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Introduction to Line Scan Cameras

Line scan cameras are semiconductor cameras with a single line of photosensitive pixels that have a multitude of uses in various industrial applications. Line scan cameras can be used for one-dimensional determinations, such as measuring the width of a gap, or for the acquisition of two-dimensional images when in combination with a scanning motion, rather like a fax or scanner.

Line scan cameras offer substantial advantages in opto-sensor and measurement technology applications, including high optical resolution, high speed and the tunable synchronization of individual line scans.

Optical resolution

The optical resolution of a line scan camera is given by the number of pixels, the photosensitive elements of the line sensor. Currently, line scan cameras with up to 12,000 pixels are available.

During a scanning run, the effective resolution perpendicular to the line scan camera is determined by the velocity of the scan and by the line scan frequency, i.e. by the number of line scans per second.

Substantially higher resolutions are reached with line scan cameras in comparison with conventional CCIR video cameras. For rotating drum and conveyor belt systems, line scan cameras offer the additional capability of continuous unending image acquisition in real-time.

Exposure and integration time

Exposure means illumination of the line scan sensor for a set time period. The light sensitive elements of the sensor transform the radiation into freed charges. The time period of accumulation of these charges during one exposure is called the integration time. The accumulated charges are read out of the sensor, pixel-by-pixel, using a scrolling shift register. In continuous mode, the next exposure is simply begun at the time of read-out of the previous exposure and the charges accumulated at each camera pixel are transformed into appropriate voltage values for further use.

Pixel and line frequency

The pixel frequency is the rate at which the freed charges are read out of the sensor and the maximum pixel frequency is a function of each individual sensor.

The minimum exposure period of a sensor is the minimum time required for the read-out of a whole line scan and is dependent on the maximum pixel frequency and the number of pixels (plus a sensor-dependent overhead of passive pixels).

The reciprocal of the minimum exposure period determines the maximum line frequency.

Cameras with integration control can alter the integration time within an exposure period (emulating a shutter mechanism).

The number of pixels and maximum line scan frequencies of the available cameras are listed in the tables.

Interfaces for line scan cameras

Schäfter+Kirchhoff mainly provide digital line scan cameras, although legacy analog line scan cameras can be supplied for backward compatibility. The line scan signal is digitized with 8 or 12-bit resolution directly in the digital line scan camera, which can be supplied with either GigE Vision™, LVDS, CameraLink® or USB 2.0 interfaces for data output.

Schäfter+Kirchhoff now also provides the latest industrial standard interface, GigE Vision™, for monochrome and color line scan cameras, for TDI and the other special-purpose line scan cameras with integrated bright-field illumination. Signal preprocessing, such as thresholding and shading correction, is performed within the camera and does not impinge on CPU use. These cameras work with standard network components, a grabber board is unnecessary and data cables of up to 100 meters long are possible.

Cameras with the LVDS interface are operated by the Schäfter+Kirchhoff SK9192D line scan PCI grabber board, especially developed for line scan cameras. Signal preprocessing, including thresholding, windowing and shading correction are all performed on-board. A Schäfter+Kirchhoff merger box enables the line-synchronous operation of multiple line scan cameras using one grabber board.

Cameras with the CameraLink® interface fit seamlessly into systems where a CameraLink® grabber board is already used.

Cameras with the USB 2.0 interface are the primary choice for mobile applications or those involving multiple PCs, such as transportable measurement systems or educational and scientific experiments operated from a laptop.

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