CMOS sensors: A comprehensive guide

When you open the camera app on your smartphone, click that shutter button and the resulting image is nothing short of magic. However, that magic is made possible by a CMOS (complementary metal-oxide-semiconductor) sensor, a critical piece of hardware that makes the imaging experience possible on digital devices.

As photography shifted from film photography to digital photography, sensors played a vital role in making the transition not only possible but also seamless. When digital photography first arrived in the mid-1980s, the sensor making it possible to capture images without the use of film was CCD (Charge-Coupled Device) sensors. These sensors are composed of an integrated grid of semiconductor capacitors capable of holding an electrical charge.

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When light reaches the sensor, these capacitors would act as an individual photosite to absorb the light and convert it into an electrical charge. The CCD sensor transferred charges across the sensor to a single output node, which meant the need for nearly perfect charge transfer in read out. CMOS sensors arrived as an alternative to charge-coupled device (CCD) image sensors and by the mid-2000s, it became the most dominant type of image sensor.

They are now used in smartphone cameras, digital point and shoot cameras, DSLRs, and mirrorless interchangeable-lens cameras. Here is a comprehensive look at CMOS sensor, how it works, and benefits in photography.

CMOS sensor: What is it and how does it work

A CMOS sensor is the most widely used image sensor, used in digital cameras, digital video cameras and digital CCTV cameras, to create images. It consists of an integrated circuit that records an image by capturing light and converting it into digital data. In other words, these sensors are an electronic chip that converts photons to electrons for digital processing.

Like many other semiconductor technologies, CMOS sensors are also produced using the process of photolithography. A CMOS sensor is composed of photosites, which are the individual pixels that capture light. Over each photosite, there is a pattern of red, green, and blue colour filters, also known as colour filter array. When light enters the camera through the lens, it first passes through the colour filter array and reaches the photosite.

At this moment, each photosite records the intensity of the light for a specific colour and the data is then combined to create a full-colour digital image. In addition to converting light into a digital image, the CMOS sensor is also responsible for minimising noise and improving image quality.

CMOS image sensor: Meet the active pixel sensor

In 1968, Peter J.W. Noble invented an active pixel sensor (APS) as an image sensor where each pixel sensor unit cell had a photodetector (a pinned photodiode) and one or more active transistors. CMOS is the most common type of APS in use right now where the efficiency of these photodiodes greatly impacts the overall performance of the sensor.

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The CMOS process came to be known as a well-controlled stable semiconductor manufacturing process and it became the baseline process for all logic and microprocessors by the late 1980s to early 1990s. Between 1988 and 1991, Toshiba developed a sensor with a lateral APS structure and fabricated it using PMOS transistors.

In 1993, NASA’s Jet Propulsion Laboratory (JPL) fabricated a CMOS-compatible active pixel sensor, which had a lateral APS structure similar to the Toshiba sensor but was fabricated with CMOS transistors. This sensor became the first CMOS sensor with intra-pixel charge transfer and by 2000, CMOS sensors were used in various applications.

Benefits of CMOS sensor

When compared to a CCD sensor, a CMOS sensor’s benefits include resolution, frame rate, power consumption, and dynamic range. Let’s understand these benefits in detail.

1. Resolution:

Resolution is one of the important parameters of digital camera sensors where higher resolution allows the capture of more details in images. With a CMOS sensor, it is possible to support higher resolution by increasing the number of pixels in a given area of the sensor. A modern CMOS sensor is now produced with higher resolution and better image quality.

2. Frame rate:

Another key aspect of camera sensor performance is frame rate, which defines the number of individual images captured per second. It is measured in frames per second (fps) and higher frame rates produce smoother video motion. Compared to a CCD sensor, a CMOS sensor can offer faster frame rates for applications like wildlife photography or sports photography.

3. Power consumption:

CMOS sensors consume much less power than their CCD counterparts. This is because of their ability to convert photons directly into digital signals at the pixel level. This reduced power consumption also helps with delivering longer battery life.

4. Dynamic range:

Dynamic range defines the capability of an image sensor to capture a wide range of light levels in a single image. CMOS sensors are not only able to capture greater dynamic range but also support high dynamic range (HDR) to deliver images with vivid colours.

Role of CMOS sensor in photography

One can say that modern digital photography would not have been possible without CMOS sensors. Access to CMOS sensors had a significant cultural impact leading to the wide availability of digital cameras and camera phones. By putting a camera in every person’s hand through camera phones, CMOS sensors became an unsung hero in the rise of social media and selfie culture.

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The CMOS sensor has not only led to significant improvements in digital photography but has also made it possible to turn every person into a photo enthusiast. Today, we are all able to capture images with better quality, low noise, and high dynamic range thanks to CMOS sensors. Their versatility and adaptability have led to CMOS sensors being used in machine vision, biomedical imaging, and security applications.