When designing solutions using active pixel array CMOS digital image sensors, a multitude of sensor specifications must be considered. For instance, the sensor's resolution, optical format, shutter type, maximum frame rate, dynamic range, signal-to-noise ratio (SNR), pixel structure, and so on. To make things more complicated, sensor characteristics/functions such as power consumption, interface, package type, on-board HDR processing, and regions of interest also need to be taken into account. The optimal choice is not always immediately apparent.

To aid in the selection of these specifications and features, an important consideration is the intended application of the sensor. Some applications require very high resolution to capture stationary objects, while others necessitate the detection of rapidly moving objects and the ability to reproduce a "freeze-frame" effect. Another significant application consideration is power consumption requirements. For fixed installations, power consumption of the sensor may not be critical. However, in portable applications, where the sensor must operate on batteries, its energy efficiency becomes paramount.

When selecting a sensor, the most appropriate starting point is the speed of application, that is, the speed at which the object moves. Because this will determine the type of shutter required. In the field of digital image sensors, there are mainly only two choices: rolling shutter and global shutter.

Precautions for image sensors

Rolling Shutter

A digital image sensor consists of a pixel array arranged in rows. When using a rolling shutter image sensor, each row in the array is exposed sequentially from the top to the bottom of the array. In other words, the exposure time for adjacent rows is slightly different (referred to as row time), with a time difference of approximately 10 microseconds between adjacent rows.

Unlike rolling shutters, global shutters expose every pixel in the array simultaneously. These sensors must have pixels with "storage nodes" that can store charges throughout the sensor readout process. Both rolling shutters and global shutters have their advantages and disadvantages.

Compared to global shutters, rolling shutters are more cost-effective and easier to implement. In global shutter sensors, the storage nodes are susceptible to stray light, often resulting in higher noise. Additionally, the storage nodes are located next to the pixels, which imposes limitations on pixel size. Conversely, the disadvantage of rolling shutters is that they tend to produce motion artifacts when capturing fast-moving objects.

Due to the sequential exposure method employed by the rolling shutter array, spatial distortion may occur when capturing moving objects. Similarly, as different regions of the array are captured at varying times (potentially under different lighting conditions), they may also be affected by unrelated illumination

Therefore, the rolling shutter array performs poorly in capturing moving objects, but it is an excellent choice for static high-resolution applications.

Global shutter

The global shutter is suitable for scenarios where the rolling shutter performs poorly, including fast-moving objects, especially those with high angular velocity. Applications where the global shutter excels include augmented vision (AV), virtual reality (VR), machine vision (MV), and any environment with high vibration, such as barcode scanners and robotic applications.

The global shutter also boasts other advantages: since the entire array is exposed simultaneously, the global shutter can be directly synchronized with other global shutters or light sources (such as flash lamps). Since the global shutter does not need to deal with irrelevant illumination, it is also easier to achieve automatic exposure control.

Considerations for global shutter performance

The first step in evaluating the performance of a global shutter is to refer to its demonstrated specifications. When evaluating global shutter sensors, it is necessary to consider both application-oriented and performance-oriented metrics. Application-oriented metrics can help you select the sensor suitable for a specific application within a particular product series, while performance-oriented metrics can assist you in comparing products from different manufacturers.

The simplest approach is to start with application-oriented indicators. The five main indicators determined by the end use are as follows:

1. Resolution

2. Optical format

3. Global Shutter Efficiency (GSE)

4. Frame rate

5. Power consumption

The priority of these five indicators will vary depending on the specific requirements of the end use. For example, a high-resolution application may require a resolution of 2 million pixels (MP) and an optical format of 1/2.8 inch, while a low-resolution application may only require VGA resolution and an optical format of 1/8 inch.

Frame rate

Frame rate is measured in frames per second (fps), indicating the number of images that a sensor can capture within one second. A higher frame rate is required when capturing objects moving at a faster pace to avoid blurring.

Global Shutter Efficiency (GSE)

As mentioned above, GSE is a ratio indicating the ability of the global shutter to suppress stray light. It is typically specified under specific wavelength and aperture (f/stop) settings. A higher value indicates better performance.

energy efficiency

Low-power optimization in consumer applications is crucial during development, especially for applications such as augmented reality (AR), virtual reality (VR), and mixed reality (MR) head-mounted devices. Additionally, autonomous mobile robots (AMR) and handheld barcode scanners are also examples of battery-powered devices in the industrial sector. By improving the energy efficiency of these devices, their operational lifespan can be significantly extended, thereby reducing the frequency of charging and improving the overall user experience. The remaining metrics are performance-oriented and can be used to compare products from different manufacturers.

Signal-to-Noise Ratio (SNR)

The signal-to-noise ratio (SNR) is measured in decibels (dB) and is defined as a maximum value. It is a method of measuring the performance of a sensor in the presence of small (i.e., low-light) signals. The higher the value, the better the performance. The maximum SNR value, SNRmax, truly reflects the linear full well (LFW), or essentially the number of photons that a pixel can capture.

dynamic range 

The dynamic range is also measured in dB, representing the ratio between the maximum measurable input signal and the minimum measurable input signal (i.e., the noise level). It indicates the sensor's ability to handle input signals of different intensities in the same scene. A higher decibel value is better. Tunnels are a good example of a scene requiring a high dynamic range, as the interior of a tunnel may be dark while the exterior is brightly lit. The sensor needs to be able to adapt to both situations within the same scene.

In addition to performance metrics, certain applications may require sensors to possess specific characteristics, enabling them to perform particular functions or exhibit unique capabilities. Not all global shutter sensors are equipped with these features. The requirements of the application will dictate which functionalities are necessary and which sensors to consider.

Synchronous sensor

One of the advantages of using a global shutter for a single exposure of the entire sensor array is that the exposure of the array can be precisely synchronized with other events, such as other sensors and flashlights.

Through the "trigger" mode of the synchronized sensor, the flash can be controlled for precise active lighting, or multiple cameras can be synchronized to achieve stereo or wide-screen shooting.

Embedded automatic exposure

The automatic exposure function enables the sensor to automatically control gain and exposure based on given lighting conditions. Automatic exposure is a basic function of the sensor to adapt to dynamic lighting conditions.

By embedding this function directly into the sensor, exposure control can be accelerated, enabling real-time response, whereas relying on host control results in slower response times. For most high-speed applications, embedded automatic exposure is essential.

scene change

The scene switching function enables the sensor to quickly switch settings based on different resolutions, gains, exposures, and frame rates to adapt to various imaging scenarios. In many sensors, these "scenes" are already stored and can be dynamically changed within a single register setting.

Programmable and switchable regions of interest (ROI)

The region in the image is a collection of related pixels, primarily used for object analysis. ROI enables the sensor to focus on a specific area by filtering out other parts. This is a method to optimize data transmission and processing. Programmable ROI makes real-time computer vision applications possible.

In summary, application-oriented indicators, performance-oriented indicators, and specific functions can be used in combination to help select a specific global shutter sensor from a range of sensors and determine the sensor manufacturer that meets the final usage requirements.

Hyperlux SG Series Global Shutter Sensor

ON Semiconductor has developed a series of high-performance, small-sized global shutter sensors named Hyperlux SG, including the ARX383, AR0145, and AR0235. The Hyperlux SG series of sensors combines industry-leading global shutter efficiency (GSE) with low-power operation, making them ideal for portable and high-vibration applications.

The Hyperlux SG series incorporates a novel and innovative global shutter pixel design optimized for accurately and quickly capturing motion scenes. It captures clear, low-noise images in both low-light and bright scenes.

The Hyperlux SG sensor series possesses the following characteristics:

· Horizontal/vertical mirroring, windowing, and pixel merging

· Programmable Region of Interest (ROI)

· On-chip trigger mode for synchronization

· On-chip automatic exposure

· Built-in flash control

· Scene transition

· Flexible control of skip-row and skip-column modes

The resolution of the sensor combination ranges from VGA (640 x 480) to 2.3 million pixels (1920 x 1200), with optical formats ranging from 1/8 inch to 1/2.8 inch, and frame rates up to 120 frames per second, making them suitable for various high-speed imaging applications. Each specification has advantages compared to other similar products, and it is precisely because they combine excellent performance and functionality that the Hyperlux series of sensors stands out in the market. These sensors are highly suitable for high-speed applications, including barcode scanning, machine vision, AMR, AGV, AV/VR/MR, drones, and 3D scanning.

To facilitate product education and system design, a comprehensive development platform is also provided, enabling rapid system development. It includes complete testing capabilities to evaluate the product before design, and provides reference designs for use during the design phase.

Global shutter sensors are the optimal choice for high-speed imaging applications. Once the shutter type is selected, there are still various specifications and features to choose from, ensuring that the sensor is suitable for the intended application.

When evaluating global shutter sensors, an important consideration that must always be kept in mind is the Global Shutter Efficiency (GSE). Without a sufficiently high GSE, all other specifications combined may still produce unacceptable motion artifacts in the image. For applications demanding low power consumption, high performance, and high GSE, ON Semiconductor has developed the Hyperlux SG series of global shutter sensors.

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