What Is a CCD (Charge-Coupled Device) and How Is It Used?
Reading Time: 4 minutesCCDs are used in specialized areas of photography as they’re better than CMOS sensors in some cases. Here’s everything you need to know.
To digitally take a photo, a modern camera needs to capture light and convert it to digital information. To do this, a camera would require a sensor that accurately and speedily records photons from the environment.
You are probably already aware of the CMOS sensor used in smartphones and consumer digital cameras. But did you know that there is another type of sensor that provides higher levels of detail and dynamic range? These camera sensors are known as CCDs.
So, what exactly are CCDs? How does it work, and how is it used? Let’s talk about it.
What Is a CCD (Charge-Coupled Device)?
A CCD, or charged-coupled device, is an electronic sensor that converts light to digital signals through charges generated by bouncing photons on a thin silicon wafer.
CCDs were the gold standard for camera sensors from the early 80s till the late 2000s. This is because around 2010, CMOS sensors gained significant technological innovations that would make them cheaper to manufacture as a system on a chip (SoC) while having comparable image quality to a CCD sensor.
Since CMOS gained popularity, it has become rare to see CCD sensors on smartphones and cameras this past decade. However, CCD sensors aren’t exactly obsolete. Although they may have been phased out of the consumer camera market, CCD sensors are still the preferred sensor used in certain areas of photography.
Applications of CCD Technology in Photography
Aside from being expensive to manufacture, CCD also had other problems that caused it to be phased out of the consumer market. This would include its high-power requirement, which is 100 times more than what CMOS would use, and slow image processing, which is a problem when taking photos in bursts and shooting video.
Despite all these disadvantages, CCDs are still thriving in various industrial and scientific applications that need machine vision. This is because CCDs still provide higher quality low-noise images that these areas of specialized photography require. Plus, the cost of buying and operating CCD cameras isn’t really a problem for well-funded institutions and businesses.
So, what exactly are these specialized areas of photography that still use CCD? Let’s find out below:
Optical Microscopy
CCDs are used in various microscopy applications to observe food, chemistry, engineering, and other applications where clear visuals of microscopic objects are necessary. A CCD is chosen for optical microscopy because it can record objects with over 10 pixels with high sensitivity and low noise ratios.
Space Photography
Taking photos of space is best done on CCD cameras. This is because CCD sensors have the highest quantum efficiencies, resulting in low noise, high dynamic range, and better uniformity—all critical aspects of space photography.
Near-Infrared Imaging
CCDs are used in various industrial imaging applications, one of which is near-infrared imaging. A sensor needs to have highly efficient photon absorption to do near-infrared imaging, as infrared photons are less visible than regularly visible photons. Since CCDs provide highly sensitive sensors that can capture infrared photons better, they are always used in these applications.
CCDs thrive in the scientific, industrial, and medical photography space primarily because of their high quantum efficiencies, low noise imagery, and high level of uniformity. But how exactly do CCD sensors provide such qualities? You’ll first need to learn how CCD sensors work to understand this better.
How Does a CCD System Work?
CCD is just one of the various types of camera sensors. And just like other camera sensors, CCDs capture light and convert it into digital signals, which are then processed and displayed as pixels when viewed on an electronic display such as a monitor.
Although all imaging sensors have the same task of capturing the analog to make digital signals, the mode or process it takes to achieve said tasks would be different from other sensors.
For a CCD sensor to capture images, it goes through a five-step process, starting with light-to-charge conversion, charge accumulation, charge transfer, charge-to-voltage conversion, and then signal amplification. Let’s go through the process step by step:
Step 1: Light-to-Charge Conversion
A CCD sensor captures light by allowing photons (energy from light) to bounce off a thin silicon wafer which then releases an electron. A tiny positively charged capacitor then acts as a bucket that collects and stores the released electrons. A unit of this thin silicon wafer on top of a tiny capacitor is known as a photosite.
Steps 2 and 3: Charge Accumulation and Charge Transfer
A CCD sensor continues to collect and store such electrons until the camera shutter closes. All the stored electrons from the capacitor are what make the charge.
When the camera shutter closes, all the charge from the photosites is transferred to a sense capacitor circuit. The transfer is done by shifting the charges horizontally to the edge of the sensor and then vertically until each charge is sent to the sense capacitor circuit.
CCD sensors use this shift register mechanism to transfer charge, while CMOS sensors use local voltage conversion and signal amplification. Although this makes CMOS the faster sensor, it also makes their output quite noisy as the sheer number of local amplifiers create noise or artifacts in an image. In contrast, a CCD only uses one amplifier circuit to amplify signals.
Another disadvantage of using local amplification at high speeds is that it causes unevenness to the imagery. CCD sensors don’t have such problems because of their linear process when processing charges in each photosite.
Steps 4 and 5: Charge-to-Voltage Conversion and Signal Amplification
Analog charges sent to the sense capacitor are automatically converted into voltages which makes the raw digital data used to make images. After the charge-to-voltage conversion, the digital signals are still too low for a processor to use.
To boost the digital signals, a signal amplifier is used. This amplified signal is then sent to an image processor which then assembles the image.
CCDs Are Here to Stay
Once the gold standard for digital camera sensors, CCDs are now discontinued for regular consumer use. But with their high quantum efficiencies, low noise imaging, higher dynamic range, and excellent uniformity, CCDs are still used in many scientific and industrial applications.
And although it is unlikely that manufacturers will bring back consumer-grade CCD cameras in the near future, CCDs will continue to be a staple in scientific research.
Reference: https://www.makeuseof.com/what-is-a-ccd-charge-coupled-device/
Ref: makeuseof
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