How to use a line scan camera

Line scan cameras often do not get the deserved attantion in the world of machine vision. Because even in cases where the use of a line scan camera would be technically and economically of advantage, a matrix camera is used.

But despite that, line scan cameras are the only efficient solution for the inspection of endless materials such as steel bands, paper webs and glass sheets, etc.

Therefore line scan cameras such as the EyeCheck ZLS are for example used in the printing industry for the production of paper, when producing steel sheets, as well as in textile production, to detect and classify flaws in the material.

For example steel sheets are used differently according to their quality. The image processing makes it possible that the flawless parts of steel sheet is used for the bonnet and the doors and the sheet with small flaws is used for the underbody.

Line Scan Cameras are the better alternative

Line scan cameras are the better alternative compared to matrix cameras, when inspecting moving sheet material for defects. The reason is a result of the difference those two technologies:

Matrix cameras either capture a fixed (synchronous) or variable (asynchronous) image sequence of a moving object. To capture endless material with this technology and check for defects, means that an overlapping of images is necessary. And that is not enough. The software has to laboriously rectify, cut and string together the images afterwards.

But then why lose time, when line scan cameras offer a cost effictive way to capture high resolution images, which then are passed onto the software on a standard PC for the evaluation.

What is the difference?

a) Only one line and that continously

Line scan cameras only have one line of light sensitive image elements, which scan moving objects continously in a high line (frame) rate. This means that in use the single pixels are read into a parallelly arranged horizontal register and then each pixel is converted into digital values. Those can then be processed by a PC and evaluated with an image processing software.

b) Synchronous with the speed

When using a line scan camera, the object has to move in a reasonable speed below the camera, or in some cases the camera is moved over the object. The challange is to get the same resolution in the direction of the movement (y-direction) as in the direction of the object width (x-direction). Therefore the line fequency of the camera has to match the speed of the object, because otherwise the image is either stretched or compressed.

In practice the band-conveyor change speed according to different influences such as load transfere, brake or speed-up application and therefore do not run with the same speed all the time. This means in most cases that an incremental encoder is used to match the line frequency to the speed of the inspected material. Because only then is an evaluation of the image with the software sensible an exact.

c) Setting for the light

This successfully done, the light is another thing to mind. Because the system as it is now will deliver images in varying brightness, depending on the speed. The reason is the varying exposure time of the line scan sensor with varying line frequency.

The integration time of the sensor remains constant with a timing signal. The configuration is carried out on the frame grabber and is conform with the maximally expected line frequency.

In the end it need two things to work smoothly: steady resolution and steady lighting even when the line frequency changes.

The software that fits the needs

Besides the many hardware components such as lighting, lens and frame grabber, also a powerful image processing software is important. The software is taking on the challenge of evaluating the plentiful data.

The EyeVision software offers high flexibility and accuracy as well as user friendliness. Additionally the EyeVision software supports many hardware platforms.

Intelligent Line Scan Camera

With the intelligent line scan camera EyeCheck ZLS EVT offers the ultimate combination. The camera has an integrated FPGA, high resolution, evaluation directly on the camera, completely without a PC. Also the EyeVision software is already installed an the user gets a complete package: camera, processing unit, evaluation software. As fast and as easy to use as possible.

EVT also has the matching lighting to complete the combination.

This is not only true for line scan cameras. EVT offers its customers the EyeVision image processing software as well as cameras, lenses, lighting and the matching cables and accessories. In short every component for an image processing solution.

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