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A feature of line scan cameras is that they capture images in a line shape and create a single image. Area scan cameras, which are often compared, are characterized by capturing the entire field of view at once.
With a line scan camera, you can acquire a flat image by continuously shooting while moving the subject vertically with respect to a row of photodiodes .
Compared to area scan cameras, there is no need for image composition when acquiring horizontally long images, so it is good at capturing slight changes that occur in pixel units.
The objects to be photographed, which line scan cameras are good at, include those with a large size and those that require high-precision resolution.
For example, when shooting a large-sized subject, if it is an area sensor, there is a method of splitting and shooting, but in that case it is necessary to stitch multiple images together, and if an inspection is performed to detect defects , The work of connecting becomes very important and difficult because there may be defects in the joints. On the other hand, if you use a line sensor, you can shoot as a single image, and there is no need to connect the images.
Specifically, it covers a wide range of fields, from inspections of social infrastructure such as roads and outer walls, to industrial inspections of non-woven fabrics, gears, semiconductor parts, etc., analysis of fine arts, and selection of fruits that have been visually inspected until now. It can be used for any purpose.
Similar to a normal camera, a line scan camera converts the light coming from the lens into an electronic signal by forming an image on a photographing element such as a CCD or CMOS and outputs it as an image.
A horizontally long image is acquired by continuously shooting while the subject moves in the direction perpendicular to the shooting element consisting of a row of photodiodes.
Line scan cameras are roughly divided into models that can acquire monochrome images and models that can acquire color (or invisible light) images.
In the model that can acquire a color image, the photodiode arrangement has a multi-layer structure of about 1 to 3 rows. This is because there is only one color information available from one photodiode.
A three-row color sensor captures a specific pixel with three photodiodes that can acquire blue, green, and red color information. On the other hand, a single row of color sensors captures a specific pixel with a single photodiode, so only a single color information can be obtained. Therefore, since the color information of a specific pixel is estimated from the surrounding color information, the color accuracy is inferior to that of the three-row color sensor.
Line scan cameras are often compared to area scan cameras. Just like the line scan lens compared to area scan lens.
Whereas the area scan camera shoots the entire specified area at once and images it, the line scan camera shoots on a straight line and makes one image.
Therefore, it is inferior to the area scan camera in terms of the overall number of pixels and performance, but unlike the area scan, the desired image information is limited to the line shape.
Also, if the combination is sufficient, the accuracy will be partially improved and the cost performance will be excellent, so it can be said that it is a scan camera that can be used sufficiently in practical terms.
The important thing in selecting a line camera is to make a total judgment based on the resolution of the target system to be handled, the distance from the workpiece, the amount of light, and so on.
The features and trends of recent line cameras are shown below as a reference when making that decision.
The first is exposure control. Since ancient times, most line scan cameras have no exposure control and have manually changed the brightness of the light source in response to speed fluctuations.
In addition, methods such as changing parameters according to speed were also adopted.
However, by having an electronic shutter, I was able to always take pictures with the same exposure time, and even if there was a change in speed, I was able to take pictures with the same brightness.
The second is high-speed support. Cameras with the highest speed levels are now on the market, including cameras with a throughput of 640 Mbyte / s.
The third is high sensitivity. Conventional line cameras require a stronger light source because they can only take the exposure time for line scans.
Therefore, the sensor itself uses an efficient sensor with an aperture ratio of 100%.
In addition, there is a camera that uses time delay integration technology to improve the sensitivity by several tens of times or more, making it ideal for places where the amount of light cannot be increased and for high-speed scanning.
Finally, shading correction. Modern cameras have the ability to correct small differences in sensitivity within a pixel in real time.
Shading correction, which is the correction of changes in the amount of light in the width direction due to uneven lighting, is also possible using this tool.
For more information, check on https://www.dzoptics.com/en/
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Published on October 21, 2021
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