Illumination techniques for line scan cameras

For the illumination of a line scan camera application, ultimately only a thin line of light is required, since only one, two, four (or in extreme cases up to 196 pixels: TDI line scan cameras) wide line is captured simultaneously by the sensor. It is very important for all line scan camera applications, that light has to be as bright and extremely high-frequency as possible, so that no image flickering occurs.

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

• Direct LED line lights can be used as incident light or dark field illumination
• Wider, less bright line lights with diffuser serve as transmitted light
• Tunnel lights or coaxial lights for diffuse illumination are also in use today.

Incident light illumination for line scan cameras

• This is the standard method for the surface inspection using line scan cameras.
• Camera and lighting are arranged in such a way that the camera is at the reflection angle of the lighting (angle of incidence = angle of reflection). In this way as much incident light is detected as possible.
• This works especially well in case of reflecting materials. Bright areas appear bright, dark areas dark in this way.
• If the angles of surface structures are not suitable aligned but tilted, they also appear dark in the camera image. Scratches in the surface or inclined phases, etc. have darker shades of grey in the image.

Dark field illumination for line scan cameras

• Line scan cameras, too, can be used to achieve a dark field effect. For this purpose, the illumination is not aligned onto the component at the angle of reflection, but clearly flatter.
• The directly reflected light does not hit the camera, the image field appears dark.
• Only the faint stray light is detected by the camera, therefore a lot of light is required for this technique.
• Scratches, slots, elevations or embossments appear bright as they stray the light due to their uneven surface. Ideal method to detect surface defects on smooth materials.

 

Backlight illumination for line scan cameras

• In this illumination scenario, camera and illumination are exactly opposite of each other, the test object is right in between.
• The contours of non-translucent materials become visible in this way, edge chippings can be detected and the component can be measured.
• In case of transparent objects (like glasses and foils, etc.) this method can be used to make bubbles or cracks in the material or edge chippings visible.

 

 

Tips for the alignment of line illumination

The line scan cameras often detect only a few 1/10 mm wide stripes of the image field. This is to be hit and illuminated exactly by means of the equally thin line 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.
• Take 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.