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Charge-coupled devices (CCDs) are now one of the most common sensors used in optical microscopy, as well as many other characterization techniques. It was never the original purpose of the devices - they were originally designed back in the 1960s for use in memory storage applications. In this article, we look at what CCDs are and how they are used in modern-day optical microscopes.
Until recently, film cameras were used in optical microscope detectors, but there has since been a shift to CCDs. As well as viewing samples through the eyepiece, many modern optical microscopes now have an attached camera that allows images and videos to be taken. CCDs are essentially an array of photodiodes (a type of sensor which captures photons of light) that can be used to build up an image of a sample.
CCDs offer a way of digitally capturing photons to generate an image of a sample and are essentially doped silicon wafers, which are etched with photoactive regions to capture photons. These photoactive regions are metal-oxide-semiconductor (MOS) capacitors, and each of these capacitors acts as a photodiode, which in turn behaves as a pixel.
These photoactive regions can be built into arrays to represent many pixels which are then used to build the image of the sample. This is achieved when the photons of light from the microscope hit the detector (after passing through the sample), creating a surface charge in each capacitor pixel.
This surface charge is generated because the photon creates a hole (through the ejection of an electron under photoirradiation) in the silicon lattice. As each pixel is separated from the next, the induction of holes essentially generates an array of potential wells which store these charges.
The change in charge is proportional to the intensity of light hitting it, where the absorption of one photon equates to the generation of one hole. This change in electrical charge is then read by the electronics in the CCD camera, where the electronic charge is shifted along transfer channels under an applied voltage. It is this mechanism that gives CCDs their name, as it is in this transfer process where the translation of charge in the silicon is coupled to the voltage patterns from the gate.
Additionally, because the charges are stored in potential wells, they can be stored for relatively long time periods before being read by the chip within the camera system. The intensity and positional values of the electronic charges from each pixel are rapidly digitized by the system, which enables the image to be constructed and outputted onto the monitor.
The whole imaging process happens instantaneously, which allows rapid images to be taken of samples. This is particularly important when processes within the sample are being imaged rather than the structure of the sample, as delays in the imaging process could fail to capture what is happening when the user tries to image it. Like many cameras, the pixel arrays and subsequent images can adopt many aspect ratios, but the most common is 4:3.
There are many different CCD cameras used in optical microscopes today, with many cameras possessing over 10 million pixels, low noise interferences, high sensitivities, high spatial resolutions high spectral bandwidths, high dynamic ranges, the ability to render color (with the use of filters or extra optical components), high quantum efficiencies (for imaging fluorescence) and the ability to image at many frames per second. They are far more advanced than the cameras and image methods that preceded them and, like all cameras, are advancing with modern-day technology.
As it stands, the CCD cameras used in optical microscopes can range between 0.1 and 20 MHz, but this adjustable depending on the specific requirements of the user. In CCD cameras, it is not the imaging sensors that are the limiting factor in terms of image quality, but the speed in which the electronics can generate/digitize the image through the analog to digital convertor. So, there is still room for CCD camera systems to improve, provided all the components around the image sensors do too.