Image sensor
Sensor that converts light into electrical signals for imaging.
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An image sensor, also called an imager, is a device that captures visual information. It works by turning changes in light—as light passes through or bounces off objects—into electrical signals, which are small bursts of current.
These signals can come from visible light or other types of electromagnetic radiation. Image sensors appear in both analog and digital electronic imaging devices, such as digital cameras, camera modules, camera phones, optical mice, medical imaging gear, night vision equipment like thermal imagers, radar, sonar, and more. Over time, electronic and digital imaging has increasingly replaced chemical and analog methods.
The two main kinds of electronic image sensors are the charge-coupled device (CCD) and the active-pixel sensor (CMOS sensor). Both rely on metal–oxide–semiconductor (MOS) technology: CCDs use MOS capacitors, while CMOS sensors use MOSFET amplifiers.
Analog sensors for invisible radiation often involve vacuum tubes, whereas digital sensors include flat-panel detectors. Usually, an image sensor needs a lens to project an image onto it, like in a camera, though lensless sensors have also been developed. High-power lasers can damage the pixels in image sensors.
Comparison between CCD and CMOS sensors
CCD and CMOS sensors both capture light and turn it into electrical signals, but they differ in design and use. CCDs are analog devices built from pinned photodiodes. When light hits the chip, each photodiode holds a small electrical charge.
The charges in the row of pixels nearest the output amplifiers get amplified and sent out; then each row shifts its charge one step closer to the amplifiers, and the process repeats until all rows are read. CMOS sensors, on the other hand, have an amplifier for every pixel, unlike CCDs which have only a few amplifiers. This means less area for capturing photons, but microlenses placed in front of each photodiode focus light that would otherwise hit the amplifier and be lost. Behind each microlens sits a color filter made from colored photoresist.
Some CMOS sensors also use back-side illumination to boost photon capture. CMOS sensors can potentially use fewer components, consume less power, and offer faster readout than CCDs. They are also more resistant to static electricity.
Quick Facts
- Main types
- charge-coupled device (CCD) and active-pixel sensor (CMOS sensor)
- Technology base
- metal–oxide–semiconductor (MOS) technology
- Ccd building block
- MOS capacitors
- Cmos building block
- MOSFET amplifiers
- Common color filter pattern
- Bayer pattern (checkerboard arrangement of two green pixels for each red and blue pixel)
Facts from the source article.
Lore & Background
The two main types of electronic image sensors are the charge-coupled device (CCD) and the active-pixel sensor (CMOS sensor). Both are based on metal–oxide–semiconductor (MOS) technology, with CCDs based on MOS capacitors and CMOS sensors based on MOSFET amplifiers. Each cell of a CCD image sensor is an analog device, a pinned photodiode.
When light strikes the chip it is held as a small electrical charge in each photodiode. The charges in the line of pixels nearest to the output amplifiers are amplified and output, then each line of pixels shifts its charges one line closer to the amplifiers, filling the empty line closest to the amplifiers. This process is repeated until all lines of pixels have had their charge amplified and output.
A CMOS image sensor has an amplifier for each pixel compared to the few amplifiers of a CCD. This results in less area for the capture of photons than a CCD, but this problem has been overcome by using microlenses in front of each photodiode, which focus light into the photodiode that would have otherwise hit the amplifier. Behind each microlens sits a color filter made of special colored photoresist. Some CMOS imaging sensors also use back-side illumination to increase the number of photons that hit the photodiode.
CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors. They are also less vulnerable to static electricity discharges. Another design, a hybrid CCD/CMOS architecture (sold under the name 'sCMOS'), consists of CMOS readout integrated circuits bump bonded to a CCD imaging substrate.
Reader's Guide
Image sensors are fundamental to modern electronic imaging, having largely replaced chemical and analog imaging in many applications. The two dominant types, CCD and CMOS, both accomplish the task of capturing light and converting it into electrical signals, but they differ in architecture and typical use. CCD sensors are used for high-end broadcast quality video cameras, while CMOS sensors dominate in still photography and consumer goods where overall cost is a major concern. Cameras integrated in small consumer products generally use CMOS sensors, which are usually cheaper and have lower power consumption in battery-powered devices than CCDs.
Performance parameters such as dynamic range, signal-to-noise ratio, and low-light sensitivity improve as sensor size increases, because more photons hit the pixel with larger area in a given exposure time. Color separation is achieved through various mechanisms: integral color sensors with a color filter array (most commonly the Bayer pattern), the Foveon X3 sensor using layered pixel sensors, or 3CCD systems using a dichroic prism. Specialty sensors serve applications including multi-spectral imaging, x-ray detection, thermography, and astronomy. Lensless sensors substitute alternative optical components such as coded apertures or diffraction gratings to form images, potentially reducing manufacturing cost and allowing thin optical paths.
Frequently Asked Questions
What exactly is an image sensor?
An image sensor (sometimes called an imager) is the core component that grabs visual data by converting incoming light into tiny electrical current pulses. It can respond to visible wavelengths as well as other parts of the electromagnetic spectrum, depending on its design.
How does a sensor capture color if each pixel only sees one wavelength?
Most color sensors use a Bayer pattern—a checkerboard overlay where two green filters sit for every one red and one blue filter. The camera's processor then interpolates the missing color values from neighboring pixels to reconstruct a full-color image.
Where do image sensors show up outside of regular photography?
They power everything from optical mice and smartphone camera modules to medical tools like video laryngoscopes and flat-panel X-ray detectors. Thermal imagers with microbolometer arrays, radar, sonar, and gamma cameras are other well-known applications.
Why should a photography enthusiast care about the sensor rather than just the lens?
The sensor determines the fundamental resolution, dynamic range, and light sensitivity of every frame you capture, regardless of how premium the glass in front of it is. Understanding whether you're looking at a CCD or CMOS design, and its pixel architecture, helps explain why two cameras with similar megapixel counts can produce very different images.
More in Camera Image Sensors
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Image sensor (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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