Image Sensor: CCD vs. CMOS

There are two image sensors that dominate digital photography today: CCD and CMOS.  Each image sensor has its place in the world but comes with very distinct advantages and disadvantages.

Fundamental Construction

Image sensors by definition convert electrons into voltage. However, there are different ways to get from point A to point B. A CMOS sensor has circuitry at every photo sensor. So each pixel is read simultaneously and then transmitted as digital information at once. This set up leaves the chip relatively crowded with circuitry but is extremely efficient. In a CCD imager, the pixels are recorded on the chip and then one by one sent through the analog to digital converter to build the data. This takes more energy than the CMOS process, but delivers much cleaner images.

Power

Because CMOS sensors convert visual information to digital information so efficiently, the power required to operate these devices is relatively low. In a CCD camera, the amount of work required to convert all the pixels to data sucks lots of battery power. Though this may seem trivial to most photographers, the world has many uses for low energy photography. Security cameras, cell phone cameras and most computer electronics would suffer a disadvantage if battery life were a consistent problem for the consumer. These devices are also not necessarily required to deliver pristine images, so they are designed with the cheaper option, CMOS.

Image Quality

CMOS sensors generally record less resolution than CCD cameras because they cannot physically sustain as many pixels on the plane of the chip. Each CMOS pixel is packaged with the circuitry to convert it to a digital signal, thus each sensor takes up more space. Even though a CCD sensor has to spend more time and energy converting each pixel, it can hold more across its surface.  So the noise level on a CCD sensor is inherently less than that on a CMOS sensor of the same size. This difference in layout also leads to more dynamic range in CCD sensors. The colors tend to be more saturated and vibrant in contrast to those that are captured through a CMOS sensor.

Light Sensitivity

CCD sensors tend to respond better to low light conditions than CMOS sensors. The clutter on CMOS sensors reduces the light sensitivity of the chip. It takes more light to penetrate the thick layers, so dim light will not make it through. However, the advantage is that CMOS sensors facilitate adding gain to an image. Because circuitry is so close to each pixel, the camera can boost the exposure as it is recorded. On a CCD camera, the image will need to be enhanced through the same energy consuming process as it is recorded. This in turn makes it more difficult for CCD cameras to run efficiently.

In conclusion, CCD sensors deliver higher image quality while CMOS sensors are more efficient and consumer friendly. Both systems have roles to play in photography today, though it is arguable whether they truly compete with each other.

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