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400 500 600 700 800 900 1,00) 1,100 Wavelength (nm)

Fig. 9-15. Spectral Response Function of a Silicon Line Imager. The spectral response Is shown in terms of the output voltage resulting from illumination by energy density vs. the wavelength from 400to 1,100 nm. The imager reaches a quantum efficiency (generated electrons per incident photon), denoted by tj, above 50% between 400-800 nm. Outside this wavelength region the quantum efficiency drops to small values making the imager less suitable above 900 nm.

400 500 600 700 800 900 1,00) 1,100 Wavelength (nm)

Fig. 9-15. Spectral Response Function of a Silicon Line Imager. The spectral response Is shown in terms of the output voltage resulting from illumination by energy density vs. the wavelength from 400to 1,100 nm. The imager reaches a quantum efficiency (generated electrons per incident photon), denoted by tj, above 50% between 400-800 nm. Outside this wavelength region the quantum efficiency drops to small values making the imager less suitable above 900 nm.

Area array imagers, or matrix CCD imagers, provide an alternative to line imagers. The principles of operation are essentially the same as line imagers. Area array imagers offer the advantage of undistorted geometry within the image. A disadvantage compared to line imagers is the possible smear effect during frame transfer. There are a variety of read-out techniques to compensate for the smear effect Matrix imagers can also suffer from a relatively poor fill factor of pixels in the array. Table 9-11 summarizes line and matrix imager capabilities for current systems that are at least partially space qualified.

When radiometric performance of the optical instrument is paramount, we use time delay and integration (TDI) methods. TDI describes an imaging principle that uses the image motion along the rows of a matrix imager to extend, the integration time. Integration time is extended by electronically shifting the integrating pixel cell synchronously to the movement along the row. The signal-to-noise ratio of this concept is improved by the square root of the number of TDI stages. The primary advantage of TDI imager systems compared to line imagers is the improved signal-to-noise ratio. The disadvantage is the increased requirement for spacecraft attitude and orbit stability (due to the required synchronization of the shifting pixel).

We classify and select infrared detectors according to their spectral band of operation and a figure of merit called specific detectivity or quantum efficiency for photon detectors. The operating temperature of the detector dictates the cooling requirements

TABLE 9-11. Characteristics of Imagers. Typical parameters for available line and matrix Imager systems. Photo response nonuniformity Is the difference between the most and least sensitive element under uniform illumination. Dark signal uniformity Is equivalent to photo response nonuniformity, but without Domination. Dynamic range Is the saturation exposure divided by the rms noise-equivalent exposure. Read-out speed Is given in million samples per second (Msps) per output port

TABLE 9-11. Characteristics of Imagers. Typical parameters for available line and matrix Imager systems. Photo response nonuniformity Is the difference between the most and least sensitive element under uniform illumination. Dark signal uniformity Is equivalent to photo response nonuniformity, but without Domination. Dynamic range Is the saturation exposure divided by the rms noise-equivalent exposure. Read-out speed Is given in million samples per second (Msps) per output port

Characteristic

Line Imager

Matrix Imager

Pixels

6,000 - 9,000 pixels

Up to 1,024 x 1,024 Image pixels In frame transfer mode

Photo response nonuniformity

5%

5%

Dark signal nonuniformity

5%

5%

Dynamic range

10,000

5.0(H)

Limitations on read-out speed

-10 Msps

For example, 4 ports each at 20 Msps

for the sensor focal plane. Infrared sensors often have nonnegligible time constants for response with respect to integration time. Because of technical difficulties with combining detectors and read-out structure, the total number of pixels in an IR detector array is limited in practice to several hundred.

We detect infrared wavelengths with thermal detectors or photon detectors. Thermal detectors exploit the fact that absorbed heat raises the temperature of the detector, which changes its electrical characteristics. The advantage of thermal detectors is uniform response with respect to wavelength. Thermal detectors can also be operated at ambient temperatures, although they have lower sensitivity and slower response times.

Photon detectors use absorbed photons to generate charge carriers. These systems offer the advantages of higher sensitivity and shorter time response, but they must be operated at low temperatures.

Infrared detectors are often rated by the specific detectivity, D*, given by

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