CCD Primer

Bracket Pulsing
CCD Grading
Cosmic Rays
Dark Current
Deep Depletion CCD
Detection Modes
Dual Capacity Mode
Dual Readout Mode
Dynamic Range
Etaloning in CCDs
UV Extension
Fiber Optics
Flat Fielding
Full Well Capacity
Image Calibration
Imager Architectures
Image Intensifiers
Kinetics Mode
Matching Resolution
MPP Mode
Noise Sources
On-chip Multiplication Gain
Open Poly CCD
Optical Window
Quantum Efficiency
Readout vs Frame Rate
Reducing Dark Current
Saturation/ Blooming
Signal to Noise Ratio
Spurious Charge
XP Cooling


CCD and CMOS Imager Architectures

Charge Coupled Device (CCD) detectors come in three major readout architectures, Full Frame (FF), Frame Transfer (FT) and Interline (IL). Each of these formats has certain advantages as well as limitations that will be considered here. This discussion will focus on the most common forms of these CCD types which are single output devices that can be run as low noise CCD detectors.

The Full Frame CCD
The Full Frame CCDs are devices in which the total area of the CCD is available for sensing incoming photons during the exposure period. During readout of the CCD, charge is shifted sequentially across the array necessitating the use of a shutter to prevent smearing for almost all exposure lengths. (Technically,if the exposure time is much longer than the actual readout rate, then the level of smearing can be quite small.) This format has 100% fill factor, which means that 100% of each pixel area is being utilized to detect photons during the exposure.

The frame transfer CCD
The frame transfer CCD imager has a parallel register divided into two distinct areas. The upper area is the image array, where images are focused and integrated. The other area, the storage array, is identical in size and is covered with an opaque mask to provide temporary storage for collected charge. After the image array is exposed to light, the entire image is rapidly shifted to the storage array. While the masked storage array is read, the image array integrates charge for the next image. A frame transfer CCD imager can operate continuously without a shutter at a high rate.

The Interline Transfer CCD
The interline transfer CCD has a parallel register that has been subdivided so that the masked storage area fits between columns of exposed pixels. The electronic image accumulates in the exposed area of the parallel register, just as it does in the frame transfer CCD. At readout, the entire image is shifted under the interline mask. The masked pixels are read out in a fashion similar to the full frame CCD.

Other CCD Architectures

Spectral Framing
In a mode particularly suited for spectroscopy, the CCD is masked so that only a single row of the parallel register is exposed. In this mode, one dimensional line images can be acquired at very high speed until the parallel register is filled up. Spectral framing CCDs are used in time resolved spectroscopy. An observation could consist of hundreds of individual spectra, distributed over time.

Fast Framing Mode
The frame transfer concept can be extended to multiple frames by masking most of the parallel register and using only a small region as the image array. A scene is focused on the image array and a high speed shutter or strobe light is used to time the exposure. After each exposure, charge from the image array is quickly shifted under the mask and a new image can be acquired. Once the parallel register is filled with images, it is read out. Because fewer rows are clocked to shift the image array into storage, this mode works much faster than standard frame transfer.

Time Delay Integration
Time delay integration (TDI) is an integration and readout mode which allows the acquisition of long swaths of a moving imaged. For example, a moving image is focused on an unshuttered CCD imager. The parallel register is clocked in step with image motion, so that charge packets always correspond to the same image region as they move across the parallel register. Charge accumulates and signal strength increases as the pixels approach the serial register. When pixels reach the serial register, they are transferred out, digitized, and stored in the normal fashion. The exposure time for each pixel is exactly the length of a full parallel shift sequence, which is determined by the velocity of the scene. Compared to a simple exposure, TDI increases sensitivity in proportion to the number of rows in the parallel register.