What is a CCD Camera?
A CCD camera is any type of digital camera with a charge coupled device (CCD) image sensor. This includes the vast majority of consumer and professional still cameras, video cameras, security cameras, cell phone cameras and medical cameras. CCDs are very efficient, generally capturing about 70 percent of incident light, unlike photographic film which only responds to about two percent of incident light. CCDs are also sensitive to infrared light, which makes them ideal for night-vision surveillance cameras and astronomy applications. While some cameras use a complementary metal-oxide semiconductor (CMOS) image sensor, the CCD is the most common type.
Most CCD cameras use a single charge coupled device to collect image data, whether the camera is designed for monochromatic, color or infrared operation. In this case, light enters through the lens, is filtered and then focused onto the surface of a single photoelectric image sensor array. Many professional video cameras, known as "three-CCD" or "three-chip" cameras, contain three CCD arrays. With these, the incoming light is split by a prism into its red, green and blue components, each focused on its own CCD sensor. This improves color separation and increases light sensitivity, resulting in more accurate color shading in general and more detail in lower-light situations.
Fax machines, scanners and other types of linear-scan cameras use a one-dimensional CCD image sensor to gather data, moving either the sensor or the object being scanned in order to capture the entire image. Every other type of CCD camera uses a fixed two-dimensional area matrix. The CCD sensor is an array of coupled, photoactive capacitors which build up charges based on the intensity, duration and wavelength of the light being focused upon them. Once exposed to an image, the sensor's controller shifts the charge of each capacitor to its neighbor in the array. This creates a ripple effect across the whole matrix, shifting the last set of charges off-chip to a separate digitizer; this digitizer converts them into numeric values to be stored in the camera's memory.
How the CCD camera stores and retrieves image data typically affects the design of the system. The full-frame method uses the entire CCD for light collection and requires a mechanical shutter to prevent smearing when the image data is transferred off-chip. This design is ideal when collecting the most light and the best image is more important than cost, time and power consumption. The interline method uses every other column of the CCD to quickly store image charge data with a one-pixel shift, preventing smear and removing the need for a mechanical shutter at the cost of efficiency. Alternatively, the frame-transfer method can be implemented with an acceptable amount of smearing and no mechanical shutter. Frame-transfer utilizes half of the CCD for charge storage and retrieval, while the other half is accumulating a new image, thus it requires twice the amount of silicon to handle the same size image.
Specialized CCD cameras are used in astronomy because they are sensitive to light wavelengths, from ultraviolet to infrared. They are so sensitive, in fact, that many extra steps must be taken to reduce the amount of image-distorting "noise," including cooling the CCD to liquid nitrogen temperatures. With the right amount of compensation and image processing, observatory-quality astrophotography has become accessible to serious, dedicated amateurs armed with CCD camera equipment.
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