Key differences between CCD and CMOS imaging sensors

Once considered the gold-standard for performance in machine vision, CCD sensors (Charge Coupled Device) are being discontinued in favor of modern CMOS imaging sensors (Complementary Metal-Oxide Semiconductor) in many applications. Why is this, and how do you know which sensor type is right for your project?

Here’s a simplified explanation of how the technologies work: 

Both CCD and CMOS image sensors convert light into electrons by capturing light photons with thousands—or millions—of light-capturing wells called photosites. When an image is being taken, the photosites are uncovered to collect photons and store them as an electrical signal.

The next step is to quantify the accumulated charge of each photosite in the image. Here’s where the technologies start to differ: in a CCD device, the charge is transported across the chip and read at one corner of the array, and an analog-to-digital converter turns each photosite’s charge into a digital value. 

In most CMOS devices, on the other hand, there are several transistors at each photosite that amplify and move the charge using more traditional wires. This makes the sensor more flexible for different applications, because each photosite can be read individually.

A special manufacturing process gives CCD devices the ability to transport charges across the chip without distortion, leading to high-quality, highly sensitive sensors. CMOS chips use more conventional (and cheaper) manufacturing processes.

This all adds up to several main differences between CMOS and CCD sensors: 

• CCD sensors create high-quality, low-noise images. CMOS sensors are usually more susceptible to noise.
• Because each photosite on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip tends to be lower, as many of the photons hit the transistors instead of the photosite.
• CCD sensors consume as much as 100 times more power than an equivalent CMOS sensor.
• CMOS sensors can be manufactured on most standard silicon production lines, so are inexpensive to produce compared to CCD sensors.

Overall, CMOS sensors are much less expensive to manufacture than CCD sensors and are rapidly improving in performance, but CCD sensors may still be required for some demanding applications.