Q: Could you explain the basics of CCD and CMOS technology?
CCD and CMOS technology are fundamentally different; their underlying architecture is different. In a CCD device, the charge is transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel’s value into a digital value. In most CMOS devices on the other hand, there are several transistors at each pixel that amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually.
Traditionally, CCD sensors create higher quality, lower noise images and provide higher sensitivity. CCDs use a more mature manufacturing process than CMOS and tend to have higher quality and more pixels. Having said that, CMOS sensors have their benefits: CMOS traditionally consumes lower-power; they’re less expensive because they’re cheaper to manufacture; and smear and bloom aren’t an issue with CMOS devices.
I think the market currently is roughly split 75% CCD vs. 25% CMOS. CCD is more common in machine vision simply due to its high imaging quality and low
Q: What are the latest developments with CCD and CMOS?
Both sensor technologies have evolved in the last years. On the CCD side, some of these developments include multi-tap output, improved imaging performance and higher sensitivity. For CMOS technology, one key development is the availability of more global shutter CMOS sensors on the market. A traditional camera has a mechanical shutter, which lets the light in and then closes the shutter to stop light from coming in. With CCD and CMOS technology, there’s a concept of electronic shutter. One fundamental difference is that CCDs use a global shutter mechanism, which means that all the pixels on the CCD expose light at the same time, and then all the pixels stop exposing light at the same time. Traditional CMOS uses a rolling shutter. This means that pixels are exposed to light at different times, on a line by line basis. Rolling shutter creates a distortion effect when imaging fast-moving objects. This happens because as you’re exposing each sensor row to light, the object being imaged gets captured at different stages of its movement for each row. So you get distortion artifacts. That has been an issue with using rolling shutter CMOS in machine vision cameras. However in the last five years we’ve seen an increase in global shutter CMOS sensors. Just recently Sony released their version, the IMX174 sensor, and it is featured in Point Grey’s Grasshopper3 camera. This has the ability of pixels to be exposed in a similar way to CCD. Why CMOS is starting to become more common and gaining market share is that now you’ve got some of these benefits of CMOS mentioned earlier plus the global shutter functionality that you get with CCD that’s really required for machine vision applications.
Q: Could you discuss the applications for both CCD and CMOS?
There are so many applications where both types of technology are important. I think in general where we see demand for new CCD technology is on the life science side as well as high-end inspection type of applications. Applications where high image quality is required, like microscopy applications, and where longer exposure times are important, e.g. low dark current that CCDs offer.
With the global shutter CMOS, there’s a broad spectrum of applications. Everything from traditional factory line automation inspection-type tasks all the way through to traffic applications. We’re also seeing good interest in a lot of 3-D scanning applications. They like CMOS because it’s lower power and sometimes lower cost. Rolling shutter is more difficult to work with for 3-D scanning—it’s not impossible but it’s very difficult—so having global shutter CMOS is quite valuable for any sort of 3-D scanning applications.
- According to estimates, the market currently is divided into 75% CCD vs. 25% CMOS.
- CCD is more common in machine vision due to its high imaging quality and low read noise.
- However, both technologies have continued to evolve over the past few years.