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Tuesday, January 30, 2007

Understanding Image Sensors

The sensor is the soul of your digital camera and knowing how it works will help you to compose better images

Labels: Learning Center

Let's examine how CCDs operate. The CCD is comprised of a series of photosites in a grid pattern. Each photosite contains a light-sensing device (a photodiode) and a storage area to hold the charge created by the photodiode. When light hits the photodiode, it converts the light into electrons, or a charge. The more light that hits the photodiode, the greater the charge.

The next step in the process is to move the charge out of the photosites and into another area of the chip called a transfer register. Row by row, each row of charges is moved vertically and passed into the transfer register. When a row reaches the transfer register, the photosite charges are read out (horizontally) one by one, converted into a voltage and amplified. Once one row has been read out, its charges are deleted and the next row drops down into its place to be read out. This coupling of one row to the next is where the device gets its name. Further processing of the signal is required to convert the analog voltage into a digital signal. This A-to-D processing is done on a separate chip.

In order to sample the color information of the light hitting the photodiodes, a series of filters are embedded into the chip so that some photosites are reading red light, some blue and others green.

Now let's look at a CMOS. Like a CCD, a CMOS image sensor is also comprised of a series of photosites arranged in a grid pattern; however, the makeup of each photosite differs from the CCD. A CMOS photosite contains a photodiode for converting photons or light to electrons, but rather than storing the charge and processing the data in another part of the chip (or even outside of the chip), some of the processing is done at the photosite itself. Each photosite contains a converter for changing the charge to a voltage, an amplifier to increase the very low signal coming from the photodiode, and circuitry to reduce noise in the signal. There's a series of grid connections among the photosites in order to read out the data, and while they might be arranged in rows and columns, the design allows for accessing each photosite directly as opposed to reading out row by row.




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