Exploring Camera Sensor Types – Sensors can be classified in several ways, such as structure type (CCD or CMOS), color type (color or monochrome), or shutter type (global or rolling shutter). They can also be classified by resolution, frame rate, pixel size and sensor format. Understanding these terms can help better understand which sensor is best for their application.
However they are classified, the purpose of image sensors is the same; to convert incoming light (photons) into an electrical signal that can be seen, analyzed or stored. Image sensors are semiconductor devices and serve as one of the most important components inside a machine vision camera. New types of sensors are produced every year with improvements in sensor size, resolution, speed and light sensitivity. In this article, we look at some of the basics of the image sensor technology found inside machine vision cameras and how they relate to their classifications.
Exploring Camera Sensor Types
Below is a typical CMOS image sensor. The sensor chip is kept in a package with a protective glass. The package has contact pads that connect the sensor to the PCB.
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Different sensors come in different packages. For example, the photo above is a sensor with a ceramic PGA package.
The solid-state image sensor chip contains pixels consisting of light-sensitive elements, micro-lenses and micro-electrical components. The chips are manufactured by semiconductor companies and are cut from wafers. Wire connections transmit the signal from the matrix to the contact pads on the back of the sensor. The packaging protects the sensor chip and wire connections from physical and environmental damage, provides thermal dissipation, and includes interconnect electronics for signal transmission. A clear window on the front of the package called a cover glass protects the sensor chip and wires while allowing light to reach the light-sensitive area.
Sensor matrices are produced in large series on silicon wafers. The wafers are cut into many parts, each part containing one sensor die. The larger the size of the sensor matrix, the smaller the number of sensors per tile. This usually leads to higher costs. A single chip defect will be more likely to affect a larger image sensor.
The manufacturing process from the bare silicon wafer to the individual parts of the image sensor can take up to several months.
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In a camera system, the image sensor receives incident light (photons) that is focused through a lens or other optics. Depending on whether the sensor is CCD or CMOS, it will transfer the information to the next stage as either a voltage or a digital signal. CMOS sensors convert photons into electrons, then into a voltage, and then into a digital value using an on-chip analog-to-digital converter (ADC).
Depending on the camera manufacturer, the overall appearance and components used will vary. The main purpose of this arrangement is to convert light into a digital signal that can then be analyzed to trigger some future action. Consumer level cameras would have additional components for image storage (memory card), viewing (built-in LCD), and control buttons and switches that machine vision cameras do not have.
CCD sensors (Charged Couple Device) start and stop exposure for all pixels at the same time. This is known as a global shutter. The CCD then transfers this exposure charge to a horizontal shift register where it is then sent to a floating diffusion amplifier.
Note: In 2015, Sony announced plans to discontinue CCD production and end support for CCDs by 2026.
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In the past, CMOS (complementary metal-oxide semiconductor) sensors were only able to start and stop exposure one row of pixels at a time, known as rolling shutter. This has changed over time, with many global shutter CMOS sensors now available on the market. CMOS sensors use smaller ADCs for each column of pixels allowing for higher frame rates than CCDs. CMOS sensors have undergone major improvements over the years making most modern CMOS sensors equal or superior to CCDs in image quality, frame rate and overall value.
For visible light (not infrared, UV or X-ray) sensors, there are two main types; color and mono. Color sensors have an additional layer underneath the micro lens, called a color filter, that absorbs unwanted color wavelengths so that each pixel is sensitive to a specific color wavelength. For mono sensors, there is no color filter, so each pixel is sensitive to all wavelengths of visible light.
For the color sensor example shown above right, the color filter array used is a Bayer filter pattern. This filter pattern uses a 50% green, 25% red and 25% blue array. While most color cameras use a Bayer filter pattern, there are other filter patterns that have different pattern arrangements and RGB breakdowns.
For some sensors, especially sensors with smaller pixel sizes, additional micro lenses are used to help guide the photons into the photodiode.
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Image sensors come in different types of formats (also known as optical class, size or sensor type) and packages. Resolution and pixel size will dictate the overall size of the sensor with larger sensors having either higher resolutions or larger pixel sizes than smaller sensors. Knowing the sensor format is important for choosing lenses and optics for the camera. All lenses are designed for specific sensor formats and resolutions. Note that sensor formats only describe the area of the sensor chip, not the entire sensor package.
Below is an example of a CMOS sensor categorized with a 2/3″ format type. However, the actual die diagonal is only 0.43″ (11mm). The current “inch” sensor types are NOT the actual diagonal size of the sensor. Although the sensor format types may appear to be somewhat ambiguously defined, it is actually based on old video camera tubes where the inch measurement referred to the outside diameter of the video tube. Below is a chart with the most common sensor format types and their actual sensor diagonal sizes in mm.
Here is an example of an old video camera tube. The diameter of these old tubes serves as the sensor format classification for today’s modern sensors.
Pixel size is measured in micrometers (µm) and includes the entire surface area of the photodiode and surrounding electronics. A CMOS pixel consists of a photodiode, an amplifier, a reset gate, a transfer gate and a floating diffusion. However, these elements do not always have to be inside each pixel as they can also be shared between pixels. The diagram below shows a simplified layout of CMOS mono and color pixels.
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Usually, a larger pixel size is better for increased light sensitivity because there is more surface area for the photodiode to receive light. If the sensor format remains the same but the resolution increases, the pixel size must decrease. While this could reduce the sensor’s sensitivity, improvements in pixel structure, noise reduction technology and image processing have helped mitigate this. To get a more accurate understanding of sensor sensitivity, it is best to refer to the sensor’s spectral response (quantum efficiency) as well as other sensor performance results.
Due to physical differences between mono sensors and color sensors, as well as differences between sensor manufacturer technologies and pixel structure, different sensors will sense light to different degrees. One way to get a more accurate understanding of a sensor’s sensitivity to light is to read its spectral response graph (also known as a quantum efficiency graph).
The 2 graphs below are mono and color versions of the same sensor model. The left shows the spectral response of the mono sensor, and the right of the color sensor. X axis is wavelength (nm) and Y axis is quantum efficiency (%). Most machine vision color cameras have built-in IR filters to block near-IR wavelengths. This removes IR noise and color crosstalk from the image, which best matches the way the human eye interprets color. However, in many applications it can be useful to take pictures without an IR filter. Regardless of whether an IR filter is installed or not, a color sensor will never be as sensitive as a mono sensor.
Above: 2 examples of spectral response curves using the same sensor family. Mono sensor (left) and color sensor without IR filter (right)
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The higher the quantum efficiency, the better the sensor is at detecting light. The graphs above are one of many performance results based on the EMVA 1288 measurement standards. The EMVA 1288 standard dictates how to test and display performance results so users can better compare and contrast models across vendors. Visit the EMVA 1288 page for more information.
An important function of the sensor is its shutter type. The two main types of electronic shutters are global shutters and roller shutters. These types of shutters differ in their operation and final shooting results, especially when the camera or target is in motion. Let’s take a detailed look at how they work and how it affects recording.
The diagram on the left shows the exposure timing of the global shutter sensor. All pixels start and end exposure at the same time, but readout still occurs line by line. This time adjustment produces distortion-free images without wobble or skew. Global shutter sensors are essential for capturing fast moving objects.
Diagram to
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