Time delay and integration

A time delay and integration or time delay integration (TDI) charge-coupled device (CCD) is an image sensor for capturing images of moving objects at low light levels. While using similar underlying CCD technology, in operation it contrasts with staring arrays and line scanned arrays. It works by synchronized mechanical and electronical scanning, so that the effects of dim imaging targets on the sensor can be integrated over longer periods of time.

TDI is more of an operating mode for CCDs than a separate type of CCD device altogether, even if technical optimizations for the mode are also available. The principle behind TDI—constructive interference between separate observations—is often applicable to other sensor technologies, so that it is comparable to any long term integrating mode of imaging, such as speckle imaging, adaptive optics, and especially long exposure astronomical observation.

Detailed operation

It is perhaps the easiest to understand TDI devices by contrast to more well-known types of CCD sensors. The most well-known type probably is the staring array one. In it, there are hundreds or thousands of adjacent rows of specially engineered semiconductor which react to light by accumulating charge, and slightly separated in depth from it by insulation, a tightly spaced array of gate electrodes, whose electric field can be used to drive the accumulated charge around in a predictable and almost lossless fashion. In a staring array configuration, the image is exposed on the two-dimensional semiconductor surface, and then the resulting charge distribution over each line of the image is moved to the side, to be rapidly and sequentially read out by an electronic read amplifier. When done fast enough, this produces a snapshot of the applied photonic flux over the sensor; the readout can proceed in parallel over the several lines, and yields a two-dimensional image of the light applied. Along with CMOS detectors which sense the photocharge accumulation pixel by pixel instead of moving the charge out line by line, such sensors are commonly known as parts of digital cameras, very small and very large.

A scanning array on the other hand involves just one such CCD line, or at most a couple of them. Its principle of operation is to rely on mechanical scanning, so that a single linear CCD element gets exposed to different parts of the object to be imaged, sequentially. Then the whole image is assembled from equally spaced lines through the field of view. Typical examples of this scanning mode are fax machines and other document scanners, where the imaging target is fed through at a constant linear velocity, and satellite sensing, where the constant orbital velocity of a satellite naturally exposes line after another of the underlying terrain to the transversely positioned sensor.

The advantage of using a CCD sensor this way is reduced complexity, and so price, or vice versa the possibility of utilizing much more refined and so more expensive CCD technology for the single line sensor array, for higher fidelity. CCD's can also be manufactured in configurations which are tolerant to the wide fluctuations in radiation and temperature, characteristic of space environments, and scanning one can be made extra robust by the inclusion of multiple lines. Since the out-clocking mechanism of a well-phased CCD line is a continuous process, not divided to pixels, the eventual line-wise resolution of the image can also exceed the resolution of the gating infrastructure, leading to higher resolution than a pixel based sensor.

At the same time, that continuous operation and discrete readout by line also leads to a problem: if anything moves within the scene to be imagined, there will be blurring. Wherever some accumulated packet of charge within a CCD line is moving on the sensor chip, any extra light on it will lead to more charge, even if it comes from a wrong direction, or a newer moment of acquisition than intended, it'll also register just the same. It will integrate over time. This leads to what is in the cinematic arts called motion blur.

In TDI mode, motion blur and the pseudo-analogue nature of CCDs is turned from a fault into an asset. The line or 2D array is turned so that the lines in the CCD sensor follow the expected trajectory of the object in the field of view, and the rotation of the sensor platform is adjusted to keep the viewing of the object constant. This way, photons landing on the sensor sum (integrate) over time, instead of spreading out. With the high sensitivity of CCD sensors, into the photon counting regime, this can lead lead to extremely low detection and measurement sensitivity.

Specific technology for TDI

While the basic theory of TDI only mentions single row CCDs, specifically designed parts and algorithms utilize everything from a few lines to entire staring arrays, with integration taking place over multiple lines, in software, as well. A designated TDI CCD improves upon the single-line-scan system by adding up multiple measured photocharges over its more complicated sensor, and by more comprehensive analysis of the interaction between continuous lines and discrete column structure.

Since CCD technology is used even in x-ray astronomy where imaging losses are high, calling for TDI, and the imaged particles are high enough in energy to immediately ionize and otherwise degrade the imaging machinery, a lot of effort in the field also goes for radiation hardening.

Applications

TDI CCD is especially used in scanning of moving objects, for example letter and film scanning, or from a moving platform, for example aerial reconnaissance.

• Astronomy
• Medical radiography
  • angiography
  • mammography
• Military imaging
  • High-altitude surveillance
  • Low-Light level observation