Push broom scanner

A line scan camera is known a type of camera having only a light-sensitive line ( line sensor ) - in contrast to the two-dimensional sensor, which has a plurality of rows. Although line sensors have approximately the same pixel size as surface sensors, the line length can be much larger. Instead of maximum 1-4 k pixel width for area sensors, a line camera having 17 k pixels or more. Consequently, a line scan camera for a given object field provides a better spatial resolution. In addition, a line can be read much faster than a plane.

• 3.1 Aerial Cameras
• 3.2 splitting of light into its spectral colors ( spectroscopy)
• 3.3 Barcode, Scanner & Fax
• 3.4 Machine Vision

Line sensors

As with the surface cameras mainly of silicon sensors are used. Until a few years CCD and NMOS sensors were mainly used, supplemented lately by CMOS sensors or partially replaced.

Spectral sensitivity

While extending the sensitivity range of the sensors based on silicon (Si) from the UV region ( from about 200 nm ) to the near infrared ( 1000 nm ), sensors are made ​​of germanium (Ge), or indium gallium arsenide ( InGaAs) in the infrared wavelength range of from 1 microns can be used up about 2 microns.

The UV sensors are usually provided with a cover glass made ​​of quartz, since ordinary glass for radiation with a wavelength below about 400 nm is no longer permeable.

Pixel

The number and shape of the pixels depends on the requirements of the application. There are cameras with monochrome and color sensors. In these sensors, a separate line with the corresponding color filter for each basic color usually available.

Operation

The CCD, CMOS, NMOS or InGaAs sensors used in the line scan cameras, although differ sometimes considerably, but there are always at least two steps are carried out: exposure and readout to be explained here using the example of a CCD sensor: A CCD line sensor composed of the light-sensitive pixels, and an analog shift register ( charge-coupled device ), that gives the sensor its name. During the exposure time, the incident photons generate electrons in the pixels that are transferred at the end of the exposure time in the shift register. This is read on a scale on the sensor clock signal. While the charges are read out from the shift register and converted to a voltage, new electrons are accumulated in the pixels by the exposure of the sensor. The read-out signals thus do not originate from the current exposure cycle, but from the past.

Simple CCD and CMOS line sensors only need two inputs: a start of scan input (SOS ) and a clock input. A simple line scan camera can therefore be implemented with a few components. With affordable driver blocks the effort is reduced to a minimum.

Line scan cameras included in addition to the required clock signals, at times an analog -to-digital converter which digitizes the analog signal from the sensor. The digital data is sometimes cached in RAM and then usually routed via a USB or Camera Link interface to the computer.

Areas of application

Aerial Cameras

Aircraft and in particular satellites move uniformly at a known speed. Image lines, which are received by a line camera in succession, can then be combined to form a two-dimensional image. For example, wins the Mars Express High-definition camera their image information line by line.

The row-wise scanning a site is referred to in English as a push -broom, based on a broom that sweeps along on the floor like a line scanner along its swath. A whisk -broom scans an image from pointwise. The cameras of the MTG weather satellites operate on this principle.

Splitting of light into its spectral colors ( spectroscopy)

With enough light can x 200 micron sized pixels good results are obtained even with simple and inexpensive line scan cameras with 14 microns in the visible range of 400-800 nm.

In order to determine the amount of light as closely as possible, the number of electrons collected should be as large as possible because the noise is determined by the square root of the number of electrons. Cheap CCD sensors can be stored in the pixels of about 100,000 electrons, high-quality CCD sensors 600,000 electrons. NMOS sensors come on 50,000,000 electrons.

The choice of sensor therefore is dependent on the requirements: For small amounts of light CCD sensors are used, precise measurements require a minimum light intensity, in order to fill the pixels of the NMOS sensors with sufficient electrons.

In order to minimize the dark current at long exposure times, the sensors are in high quality line scan cameras to about -10 ° C and sometimes cooled far below.

Line scan cameras for spectroscopy require no lens, because the picture on the spectrometer takes place.

Barcode, Scanner & Fax

Widely used are line scan cameras in barcode scanners, fax machines and scanners. In the latter devices, the two-dimensional image from a series of individual lines is assembled. In the meantime usual line sensors with over 10,000 pixels can be one A4 template with 10,000 x 14,000 pixels, so dissolve 140 megapixels. The pixels of the sensors are here rectangular in most cases.

For optical imaging an inexpensive plastic lens is used mostly for cost reasons.

Industrial image processing

Even in industrial image processing more rows cameras. Here is not the line sensor, but the object is moved similarly to the previous facsimile. So that the vertical is equal to the horizontal magnification, the sample rate of the lines ( 100 Hz to 100 kHz) has to be adapted to the speed of the passing objects.

Most high quality lenses have to be adapted to the specific tasks often.

Image-processing systems that are based on line-scan cameras, to be used both for quality assurance tasks and sorting methods. With the ability to use high object speeds, very high throughput rates can be achieved.