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Illumination

Illuminate features & make defects visible

Illumination techniques for line scan cameras

Ultimately, only a thin line of light is required to illuminate a line scan camera application, as only one, two or four (or in extreme cases up to 196 pixels: TDI) wide lines are captured by the sensor. However, it is important for all line scan camera applications that the light is as bright as possible and extremely high frequency to avoid image flicker.

Today, LED line lights are used almost exclusively. In the early days, high-power LED line lights were actively cooled with water or fans, but today they are usually passively cooled. However, good heat dissipation through a metallic mounting surface is still important today.

Lighting types for line scan camera applications

Comparable to 2D technologies

  • Direct LED line lights can be used for incident light or dark field illumination.
  • Wider, less bright line lights with diffusers are used for backlight applications.
  • Tunnel lights or coaxial lighting for diffuse illumination is also used today where objects are highly reflective and the surface is not very homogeneous.

Combined multi-lighting

Combinations of different techniques are also possible with line scan camera systems. For example, a colour line scan camera can simultaneously use illumination for incident light and blue illumination for transmitted light. This means that both image information is available at the same time.

Sequential acquisition one after the other (e.g. bright field, then dark field) are also possible with some line scan cameras, but require very fast illumination strobe controller. Then only every second line has to be extracted into a separate image. That's it!

Coaxial illumination for line scan cameras

Incident light illumination for line scan cameras

  • This is the standard method for surface inspection using line scan cameras.
  • The camera and lighting are positioned so that the camera is at the reflection angle of the lighting (angle of incidence = angle of reflection). In this way, as much incident light as possible is captured.
  • This works particularly well with reflective materials. Bright areas appear bright and dark areas appear dark.
  • If the angles of surface structures are not properly aligned, but are tilted, they will also appear dark in the camera image. Scratches in the surface or inclined phases etc. will appear darker in the image.

Incident light (Bright field)

Dark field illumination for line scan cameras

  • Line scan cameras can also be used to create a dark field effect. The illumination is not positioned on the component at the angle of reflection, but much flatter.

  • The directly reflected light does not hit the camera and the image field appears dark.

  • Only the weak scattered light is detected by the camera, so a lot of light is needed for this technique.

  • Scratches, slits, bumps or embossments appear bright because they scatter light due to their uneven surface. Ideal for detecting surface defects on smooth materials.

Dark field

Backlight illumination for line scan cameras

  • In this lighting scenario, the camera and lighting are positioned directly opposite each other, with the object to be inspected positioned between them.
  • This allows the contours of opaque materials to be seen, edge breaks to be detected and the component to be measured.
  • In the case of transparent objects (e.g. glasses, foils, etc.), this method can be used to visualise bubbles, cracks or any edge chipping in the material.

Backlight

Tips for the alignment of a line scan camera illumination

The line scan camera often only captures a strip of the image field that is a few 1/10 mm wide. This has to be precisely hit and illuminated by a very thin strip of light:

  • Use a calibration target with a slightly wider black stripe (possibly also with crosswise division) and align it at right angles to the conveyor.
  • Change the trigger type of the camera to freely running mode without the use of encoder or trigger signals.
  • Position the calibration target in such a way that the complete length of the black stripe is detected by the line scan camera. As the image block typically consists of many image lines, it must now be completely black.
  • ake a pen and draw a line over the stripe. Now you can determine the right and left positions in the camera image. If necessary, mark them also on the calibration target.
  • Now you know the exact position which is detected by the camera. Align the angle and working distance of your illumination as required and, if possible, focus the light beam for maximum brightness.

Target for calibration and adjustment

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