The transition from analog to digital allowed digital cameras to process photographs in seconds. Digital photography has become an integral part of our modern way of life.
We now have compact cameras with amazing image quality. As cameras improve, so does the way we use images in a variety of enterprises. In the age of artificial intelligence, imaging systems are taking over the role of the human eye. Imaging can collect a wealth of information from ordinary activities, including age, gender, and height. For example, we can collect data from images of people standing at a stop sign, using a vending machine, entering a store, crossing the street, passing a kiosk, registering for a conference, ordering a coffee, boarding a flight, and so on.
But how do we achieve this remarkable feat? The secret is in the image sensor.
How do image sensors work?
The simplest explanation of how an image sensor works is a device that snaps a picture of the object. Image sensors capture the image by using the light reflected or emitted by the subject. The light reflects from the object and is captured by the image sensor. The sensor is placed in the optical path of the light and passes it to a semiconductor photodiode.
Image sensor converts light energy into electrical measurements, and then converts these measurements into digital signals. Digital imaging sensors use a digital array of sensors to capture an image of the subject and store the resulting electronic data in a digitized memory array. After that, an image is created by integrating together this data. Depending on the bit depth of your camera, this process varies in precision.
The image becomes a grayscale image as a result of all of these data collection processes. By using a Bayer filter array, the image acquires its color. Each photo site has a colored filter over it that is used to determine an image’s color based on the electrical signals from nearby photo sites.
Types of imaging sensors
Imaging sensors are categorized according to their structure such as a Charge-coupled device (CCD) and complementary metal-oxide-semiconductor (CMOS), chroma type (color or monochrome), and shutter type (global or rolling shutter) etc. They are also classified based on their resolution, frame rate, pixel size, and sensor format.
This blog provides a brief review of types of CMOS image sensors used in cameras, based on the shutter mechanism used to read pixel values.
History behind the term Shutter
In 1845, a shutter-like device was invented for the first time. To take pictures of the Sun, two French physicists named Fizeau and Fucault created a gadget that was based on a guillotine. It was constructed from a board with a hole through the middle. The board dropped it into a slot in front of the camera when the picture was taken at 1/30th of a second.
The plate was briefly exposed as a result of the hole passing in front of it. Further improved this method in the 1860s, William English, who developed a mechanism using the guillotine principle and an adjustable aperture. It is acknowledged as the original focal plane shutter.
Central opening shutters enabling quicker shutter speeds were all being manufactured in the early 1920s.
Ultimately, as films become more sensitive, shutter speeds began to fall from seconds to fractions of a second, making manual exposure impossible. Innovative engineers came up with a lot of methods for making shutters open and close more quickly than once every second. And hence the camera shutter came into existence.
Regardless of modern cameras or outdated processes, we still employ different shutter systems for large-format photography. In modern technology today, the rolling and global shutter mechanisms are in demand. As a result, the following is a comparison and individual description of each.
Based on shutter mechanism, there are two main types of CMOS sensors
- Global Shutter
- Rolling Shutter
The difference is how the sensor captures the images, how it scans the pixels and integrates them before sending them out.
A global shutter is a type of CMOS image sensor that takes all the pixels on the sensor array at the same time, enabling a “freeze frame” capture of the scene.
With global shutters, all pixels on the sensor are exposed at once, capturing the entire image at once. Before the exposure begins, all pixels in the array will be placed in a ‘keep clean’ state. As soon as the exposure starts, each pixel gathers charge and continues to do so throughout the exposure. After exposure ends, each pixel sends its charge to its readout node simultaneously.
In this way, exposure and readout occur simultaneously. As a result, we can accurately capture objects that move quickly or environments that change rapidly. As a result of the control over the exposure start, numerous global shutter sensors can be synchronized simultaneously.
Benefits of Global Shutter
- Global shutter has an advantage over rolling shutter due to the consistency and accuracy of the images created by CCD sensors and global shutter.
- The photographer may be guaranteed that the photos made with this shutter will precisely depict the subject being snapped, without any segmentation or “jello effect.”
- Cameras may no longer need shutter blades, which lessens the occasional malfunction that happens in the moving parts of the shutter mechanism.
This type of sensor captures the entire image sequentially, which means that the pixels are divided into rows and exposed one after another. This means the resulting image contains a slight time delay between each row since each row requires a specific amount of time to read out. This is referred to as the “line time.” As soon as the rows are exposed, the values are integrated, and then the values are read out sequentially.
In contrast to the global shutter sensor, all rows need not be ‘clean slates’ before capturing. It is possible for the sensor to work in overlap mode. Upon completion of the read-out operations, each row shall begin the exposure for the next frame. Read-out of all rows isn’t mandatory.
Benefits of Rolling Shutter
- It is significantly more economically possible to produce it.
- Direct integration of complicated circuitry within the CMOS itself is substantially simpler.
- Since their introduction, this has made it possible for it to be utilized almost exclusively in smartphones and digital cameras.
- The luminous intensity range or range boundaries when taking a picture are referred to as the dynamic range. Wide dynamic range suggests that the final image will depict the situation more accurately in real life. Even under intense illumination, almost the entire dynamic range is preserved.
Rolling Shutter or Global Shutter?
Image via Wikipedia
Your preference for rolling shutter or global shutter mode will mostly rely on your project. Global shutters have an exposure mechanism that is “non-transient,” exactly like interline CCDs. With the use of global shutter sensors, you can take pictures, accurately control exposure duration, and synchronize numerous sensors to capture moving objects or transient occurrences over the course of a series of kinetic acquisitions with no spatial artefacts. Thus, global shutter sensors are the way to go for use cases involving image capture in motion.
In some circumstances, we need to capture static objects or just record video streams. We can use rolling shutter cameras in these situations. Rolling Shutter mode is still likely to be suitable for many scientific applications due to its improved non-synchronized maximum frame rate possibility and decreased read noise. Because of the simplicity of its architecture, rolling shutter sensors can provide more pixels at a lower cost than global shutter cameras. If the frame rate is high enough for the camera to temporally oversample object dynamics in the image region. Since it is typically undesirable to have an object traverse a substantial distance throughout a single exposure, thus oversampling is good imaging practice.
Examples of different types of Imaging Sensors
Vadzo is fortunate to be able to use imaging sensors built by Sony, Omnivision, and On Semiconductors. To gain a sense of the sensors offered in each category on the market, please refer to the list of examples below.
Each of the types has its own purpose and use case. No one type fits all requirements. According to the use case, we must choose which type of sensor best meets our needs.
Which is better?
Which is most appropriate for your needs? There is no clear victor among them. Because rolling shutters and global shutters serve distinct purposes, the shutter you choose should be based on your project.
A global shutter camera is primarily used to capture fast-moving objects without motion blur or artefacts. Applications for global shutter cameras include ball tracking, industrial automation, warehouse robotics, drones, and more.
Excellent imaging sensitivity is provided by rolling shutter sensors, which can also be employed in low-cost applications. It is mainly used to capture items that are moving slowly, such agricultural tractors, slow-moving conveyors, and independent applications like kiosks and barcode scanners.
Understanding the technologies of digital sensors can help you find the most suitable camera for your application. The proper lens, for example, will be determined by pixel size and sensor type. If you’re ready to talk about your camera needs, Vadzo’s team will be happy to assist you. Vadzo has the pleasure of working with imaging sensors from businesses such as On Semiconductors, Sony, Omnivision, and others.
Have questions? Contact us