How Line Scan Cameras Work: A Detailed Guide

What is а line scan camera?

Line scan cameras are indispensable tools for efficient and seamless inspections and quality control in industries involving continuous and moving materials, such as paper, steel sheets, textile, glass and various others.

This image recording tool allows for efficient detection of any errors and faults on the moving materials during the production.

Often times, line scanning cameras are the only solution and economically viable method for such error detection and quality control.

Line scan cameras are much more efficient than traditional area scan cameras. They are a less expensive method for making high-resolution images and feed them to the evaluation software for real time processing and viewing.

While line scan cameras are usually used for inspection of surfaces or print images, they are also useful for scanning documents and to sort post via code reading or OCR.

The line scan camera captures a single image line in a fast succession. If two-dimensional images are required, moving the camera or the object is required as well.

How line scan camera works

Line scan cameras can capture images of rotating cylindrical objects without any reflections or distortions.

Unlike area scan cameras which deliver overlapping synchronous or asynchronous sequences of images of the moving material or object, line scan cameras have one single row of light-sensitive pixels which are continuously scanning the moving material or object at a very high line frequency. With line scan cameras, the detection of faults and images is much quicker, easier and more precise which makes it much more efficient for quality control of moving materials.

Encoders are used for avoiding image distortions of the moving conveyors. For conveyors which carry individual objects which need to be scanned, the image acquisition is triggered by a photoelectric barrier which allows for the camera to capture the entire test object rather than its image spreading over two blocks of images.

Line scan camera sensors are available in resolutions starting from 512, 1, 2, 4, 7 and 12k. The pixels on the sensor are sized from 5×5µm, 7×7µm, 10×10µm to 14×14µm. Sensitivity to light is essential for line scan cameras, so in order to enhance their light sensitivity, sensors with two or more lines are used to capture images without losing physical resolution.

Line scan sensors are read out in a very fast succession and on multiple channels.

The images captured are transmitted at a speed depending on the type of camera and the image acquisition card used.

There are variations when it comes to the types and methods of evaluation of the images used. Line scan cameras can provide fixed image block scanning or continuous small image scanning, depending on the needs of the manufacturer or other user utilizing the imaging tool.

The different line scan cameras have very different technical specifications depending on the model and the purpose they serve. There are cameras which are specially optimized for high sensitivity and speed operation.

Others feature a TDI sophisticated sensor technology with a sensitivity 100x more than that of the standard models and methods used. This type of camera needs to be synchronized absolutely precisely with the movement of the conveyor and the objects so that any blurring or distortion is avoided.

There are different types of line scan cameras which are made for color detection and color application as well.

When to Use Line Scan Technology

Line scan technology is best suited for capturing high-speed, high-resolution or large images. This includes quality control of steel sheets, textiles, papers, glass tapes on conveyor lines. It is also highly efficient for providing efficient road and railway inspections, surface topology scanning and for satellite imaging.

This type of imaging technology is also suitable for free falling products, such as molten steel, glass as well as postal sorting.

It can also be used for the inspection of panel displays, silicon wafers, circuit boards, solar cells and other similar tasks where large and clear images are required.

How Line Scan Images are Formed

Line scan sensors feature one or several lines of pixel sensors, each of which accumulates photoelectric charges during the exposure time of the camera. These charges are proportional to the light from the object which is imaged to that pixel. The charges of the entire row of pixels are then transferred to a readout register which shifts them, amplifies, corrects and digitalizes them in order to create camera output.  The maximum exposure and readout rate is referred to as “line rate” and is measured in kHz.

High line rates are needed for freezing fast moving objects.

Lighting for Line Scan Cameras

Light is necessary for capturing the images and for seeing the objects. Lighting and optics are used as optical processors for enhancing the features of the objects in question. Line scan cameras usually use “line light” which comes from a line of LEDs located along the line of the object pixels currently being viewed. These line lights provide high-intensity illumination necessary for the short exposure times.

Line lights can be positioned behind the object which is being viewed in order to provide backlighting, or above the object to provide high angle illumination for detection of surface defects.

Conclusion

Line scan cameras are becoming absolutely essential, relatively inexpensive tools for highly efficient quality control and for a wide variety of inspections, sorting and detection purposes.