, graphics and video from the year 2002. The determination of the system performance is based in this study on the imaging of a calibrated gray scale test chart at varying illumination condition. At each level of illumination the system response is characterized by a signal to random noise figure. The signal is calculated as the difference of the system response to the lightest and darkest areas of the gray scale. The random noise is measured as the standard deviation of the gray values in a difference of two successive images of the test pattern. The standard deviation is calculated from 10-bit digitized images for small group of pixels (36 X 36) corresponding to the different areas of the gray scale in the test pattern images. If the random noise is plot as a function of signal (encoded in digital numbers, DN) for small group of pixels, a Photon Transfer curve is obtained. This is one of the basic performance standards of CCD sensors. However, if camera systems with nonlinear response or AGC are evaluated, the variations of the system response at different signal levels should be included to the performance measure. In these cases the signal to noise curve is useful. The signal to random noise curves were determined for a CCD and a CMOS camera characterized by similar specifications. The comparison between two camera systems shows that considerable differences between the operation of these devices especially at low light level condition can exist. It was found that approximately a sevenfold illumination level is needed in the case of the exemplary CMOS camera compared to the CCD camera in order to achieve a reasonable imaging performance.
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