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Selection of the suitable mv system

Line scan camera solutions

An industrial line scan camera is a special type of camera that has only one line of pixels. Unlike conventional cameras, which have a two-dimensional sensor, a line scan camera only exposes one line at a time.

In order to obtain a two-dimensional image of a surface, several lines are often recorded one after the other in extremely fast time while the camera moves relative to the object. This can be implemented in various ways, e.g. by using a conveyor belt, a linear unit or a robot.

Why line scan cameras?

There are many reasons for using line scan cameras: As a seamless recording method, it is ideal for inspecting longer objects and continuous materials. The price/performance ratio also favours line scan cameras when high resolutions are required. The resolutions range up to 16384 pixels in the X direction and (almost) any resolution in the transport direction.

Typical line scan camera applications:

Endless materials such as glass, steel, fabric, paper or foils, such as those used in battery cell production, can be inspected without interruption.
Even large and long objects such as flat screens and electronic assemblies can be inspected in high resolution. Typical applications include the inspection of surface defects and print inspection.
Line scan cameras are also used for scanning documents and films as well as for mail sorting (OCR, code reading).
For inspection of round bodies without perspective distortion. To do this, the component must be rotated. In this way, a true 360-degree inspection of the surface or print image can be realised.

Image creation with line scan cameras

The camera only records one image line at a time in rapid succession. In order to capture a two-dimensional image, the object must be moved for inspection: Either the object to be recorded is moved using a conveyor belt, etc. (the 'fax principle'), or the camera is moved over the stationary object (the 'scanner principle').

As the image in the Y direction is created by the movement of the conveyor belt, it is important that this movement is very constant. Technically, however, this is almost impossible to achieve, so the feed is synchronised using an encoder to avoid image distortion.

If there are individual items on a conveyor belt, image acquisition can also be triggered: A light barrier or other proximity switch sends a signal to the image acquisition hardware. The lines are only captured or transmitted at that moment, and it can be avoided that the part to be inspected is spread over two retracted image blocks.

Line scan cameras can work very quickly (10-400 kHz) so that they can scan moving objects without motion blur. Due to the short exposure time, line scan cameras require bright line illumination.

Classic line scan cameras

As with 2D image acquisition, a classic line scan camera requires a lens and (line) illumination for correct image acquisition.

There are several sensor types available for image capture. Usually a single sensor line is sufficient to produce a monochrome image. However, several monochrome sensor lines (dual line, quad line sensors) can also be used in order to capture the image several times and therefore more brightly. This reduces the amount of lighting required.

The TDI line scan cameras with up to 192 ‘stages’ are a real speciality. The result is extreme light sensitivity, but also image capture from slightly different angles, which has fewer problems with reflections.

The pixels of the multi-line sensors can also be provided with colour filters. In this way, it is possible to build dual line or triple line colour sensors for colour inspections. Quad line sensors with RGB + infrared are also used for multispectral inspection technologies.

Another speciality is line scan cameras with inGaAs detectors, which enable real SWIR applications such as contamination detection on wafers, high temperature thermal imaging inspections, etc.

Compact Image Sensors

Industrial compact image sensors, also known as line scan bars or CIS bars, acquire image information in the same way as classic line scan cameras. They expose one line after the other while the object moves relative to the camera. However, the device has fixed overall lengths and is mounted at a fixed working distance.

A key component of the Compact Image Sensor is the sensor array, which consists of many fixed-width sensor elements arranged next to each other. The sensor pixels are exposed with the aid of integrated optical elements (GRIN cylindrical rod lenses). These enable a distortion-free 1:1 image of the object on the sensor array.

Homogeneous LED lighting is also integrated directly into the sensor housing. The lighting is always optimally aligned with the sensor and there is no need for time-consuming calibration of external light sources.

Advantages of line scan bars / compact image sensors:

Thanks to its very compact design, the Compact Image Sensor takes up only a fraction of the space required by systems based on conventional line scan cameras.
Compact image sensors offer resolutions of up to 600 or 1200 dpi and deliver crisp, sharp images.
The 1:1 depiction creates a telecentric image. This eliminates shadowing and occlusions, especially at the edges. Measurement applications are also easier to realise.
Simple installation and maintenance, as this is a complete sensor unit with optics and lighting.

Benefits of Line scan cameras

1. High-resolution scans at an affordable price

Line scan cameras offer high-resolution scans in resolutions such as 2K, 4K, 8K and even 16K at comparatively affordable prices. This enables precise and detailed image capture, which is particularly important for quality control and inspection.

2. Ideal for capturing long or continuous materials

Line scan cameras are ideal for capturing webs or other long, continuous web materials. They provide seamless detection and processing without gaps, which is particularly beneficial when inspecting fabrics, paper rolls or metal strips.

3. Ideal for moving objects

The ability to scan moving objects without gaps makes line scan cameras ideal for industrial applications where products need to be inspected on a conveyor belt. This ensures that no detail is lost, even when objects are moving quickly.

4. High scan rates

Line scan cameras offer extremely fast scan rates of up to 200kHz and even 300kHz. This enables fast and efficient image processing, increasing production productivity and reducing inspection times.

5. Homogeneously illuminated images

The constant illumination and angle of incidence during scanning ensures that images are uniformly and evenly illuminated. This minimises lighting problems and provides consistent image quality. This is particularly important when detecting defects or irregularities on surfaces.

6. No image distortion on cylindrical objects

Another advantage of line scan cameras is that they do not cause image distortion when capturing cylindrical objects. This ensures that images are accurate and undistorted, which is essential when inspecting rollers, pipes and other cylindrical products.

7. Excellent colour and spectral information

Line scan cameras can provide excellent colour and spectral information, making them ideal for applications where colour accuracy and spectral analysis are critical. Technologies such as triple/quad line and 3CMOS/4CMOS sensors enable precise colour capture and reproduction. Multi-spectral solutions with up to 12 lines are also available

Potential disadvantages

1. Motion always required

Line scan cameras require the object to be captured to move in order to produce a complete image. This can limit applications and require additional mechanisms.

2. An encoder is usually required

To obtain accurate images, a rotary encoder is often required to accurately capture the position and movement of the object. This can increase the complexity and cost of the system.

3. Precise adjustment and robust construction required

Compared to conventional 2D cameras, line scan cameras require a more robust mechanical design and more precise adjustment to achieve optimal results. This increases the installation effort.

4. Often large camera housings

Line scan cameras often have larger housings than conventional cameras, which can increase space and installation requirements.

5. Larger sensors need larger lenses

Large sensors in line scan cameras often require special, larger lenses such as M42 or F-mount lenses. This can increase the cost and complexity of system integration.

6. Low depth of field

Large sensors and open apertures in line scan cameras often result in a limited depth of field. This can affect image quality and require additional focusing.

7. High speed data acquisition requires special transmission interfaces

The high data acquisition speed of line scan cameras often requires special transmission interfaces, such as CameraLink or CoaxPress, and a PC-based system. This can increase system cost and complexity.

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