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Stray light

Für fast alle optischen Fehler gilt, dass sie

Stray light is generated at any surface where light passes into another material of different optical density, e.g. from air into glass. The amount of stray light per surface is approximately 5%, but with 5 - 7 lens systems in an optical system this can add up to much more.

The result is a loss of light and a reduction in image contrast. A loss of brightness may be acceptable in a machine vision application, but a significantly reduced image contrast will usually result in less accurate measurements and detection rates.

Image series with different amount of scattered light

Appropriate anti-reflective coatings on the lenses, careful matting of the lens edges and coating of the internal housing of the optics can largely eliminate stray light.

Important for Machine Vision

  • High quality glasses, the use of high quality lenses and anti-reflective coatings are a must!
  • Anti-reflective coatings are defined for specific wavelength ranges. Especially in the infrared range, many optics are not optimised in this respect. IR images taken with standard lenses often appear "low contrast" due to the difficulty of stray light. Use optimised IR lenses!
  • In some cases, a slight loss of contrast may not be visible to the eye. Use your image processing tools (brightness / statistics tools) and the histogram as a guide. A standardised test chart with grey scales will help you to correctly determine the image contrast of the different colours and greyscales.

Test chart for measuring grey tone gradients and determining contrast

Conclusion

Don't compromise on the optics and choose high-quality lenses! The successful selection and interaction of ALL individual vision hardware components together with the software guarantee success.

Optics that are as flawless as possible are an absolute prerequisite for high-quality inspections!

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Optiken Industrielle Bildverarbeitung