The revolution of digital cameras began with the introduction of digital image sensors. In the early 1980s and late 1970s, cameras began taking digital images thanks to the adoption of CCD (Charge-Coupled Device) sensors. These sensors, which were initially developed for use in scientific applications, worked by converting light into an electrical signal that could be processed by a computer. Initially, CCD sensors were quite expensive and could take pictures at a very limited resolution. This made them unsuitable for consumer-level cameras, which were dominated by film cameras at the time. However, advancements in semiconductor technology soon made it possible to produce smaller and more affordable CCD sensors, paving the way for the development of the first consumer-level digital cameras.
Then, in the mid-2000s, CMOS (Complementary Metal Oxide Semiconductor) sensors began to gain popularity in digital cameras because of their capability of allowing each pixel to process its own data instead of transferring image data from one pixel to the next like in CCD sensors. This made using CMOS sensors more cost-effective and efficient as they required less power to operate. As a result, this technology was then later adopted into smartphone camera modules.
And thus, in this article, we are going to be looking at the different variants of camera sensors and how the technology has evolved. We are mainly going to be looking at four types of sensors:
- CCD (Charge-coupled device) sensor
- CMOS (complementary metal oxide semiconductor) sensor
- Backside Illuminated (BSI) sensor
- Stacked CMOS Sensor
What is a CCD Sensor?
As we have briefly touched upon, a Charge-Coupled Device, or a CCD sensor, is a circuit system that is used to capture pictures by converting the photons from light into an electronic signal. This signal is then processed by a computer to transform it into a digital image. CCDs were the first commercially used sensors in a wide variety of digital photography equipment. They operate by using a grid of light-sensitive pixels to capture the incoming light. When light strikes a pixel, it creates an electrical charge, which is then transferred from one pixel to the next until it reaches a charge-to-voltage converter. The converter then converts the accumulated charge into a voltage signal, which is processed by the camera’s processing chip to produce a digital image.
What is a CMOS Sensor?
CMOS, or Complementary Metal Oxide Semiconductor sensors, were introduced as a natural evolution to CCD sensors. Although both of these image sensors do the exact same thing, their mechanism of processing images is quite different. Each pixel in a CMOS sensor is equipped with its own photodiode, amplifier, and output circuit. When the photodiode comes into contact with light, it creates an electrical charge, which is then amplified and processed by the output circuit. This output circuit transforms the electrical signal into a digital signal, which is then handed over to the camera’s computing chip for processing. These digital signals are then turned into digital images (JPEGS, RAW, etc.) after being formatted from raw data.
What is a BSI CMOS Sensor?
Back Side Illuminated CMOS sensors, more popularly known as BSI sensors, are a revision of traditional CMOS sensors. They are designed to remove one of their core limitations. Traditional CMOS sensors produce images after absorbing light from the photodiodes. However, since the sensor is positioned directly under the imaging components, the quality of the final picture may diminish due to interference from the wiring and circuitry. BSI sensors alleviate this problem by placing the sensor opposite to the imaging components, which in turn produces much clearer and sharper images as it has enough room to capture all of the protons from the light. As for the mechanism of capturing images, it follows the same method as CMOS sensors. Nowadays, most modern camera systems use BSI sensors as a standard.
What is a Stacked Sensor?
Stacked Sensors are the next evolution of imaging sensors that are slowly being adopted to be included in high-end photography equipment. The key differentiating factor of these sensors is that they combine multiple layers of imaging components into a single, compact unit. In a traditional image sensor, the photodiodes, amplifiers, and output circuits are typically positioned on a single layer. This can lead to limitations in performance and complexity in design.
In a stacked sensor, the imaging components are separated into multiple layers, with each layer dedicated to a specific function. This allows for more efficient use of space, as well as improved performance and functionality compared to traditional image sensors. When light strikes a photodiode in a stacked sensor, it creates an electrical charge. This electrical charge is then transmitted to an amplifier, which is then processed by the output circuit, and finally, it is converted into a digital signal for the processing unit of the camera so that it can turn it into a high-quality image.
CCD vs CMOS vs BSI CMOS vs Stacked Sensor
In this section, we are going to be primarily talking about all the types of imaging sensors and how they differ from one another. However, we are not going to be covering CCD sensors extensively in this section as these sensors have gone somewhat extinct after the introduction of CMOS and more advanced sensors like BSI and Stacked.
Despite their age, CCD sensors still have a certain appeal to them for being the first digital camera sensor. But is nostalgia the only factor at play or can these sensors actually hold up in today’s time? Well, here are the noteworthy pros of CCD sensors.
- Image quality: Although not the best, CCD sensors have a very vintage look to the images they produce. Moreover, they were renowned for their low noise levels and accurate color reproduction. No wonder why these sensors were so popular back in the day!
- Long-lasting: Another notable feature of CCDs is their exceptionally long lifespan. People have been using cameras with CCD sensors for decades now without any significant loss in image quality.
Of course, there are several notable cons of using CCD sensors as well.
- Power consumption: One of the biggest reasons why CCD sensors have become obsolete is because of their extremely high power requirements and consumption. As a result, cameras and devices that used CCD sensors would not be able to retain their charge for long.
- Speed: CCD sensors are slow with very limited readout speeds because of which they have slower autofocus performance and increased image distortion when shooting fast-moving subjects. In comparison, CMOS sensors are significantly faster, which they used to overthrow CCD sensors.
What’s interesting about CMOS sensors is that most of the sensors that we use today are a rendition of CMOS. In other words, it means that CMOS sensors are used as the base for most camera technology that we are familiar with. Some of the pros of using cameras with CMOS sensors are:
- Power efficient: The reason why CMOS sensors were adopted so quickly is because of how much more power efficient they were than CCD sensors. This made these sensors ideal for integration with smartphone cameras.
- Compact size: CMOS sensors are built to be as small as possible. And because of their size, they are also relatively easy to manufacture.
As widely accepted CMOS sensors are, they also have certain drawbacks to them:
- Image quality: CMOS sensors do not produce the sharpest or clearest images as they feel like a step down from CCD. In addition, CMOS sensors also have very weak color reproduction capabilities and the pictures are comparatively noisier than CCD sensors.
- Degradation: CMOS sensors can suffer from image drift over time, which can lead to gradual degradation of image quality. In simpler terms, the longer the sensor is used, the more degraded the final output will become. CCD sensors perform much better in this regard.
BSI CMOS Sensors
BSI CMOS sensors are some of the most widely used camera sensors out right now. As a matter of fact, Apple has been using this sensor for their phones since the iPhone 4. And it is still being used in the latest iPhone 14 series of smartphones. Here are some of the pros that make the BSI sensors stand out:
- Low light performance: If you have ever wondered how the iPhone takes such immaculate low-light photographs, then you can thank BSI CMOS sensors for that. Since BSI sensors are not obstructed like traditional CMOS sensors, they can absorb more light and produce better-quality night shots.
- Integration: BSI sensors allow for the integration of additional features and functions into the sensor itself, such as image stabilization, auto-focus, and exposure control.
BSI sensors do suffer from a couple of drawbacks too.
- Complexity: The biggest challenge for BSI sensors is that they are very complex sensors. Hence, they are difficult to manufacture. This may lead to potential issues with consistency and performance.
- Cost: In accordance with the previous point, BSI sensors are also very expensive to manufacture because of their complex design. This is why BSI sensors are mostly used in high-end cameras, smartphones, and photography devices.
Although Stacked Sensors are still relatively new, this technology has huge potential to revolutionize the camera market. To note a few of its primary advantages:
- Improved picture quality: As of now, Stacked Sensor offers the most robust, color-rich pictures with excellent dynamic range, low noise levels, and stellar color reproduction. Compared to BSI and CMOS sensors, you will get much better image quality from Stacked Sensors.
- Versatile: Because of the immense customization capability of Stacked Sensors, they are highly versatile. Manufacturers can implement several different features within Stacked Sensors like image stabilization, zoom control, exposure control, filters, auto-focus, depth of field, and much more.
Similar to BSI sensors, Stacked sensors also suffer from similar issues, such as:
- Difficult to manufacture: Since Stacked Sensors consist of multiple layers of imaging components in one stack, they are very difficult to manufacture.
- Expensive: Not only are Stacked Sensors difficult to manufacture, but they are also very expensive. So, unless you are willing to spend a large chunk of change on a camera device, then these sensors aren’t recommended.
CCD vs. CMOS vs. BSI CMOS vs. Stacked Sensor: Which One Should You Go For?
Till now, we have seen how all of these different sensors work, what makes them different, and the pros & cons of using them. Unfortunately, there isn’t exactly a clear winner here.
“CCD sensors are great for those who love the style of classic digital images, CMOS is for those who are looking for a camera sensor at an affordable range, BSI sensors will appeal to people who enjoy taking stunning night-time photographs, and Stacked Sensors are a combination of all of the aforementioned sensors but comes at a premium cost.”
Regardless of which one you go for, you are bound to get value out of all of these sensors.