What Is a Line Scan Camera?
A line scan camera is a camera that captures an object in a line and combines it into a single image.
Area sensor cameras, which are often compared, capture the entire field of view at once.
A line scan camera captures a flat image by continuously moving the object perpendicularly to a row of line sensors, or by moving the camera.
Unlike an area sensor camera, a line scan camera can capture slight pixel-by-pixel changes when acquiring a horizontal image.
Uses of Line Scan Cameras
Objects for which line scan cameras are suitable include those that are large, require high precision resolution, are long and continuous, and have a three-dimensional appearance.
For example, when photographing a large object, an area sensor can be used to take pictures by dividing the object into sections, but in this case, multiple images must be stitched together. On the other hand, using a line sensor, the image can be captured as a single image and does not need to be stitched together.
Specifically, line scan cameras are used for a wide range of purposes, from inspections of social infrastructure such as roads and exterior walls to industrial inspections of non-woven fabrics, gears, semiconductor parts, etc. In addition, they are also used in the analysis of works of art, and in the sorting of fruit that had previously been inspected visually.
Principle of Line Scan Cameras
Like an ordinary camera, a line scan camera converts light entering through a lens into an electronic signal by forming an image on a CCD, CMOS, or other imaging element and outputs the signal as an image.
The camera continuously captures images by moving the subject vertically to the imaging element, which consists of a single row of line sensors. Many images are then combined to obtain a continuous image.
Line scan cameras can be broadly classified into models that can acquire monochrome images and models that can acquire color or invisible ray images.
Models that can acquire color images have about 1~3 rows of line sensors and are multilayered. This is because only one color’s information is available from each sensor.
In a 3-row color sensor, a particular pixel is captured by three image sensors that can acquire blue, green, and red color information. On the other hand, with a single-row color sensor, only a single pixel is captured by a single image sensor, so only a single color’s information is acquired. And since the color information of one specific pixel is estimated from the surrounding color information, the color accuracy is inferior to that of a three-row color sensor.
Selecting Line Scan Cameras
What is important in selecting line scan cameras is to make a total judgment of the resolution, exposure control, high-speed compatibility, sensitivity, and other factors of the target system to be handled.
1. Exposure Control
Old line scan cameras do not have exposure control, and the brightness of the light source is manually changed in response to speed fluctuations. By using an electronic shutter, the exposure time can be automatically changed to capture images at the same brightness even if the speed changes.
2. High-Speed Support
This is judged by throughput, which represents data processing capacity. Cameras with the highest speed level are now commercially available.
3. Sensitivity
Conventional line scan cameras require a strong light source because they can only take an exposure time of one line scan. Therefore, the sensor itself is highly efficient with an aperture ratio of 100%. Some cameras use time-delay integration technology to increase sensitivity by dozens of times or more, making them suitable for locations where light levels cannot be increased or for high-speed scanning.
4. Shading Correction
Modern cameras can compensate for small differences in sensitivity within a pixel in real-time in the camera. With this tool, shading correction, which is the correction of light intensity variation in the width direction due to uneven illumination, can be performed.