CCD versus CMOS: blooming and smear performance

For many years CCD image sensors and cameras have offered advantages in image quality such as greater dynamic range, better noise performance, and better uniformity. For this reason, CCD cameras were still used in many of the most demanding machine vision systems needing the highest accuracy or in global security systems where sensitivity in low light is critical.

Now CMOS image sensors and cameras have caught up or even surpassed performance of CCD image sensors such as in sensitivity and dynamic range.  This means the change over from CCD to CMOS is happening now in many markets.  With this, we are reevaluating the strengths and weaknesses of CCD and CMOS image sensors.

One of the areas where CMOS has a big advantage is in blooming and smear performance.  Blooming is an effect where the charge developed on a pixel leaks into adjacent pixels and corrupts the scene.  It typically occurs when there are very bright spots in the scene, and it diminishes the accuracy of the pixel data as information from one pixel is then present in adjacent pixels.

Smear can be generated directly or indirectly in the vertical shift registers (VCCD) of an interline transfer CCD (IL-CCD).   The VCCD is a light-shielded area of the image sensor used to transfer the charge off of the sensor.  Smear typically results from very bright spots in an image and is caused by either:

1. Stray electrons generated under the photodiode area (the light sensitive area) and diffused into the vertical shift registers
2. Stray photons which arrive in the vertical shift registers or
3. Scattered photons, which arrive in the vertical shift registers by multiple internal reflections.

This has the effect of a bright column of pixels extending above and below the offending pixel and results in vertical streaks in the image.  In modern IL-CCD, the smear performance is mostly set by the effectiveness of the VCCD light shield.  As a note, the light attenuation of the light shield is a function of wavelength.  For long wavelengths (say > 700nm) the smear performance is much worse than specified. Because CMOS sensors have no shift registers, as the readout technology is completely different, there are no smear artifacts. For CCD sensors, even with best-in-class blooming and smear suppression, specific camera functionality is required to reduce the effect in the image data.

Blooming and smear are especially challenges with outdoor vision of global security systems, where bright spots in a scene can originate from lights, reflections, and the sun, among others.