Frame Transfer and Interline CCD - Electronic Shutter in CCDs

Image devices with a CCD sensor, such as digital cameras and camcorders, can readily implement an electronic shutter. Such electronic shutter does not require extra circuitry, because the way a standard CCD is operated already has an inherent shutter. However, if a fast shutter speed is required, the CCD integrated circuit must be enhanced in some way.

Full-frame CCD

The full-frame is a CCD in its simplest form. It is made of an array of photosensitive elements (pixels) where electrons are created by incoming photons from the time we press the shutter release button throughout the exposure time. Then a shifting phase occurs, shifting these electrons one row at a time to a sensing circuit producing a voltage proportional to the number of electrons. The shifting takes place as follows. The electrons of the first row are shifted into an array of so-called serial registers placed at the edge of the CCD array; the electrons of the second row are shifted to the first row and so on. At this point, the above-mentioned serial register shifts its content into a sensing output amplifier one pixel at a time, converting the electrons' charge to a voltage. Once all the pixels of the first row are read by the output amplifier, the shifting phase takes place again and the whole sensing process is repeated. This is so until all the pixels in the matrix are read out. This mechanism has an inherent electronic shutter in it, in that the exposure is over once all the matrix array has been shifted out and read. On the other hand, the exposure starts with an electronic reset of the CCD, during which all electrons in the array are swept away. Therefore, a mechanical shutter is not strictly necessary.

What makes all this useless, for common shutter speed used in photography, is its slowness. The shifting of a row, its reading one bit at a time, and repeating this process for all the thousands of rows present in a CCD is very time consuming. As an order of magnitude, the time required to read a row is about 130us. Reading 2500 lines, as for an 8 megapixel camera, would require 130us*2500 = 325ms. This is the time necessary in order to read the whole array and corresponds to 1/3 of a second. Not only this is a very slow time for photography, but it is not the maximum shutter speed achievable, neither. Indeed, while reading the first row through the charge detection output amplifier, all the other rows are still in the CCD's array and collecting incoming photons. Their exposure is therefore longer than the first row. The last row is read 325ms after the first one, so it is exposed to light 325ms more with respect to the first row. Besides, each row is in a different position within the matrix at each shifting phase, so smearing occurs. Hence, a practical shutter speed must be much slower than those 325ms. This mechanism is simply too slow to be used as a useful shutter.

Frame-transfer CCD

A faster solution is attained through frame-transfer CCDs. They break up the process of shifting and reading the array into two parts. The array is duplicated: one part (photosensitive or image array) acts exactly as the standard CCD array collecting incoming photons, while the second part acts just as a temporary storage area (storage array). The storage array is shielded from light, so that no electrons are generated by incoming photons. The timing is the following. At the end of the exposure, all the electrons in the image array are shifted (transferred) to the storage array. Only when all this shifting is over the reading phase begins. So, instead of shifting one row at a time and reading it pixel by pixel as in full-frame CCDs, in frame-transfer CCDs all the rows are shifted in a transient area altogether and only then they are read. The reading, that is the conversion of the number of electrons into a voltage, is done as it normally would in a standard full-frame CCD. It takes place in the storage array, however, and not in the photosensitive one. The rows of the storage array are so shifted one by one and read pixel by pixel. The advantage of this solution is that the transfer of the electrons from the photosensitive to the storage array is pretty fast and the longer reading phase is postponed. Once all the electrons are transferred into the storage array, the exposure is over, because it is optically shielded.

As an order of magnitude, a row can be shifted by one place in roughly 100ns, which is one thousand times faster than the full-frame architecture! This time is fast enough to provide a useful shutter for camcorder application, where small CCDs are utilized. For instance, a typical 754x484 CCD would require 100ns*484 = 48.4us which corresponds to 1/20000s. Again, this is not the faster attainable shutter speed for the same reasons of the previous paragraph. However, it is fast enough to offer a 1/1000 - 1/30 electronic shutter with sufficient acquisition rate.

The drawback of frame-transfer over full-frame CCDs is evident. As the array is duplicated, the area is doubled and this implies higher cost.

Interline CCD

What about an even faster shutter speed? Interline CCD still have a photosensitive and a masked storage array, but they are interlaced, so that each storage row is adjacent to its photosensitive counterpart. Photosensitive and storage rows are alternated. This means that just one shift is required in order to store the electrons safely from the photosensitive into the storage light-shielded array, instead of a number of shifts equal to the number of the rows. For instance, for an array with 2500 rows as in the previous example, the interline CCD offers a 2500 times faster shutter than a frame-transfer one! Any practical shutter speed is attainable with such a structure, independently on the number of rows in the sensor. The slow reading phase is then accomplished exploiting the storage array, just as in frame-transfer. Interline CCDs share the same drawback of the frame-transfer: they take a large area thus rising costs.

By Andrea Ghilardelli

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