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:
- Stray electrons generated under the photodiode area (the light sensitive area) and diffused into the vertical shift registers
- Stray photons which arrive in the vertical shift registers or
- 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.
The images below show examples of smear and blooming. Both image sets are stills from real time video at Full HD and 60 frames per second and are color-processed. The CCD camera uses a top quality image sensor. The scene is just something our engineers setup with “hard to photograph” package materials to show off what can be done.
In the first set of images, both cameras have blooming from the very bright strobe light that was used. Only smear is shown as pink streaks above and below the bright spot in the image from the CCD camera. In the second set of 4 images, the CCD camera has a more severe smear artifact in the second image. This is an example of the light coming through the VCCD as the strobe light is flashed while the previous image is in the VCCD. It shows evidence of the strobe light in the image even though the light was off when the image was captured.
Typical smear problem of CCD (although best in class -120 dB), not visible in CMOS (no vertical charge transport)
Smear problem results in erroneous data in the second image of CCD camera