Line scan camera

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A line camera is a type of camera that has only one light-sensitive line ( line sensor ) - in contrast to the two-dimensional sensor, which has a large number of lines. Although line sensors have roughly the same pixel sizes as area sensors , the line length can be much larger. Instead of the maximum 1,000-4,000 pixels wide for area sensors, a line camera can have 17,000 pixels or more. Consequently, a line camera delivers a better spatial resolution for a given object field. In addition, a line can be read out much faster than an area. However, due to the usually much higher clock rates compared to an area camera, there must be significantly more light.

Line sensors

As with area cameras, silicon sensors are predominantly used. Until a few years ago, mainly CCD and NMOS sensors were used, which have recently been supplemented or partially replaced by CMOS sensors.

Spectral sensitivity

While the sensitivity range of sensors based on silicon (Si) extends from the UV range (from around 200 nm) to the near infrared (up to 1000 nm), sensors made of germanium (Ge) or indium gallium arsenide ( InGaAs ) are in the infrared Wavelength range from 1 µm to over 2 µm can be used.

The UV sensors are usually provided with a cover glass made of quartz , because normal glass is no longer permeable to radiation with a wavelength below about 400 nm.

pixel

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

functionality

The CCD, CMOS, NMOS or InGaAs sensors used in the line scan cameras sometimes differ considerably, but at least two steps are always carried out, exposure and reading, which are explained using the example of a CCD sensor: A CCD line sensor consists of the light-sensitive pixels and an analog shift register ( charge-coupled device ), which gives the sensor its name. During the exposure time, the incident photons generate electrons in the pixels , which are reloaded into the shift register at the end of the exposure time. This is read out via a clock signal applied to the sensor . While the charges are read from the shift register and converted into a voltage , new electrons are collected in the pixels as a result of the sensor's exposure. The signals read out do not come from the current exposure cycle, but from the last one.

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

In addition to the required clock signals, line scan cameras sometimes contain an analog-to-digital converter that digitizes the analog signal from the sensor . The digital data is sometimes cached in a RAM so that the image can be transmitted as a whole. The image transfer to the computer then usually takes place via USB , Ethernet or CameraLink .

Areas of application

Aerial cameras

Aircraft, and especially satellites, move steadily at a known speed. Image lines that are recorded one after the other by a line camera can then be put together to form a two-dimensional image. For example, the Mars Express high-resolution camera acquires its image information line by line.

The line-by-line scanning of an area is referred to as a push broom , based on a broom that sweeps along the ground like a line scanner along its swath . A whisk-broom scans an image point by point. The cameras of the MTG weather satellites work according to this principle.

Decomposition of light into its spectral colors (spectroscopy)

If there is enough light, good results can be achieved in the visible range from 400 to 800 nm with simple and inexpensive line scan cameras with 14 µm × 200 µm pixels.

In order to be able to determine the amount of light as precisely as possible, the number of electrons collected must be as large as possible, because the noise is determined by the square root of the number of electrons. Inexpensive CCD sensors can store around 100,000 electrons in the pixels, high quality CCD sensors 600,000 electrons. NMOS sensors have 50,000,000 electrons.

The choice of sensor therefore depends on the requirements: CCD sensors are used for small amounts of light, precision measurements require a minimum of light intensity in order to sufficiently fill the pixels of the NMOS sensors with electrons.

In order to keep the dark current low during long exposure times , the sensors of high-quality line scan cameras are cooled to around −10 ° C and sometimes well below it.

Line scan cameras for spectroscopy do not require an objective because the imaging is done via the spectrometer .

Barcode, fax and scanner

Line scan cameras are widely used in barcode readers, fax machines and scanners . In the case of the latter devices, the two-dimensional image is composed of a large number of individual lines. With line sensors with over 10,000 pixels that are now common, a DIN A4 template with 10,000 × 14,000 pixels, i.e. 140 megapixels, can be resolved. The pixels of the sensors are here in most cases rectangular.

For reasons of cost, an inexpensive plastic lens is usually used for optical imaging .

Industrial image processing

Line scan cameras are also being used more and more in industrial image processing . Similar to earlier fax machines, it is mostly not the sensor line that is moved here, but the object. So that the vertical image scale is the same as the horizontal image scale , the scanning rate of the lines (100 Hz to 100 kHz) must be adapted to the speed of the passing objects.

The mostly high-quality lenses often have to be adapted to the special tasks. As a matter of principle, the representation is orthogonal in the axis of motion. In order to obtain this distortion-free representation in the other image axis as well, telecentric lenses are used - at least in metrological applications .

Image processing systems based on line scan cameras are used for quality assurance tasks as well as sorting processes. The possibility of using high object speeds means that very high throughput rates can be achieved. In addition, line scan cameras are often used for objects that require a distortion-free display, for which a telecentric lens cannot be used for reasons of space or lighting (for example for large, elongated objects).

Because the image only has to be illuminated at the sensor line, lighting can be used much more specifically than with an area camera, in which the entire object has to be illuminated. You can also record two lighting situations at the same time by switching the lights alternately on each line and then separating them into two images again using software.

Web links

Individual evidence

  1. ↑ Data sheet CLS Controller ( Memento from January 7, 2009 in the Internet Archive )