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What Is a Line Scan Camera and How Does It Work?

Industry Knowledge 2400
Area Scan Camera CMOS Sensor VS Line Scan Camera Sensor
Area Scan Camera CMOS Sensor VS Line Scan Camera Sensor
Area Scan Camera VS Line Scan Camera
Area Scan Camera VS Line Scan Camera

Imaging principle

Imaging principle
Number of CMOS photo lines and Final number of image lines
Number of CMOS photo lines → Final number of image lines

How does a line scan camera capture an “area scan” image?

line scan camera capture an area scan image
  • Each image is stitched from each line of images
  • Each line of images is captured by a line scan camera (1-256 lines) and then obtained after image processing.

Area scan camera take a picture for area
Area scan camera: take a picture for area
Line scan camera take a picture for line
Line scan camera: take a picture for line

Shooting direction of line scan cameras

  • Frame rate is the camera’s photo-taking speed, usually defined by the number of images taken per second. Its unit is FPS, which is Frame per Second.
  • Line rate is actually a special frame rate: for CMOS with only a few lines of pixels, the shooting speed is very fast, reaching tens of thousands to hundreds of thousands of pictures per second. However, because the picture taken approximates a line and can’t be used directly, to avoid misunderstanding, it is not called frame rate, but line rate.
linear image

  • Theoretically, line scan cameras can shoot infinitely, and through stitching, we can get an infinitely long image.
  • Infinite length means infinite size. No computer can handle this amount of images and data.
  • Therefore, in practical applications, it is necessary to capture an image after taking X lines according to application requirements.
  • This X lines is the line height, which is how many pixels are “high” in this image.
Line rate means a 1-second line height
Line rate: 3000 lines/s. Line rate means a 1-second line height.

1)Line Trigger

  • The line scan camera captures one line of image after receiving the line trigger signal.
  • Line trigger is suitable for situations where the motion speed is not very uniform or where some special imaging techniques are required, such as long and short exposure HDR, time-sharing strobe, etc. There all require the use of line trigger.

2)Frame Trigger

  • Line scan camera captures one frame of image after receiving the frame trigger signal. The height of this frame can be customized. After receiving the frame trigger signal, the line scan camera continues to take pictures at the set line rate until the specified line height is reached.
  • Frame trigger is more suitable for objects in uniform motion.

What is ideal line rate
  • Customer required accuracy. When customers say the accuracy needs to reach 0.xmm, they mean that the captured image should be able to distinguish a pattern with a length or width of 0.xmm. To meet this requirement, it usually takes 2-5 pixels to present this 0.xmm. This way, when the image algorithm or the human brain sees this image, it can easily find, recognize, and detect the 0.xmm pattern.
  • Pixel accuracy. As shown in the figure below, pixel accuracy refers to the length of a segment of the object being captures, represented by the number of pixels projected onto that target. The distance of one pixel from the object determines the length (accuracy) it can express: it can be 0.1mm, several meters, or even several light-years (Webb Space Telescope).
  • The pixels are all square, so it also means “X direction accuracy = Y direction accuracy”.
  • Ideal line rate = motion speed/pixel accuracy. For example, there is an object, motion speed 2000mm/s, pixel accuracy 0.1mm, then its ideal line rate is 2000mm/s / 0.1mm = 20000/s, that is 20k(another meaning of this formula, how many 0.1mm segments can be divided from 2000m).
  • Resolution. If the object is 1000mm wide and the pixel accuracy is 0.1mm, considering redundancy on both sides, then the resolution is
    1050/0.1=10500. That is, at this level of accuracy, 10500 pixels are needed to clearly image an object of this width.

ideal line rate normal image
Picture 1 Picture 2 Picture 3

Primary colors schema
Primary colors schema-two lines

  • Some applications find it difficult to install lighting, and the condition of sufficient illumination is hard to meet, so TDI technology is used to achieve clear imaging in low-light conditions (TDI technology will be discussed later in this article).
  • Time-sharing strobe technology (will be mentioned later) requires line scan cameras in multi-lines.
  • Long and short exposure techniques (will be mentioned later) requires line scan cameras in multi-lines.

  1. Due to the need for the line scan camera’s shooting frequency (line rate) to match the motion speed, the exposure time cannot be arbitrarily increased, so Plan 2 cannot be used.
  2. In some situations, it is impossible to enhance lighting (in narrow spaces), so Plan 1 cannot be used.
  3. Using a large target CMOS sensor is too expensive or too large to be installed on-site, so Plan 3 cannot be used.

The essence of TDI
line1 line2 line3 line4 line5 line6 line7 line8 line 9

TDI imaging effect

photovoltaic industry case TDI imaging effect
Melt sheet-uniformity   Coated film-hidden cracks   Finished product – PL inspection
Time-sharing strobe
Traditional: 2 stations Time-sharing strobe: 1 station
Application in Photovoltaic Industry
Backlight-Scratch Positive light-Dirt Side light-Indentation
HDR imaged
Underexposure Overexposure HDR
HDR implementation technology

HDR Application in Photovoltaic Industry
Regular image Dynamically expand image
  1. Calculate the resolution: Width/Minimum detection accuracy = Pixels required per line (Figure A: Width >> Object; Figure B: Width > Object;Figure C: Width ≈ Object)
  2. Determine pixel accuracy: width/number of pixels = pixel accuracy
  3. Determine the line rate: Movement speed per second/pixel accuracy = ideal line rate
  4. Select the camera based on resolution and line rate
How to select a line scan camera
  • For example, if the width is 1800 mm, the customer requires an accuracy of 1 mm and a movement speed of 25000 mm/s
  • Camera: 1800/1 = 1800 pixels, minimum 2000 pixels, select a 2k line scan camera. If you want to improve the clarity, you can use 2-5 pixels to express 1mm, then multiply the existing pixel value by 2-5 times.
  • Pixel accuracy: 1800/2048=0.9
  • Ideal line rate: 25000mm/0.9mm=27.8KHz
  • Select a 2K line scan camera with an actual line rate greater than 27.8KHz
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