Image sensors such as CMOS have seen their use cases grow significantly in size across many industries today. And you can also see major differences among the designs of the image sensors. For instance, these sensors come in varying sizes. In fact, the size of the camera sensor is considered the most important factor in determining overall camera performance and image quality.
Let us now deep dive into the comparison of different sensor sizes and their suitable usage in embedded vision applications. Before we get into a camera sensor size comparison, let us first try to understand what a CMOS sensor is and how its size is measured.
What is a CMOS sensor?
CMOS (Complementary Metal-Oxide Semiconductor) is a digital device for capturing light and converting it into electrical signals. It has a photodiode and a transistor switch for each pixel. When light strikes the pixel, it creates a voltage proportional to intensity. The voltage is sampled directly at the pixel. Having an amplifier for each pixel allows the pixel signals to be amplified individually, as shown in the below figure.
Figure 1: CMOS Sensor Architecture
To learn about the differences between CMOS sensors and CCD sensors, please visit the article CMOS sensors vs CCD sensors: why CMOS sensors are ruling the world of embedded vision?
What does camera sensor size mean? How is it measured?
CMOS sensors are generally specified by their physical sizes. The size of the CMOS sensor determines the light-collecting surface area of the sensor. The dimensions of the sensor are defined by the resolution and the pixel size. As you may know, the size of a sensor is often measured in inches. The image sensor format is sometimes referred to as sensor size or optical format.
The value of optical format is the approximate multiplication of the diagonal length of the sensor and 3/2.
Optical format = (3/2) * Diagonal of Sensor
For example, consider the AR1335-CMOS image sensor from onsemi™ that has a sensor size of 4.54×3.42 mm and a diagonal of 5.68 mm. Therefore, the optical format is 5.68*3/2 = 8.52 mm, which is expressed as 1/3.2″.
Likewise, there are several sensor sizes available in the market as shown in the following figure.
Figure 2: Illustration of Sensor Size Dimensions
CMOS sensor size comparison
The below table shows the comparison of different sensor sizes and the corresponding crop factors:
|Type||Diagonal (mm)||Width (mm)||Height (mm)||Area (mm²)||Crop factor|
It is important to note that the crop factor is related to the ratio of the camera sensor’s size to a 35 mm film frame.
Crop Factor = Diagonal35mm / Diagonalsensor
How to choose the right CMOS sensor size for an embedded vision application
Each application has different sensor size requirements to produce images. Let us discuss the factors to be considered while choosing a sensor of a particular size.
- High frame rate and global shutter
- Lens mount selection
- Image circle diameter
- Low light performance
Resolution is the ability of imaging systems to reproduce the exact object detail. Many embedded vision applications like autonomous mobile robots (AMR) and autonomous vehicles demand cameras to achieve precise 3D depth measurement. This would be achieved with the high-resolution feature of that camera. Selecting sensors with large pixel sizes is likely to have higher resolution. Also, the resolution of the lens must match the pixel size of the sensor to achieve high-quality images. The quite popular camera resolution of 1600 x 1200 pixels often uses a larger sensor with a size of 1/1.8″, and now high-end 4K resolution uses a 1/1.2″ image sensor format.
High Frame Rate and Global Shutter
Embedded vision applications like automated license plate recognition, gesture recognition, robotic vision, drones, and AMR require high frame rate and global shutter features – depending on the nature of the end application. AR0234 from Onsemi is one of the most popular sensors used in such applications. It is a 1/2.6″ (Diagonal 6.8 mm) optical format CMOS sensor with a 3.0 μm x 3.0 μm pixel size. It is a global shutter sensor that is used for accurate and fast capture of moving scenes at 120 frames per second at full resolution. See3CAM24_CUG from e-con Systems is a color global shutter camera based on the ARO234 sensor.
Also, some of the other available sensor sizes are 1/2.9″ (for Omnivision’s OV2311 CMOS image sensor with 3.0 μm x 3.0 μm pixel size) and 1/3″ (Onsemi’s AR0134 CMOS digital image sensor with an active pixel array of 1280H x. 960V).
Large sensors tend to have large pixel sizes, and this indicates higher sensitivity. To achieve high sensitivity and compact design, an industrial camera usually uses a 1/2.8″ CMOS image sensor. This enables a high level of image recognition and detection performance for improved safety for smart city, surveillance, and traffic monitoring systems.
Lens Mount Selection
Mount is used for attaching a lens to a camera body. The selection of mount depends on the sensor size. For instance, the C mount, which is the type of lens mount for machine vision cameras, is appropriate for a 1.5″ sensor. S mount lens, which is commonly used in industrial applications, is appropriate for a sensor size of 1/2″, 1/3″ or smaller.
Image Circle Diameter
Industrial cameras might have issues like lens vignetting/lens shading, which is a gradual reduction of an image’s brightness or saturation from the image center to the four corners/ edges. This happens when the image format (or circle) of the lens is too small for the size of the sensor. So, to mitigate this, the image circle diameter must fit or be larger than the sensor size.
As discussed before, a large sensor contains larger photosites that are more receptive to light, thereby enhancing the camera’s ability to capture low-light images in comparison to a small sensor. Two of the popular sensor sizes targeted for low light performance are 1/1.2″ ( such as the Sony® IMX485 based 4K-resolution CMOS image sensor) and 35mm full-frame.
As sensor technology improves, there is always an onus on both smaller and larger sensors for better performance, but the reality is that in this case – bigger equals better! When the cost comes into play, sensors of large size are significantly more expensive than small sensors. We hope that this blog helped you better understand what CMOS sensor size is, and how to choose the right sensor size for your embedded vision application.
If you have any further queries on the topic, please leave a comment.
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Prabu is the Chief Technology Officer and Head of Camera Products at e-con Systems, and comes with a rich experience of more than 15 years in the embedded vision space. He brings to the table a deep knowledge in USB cameras, embedded vision cameras, vision algorithms and FPGAs. He has built 50+ camera solutions spanning various domains such as medical, industrial, agriculture, retail, biometrics, and more. He also comes with expertise in device driver development and BSP development. Currently, Prabu’s focus is to build smart camera solutions that power new age AI based applications.