Electronic Design

Choosing An Image Sensor: It's All About The Application

What kind of image sensor should you choose for your next machine-vision product—CMOS or CCD? Each has its advantages and disadvantages, depending on the application. Some companies produce both types of sensor, giving designers the flexibility they need for a particular design. 

Image sensors can be area sensors or line-scan sensors. Area sensors include a two-dimensional array of pixels that capture an image of an entire region simultaneously. Line-scan sensors use a one-dimensional linear array of pixels that build up a two-dimensional image one line at a time. Variations of the line-scan sensor include dual-line sensors (two parallel arrays of pixels) and time-delay line-integration sensors. Both CMOS and CCD image sensors can use line and area scanning.

CCDs feature pixels that produce electron packets when they’re exposed to light. These packets come in the form of an analog voltage that’s later digitized. CCDs also offer a high fill factor (the ratio of an object’s viewed area to its visible area) and low noise. They have a wide dynamic range, high uniformity, fairly good responsitivity, and moderate to high speed operation.

However, CCDs are more complex to implement in a system than CMOS imagers. They feature limited windowing capability. And, they require multiple higher voltages for operation compared to a CMOS sensor, although some progress has been made in developing sub-5-V CCD image sensors.

CMOS sensors feature pixels that produce a digital output voltage. They operate from a single sub-5-V signal and offer lower-power dissipation. Also, they provide slightly better responsivity than CCDs. They allow for extensive windowing and feature higher-speed operation.

Implementation of CMOS sensors is relatively easy. However, they don’t provide as much of a fill factor as CCDs. Noise levels are higher, and they don’t have as wide a dynamic range as CCD imagers.

CCDs are generally used in photographic, scientific, and industrial applications that require the highest image quality (in terms of quantum efficiency and noise) at the expense of system size. They also fit applications that sometimes require a tradeoff between system performance and cost. CMOS imagers find homes in applications that require small size (like mobile-phone cameras) and where system cost can be maximized with some tradeoff in performance.

Some of the latest development efforts involve producing better CMOS sensors than CCDs. As a result, certain CMOS sensor designs can be found in high-performance professional and industrial cameras, where a certain amount of noise can be tolerated.

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