Line Scan Cameras – Applications & Benefits

Line Scan Cameras – Applications & Benefits

Line scan cameras offer excellent, high-resolution images as well as the capacity to image large objects.

A line scan camera photographs an object with a single line of pixels. Either the camera or the object is moved perpendicular to the line of pixels to create a two-dimensional image of the item. When compared to "frame cameras," which take two-dimensional images (a "frame"), this method of imaging an object may appear sophisticated. A line scan camera, on the other hand, is typically a far better alternative than a frame camera when the item is huge, moving, or the task requires perfect or high resolution images.

Assume a machine vision system is used to inspect lengthy rolls of material for flaws, such as paper or plastic. To detect these flaws, a resolution of 16,000 pixels across the material is required. A rotary encoder connected to the unrolling machinery measures the material's speed as it is unrolled under the camera.

Four large-format frame cameras positioned across the web of material could be used, but this has drawbacks.

To begin with, large-format frame cameras are prone to have broken pixels, which can obscure the flaws you're looking for. Second, the exposure duration of the frame camera must be short to avoid blurring caused by material motion. To create a good image from a brief exposure, you'll need a lot of high-intensity lighting all across the imaged region. Finally, synchronizing the four cameras optically is a challenge. Finally, frame cameras make it difficult to remove offset and gain changes in pixel responses, which are required to detect low contrast problems.

A single line scan camera with 16,000 pixels is a much better option. One camera now encompasses the entire material, with no pixels that are faulty. Each line of the image is acquired using a motion encoder. To eliminate motion blur, the line exposure duration is short, but the illumination can now be focussed into a single line rather than being spread out across the frame. Time Delay and Integration (TDI) line scan cameras (described below) can be utilized if higher sensitivity is required. Finally, each pixel's gain and offset are rectified in the camera to ensure a consistent response across the line of pixels.

When To Use Line Scan Technology

Line scan technology is suitable for continuous web applications like paper, textiles, metal, or glass inspection, where big, high-resolution, high-speed picture capture is required. Inspection of flat panel displays, solar cells, and printed circuit boards, for example, requires line scan technology since big, defect-free images are required.

When the product speed is high, line scan cameras are preferable than frame cameras because you may compensate for short exposure times using concentrated illumination or a Time Delay and Integration (TDI) camera to enhance the number of photons "harvested." A line scan camera calculates gain and offset correction in the camera, but a frame camera computes this correction using the vision system's CPU, which is slower.

Line scan cameras VL series

Vieworks VL Series

High Sensitivity & High Speed Line Scan Cameras

Time Delay and Integration

Line scan cameras with Time Delay and Integration (TDI) feature 2 to 256 vertical rows of pixels. In synchronization with the product motion, photoelectrons from each pixel in each row are added together in the row of pixels "below." The object's movement rate, which is normally indicated by pulses from a motion encoder, is used to drive the shifting and summing. The exposure duration is essentially multiplied by the number of rows of pixels in TDI cameras, resulting in high-contrast images even when the exposure period is short.

Line scan cameras VT series

Vieworks VT Series

High Sensitivity & High Speed TDI Line Scan Cameras

Colour Line Scan Cameras

To detect light of different wavelengths, colour line scan cameras include rows of sensor pixels with distinct colour filters. Although most filters are what we think of as red, green, and blue (RGB), certain applications use other filter types. For evaluating printed materials, colour line scan cameras are frequently employed. The camera lines up the different colour rows. It is recommended to have the camera view perpendicular to the product's surface and use a lens with low chromatic aberration.

Line scan camera VTC series

Vieworks VTC Series

2k Colour TDI Line Scan Cameras

Lighting for Line Scan Cameras

To illuminate the object along the line of object pixels being viewed, line scan cameras often utilize a "line light," or focussed light from a line of LEDs. Short exposure times necessitate the use of line lights, which provide high-intensity illumination (fast camera line rates). Line lights can be "butted" together to create extremely long lights for seeing objects with a large field of view.

Selecting the Right Line Scan Camera for Your Applications

When selecting a line scan camera, consider the following factors:

Sensitivity

"Is the camera capturing enough photons to finish the machine vision task? "Sensitivity" inquires. It's often difficult to answer this issue only on the basis of component specifications. In practice, we estimate the required light intensity, specify more than our estimate, and then test for sensitivity.

Size (in sensor pixels)

You must specify the field of view (FOV) and the minimal defect size to determine camera size (in sensor pixels). The camera should have enough resolution to "cover" the minimum product defect size with at least 3 or 4 pixels. If the FOV is 12" and the minimum defect size is 0.005", for example:

(FOV/minimum defect size) x (3 pixels coverage)
(12/0.005) x 3 = 7200 pixels

An 8K (8,192 pixel) line scan camera could be used.

Line Rate

The FOV, part speed, and object pixel size all influence the line rate. If your FOV is 12", your product speed is 60", and you're utilizing an 8K (8,192 sensor pixels) camera, for example:

(Object pixel size) = FOV/(camera size in sensor pixels) =
Object pixel size in the FOV: 12/8192 = 0.001465"
Line rate needed: 60"/0.001465" = 40.956 kHz

Line Scan Camera Synchronization

The movement of the item must be timed with the exposure of the line scan camera. This is commonly accomplished with an encoder that generates a pulse for each unit of object movement. The line scan camera is activated to take a line image after a certain number of encoder pulses.

In most cases, we desire "square pixels," or pixels in the field of vision that are the same width and height. The camera must be activated every time the object moves a distance equal to the item's pixel size in order to acquire square pixels. Because the pixel size in the field of view in the previous example is 0.001465 inches, the camera must be triggered every time the object moves that distance for square pixels.

Pixel Correction with Line Scan Cameras

The gain and offset of each pixel in a line scan camera can be modified by hardware in the camera. This can adjust for pixel response variations, such as fluctuations in lighting intensity and pixel responses.

Summary

Line scan cameras are used in vision applications that require high-resolution, high-speed, or perfect images. Inspecting web materials, such as in plastic or paper manufacturing, inspecting continuous "objects," such as road and rail inspection, or where high-quality images are required, such as in LCD flat panel or printing inspection, are all applications where line scan cameras are the best choice.

Find out more on Line Scan Cameras or contact us about your machine vision project.