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Vision-Doctor

Illumination

Illuminate features & make defects visible

Polarised light

Light can be polarised using polarisation films in the front of any industrial lighting. Flexible films from a thickness of 0.25 mm to 1 mm are available to the user for this purpose. Thicker filters from 2 mm are usually supplied as hard plastic sheets.

In addition, a glass polarisation filter is placed in front of the optics to selectively block or transmit polarised light. The principle of polarisation and the filter effect are described in detail in the chapter on polarisation filters. More examples can be found there.

Reduction of reflections

By using a polarising film on the illumination and a polarising filter on the optics, interfering reflections can be avoided on:

  • liquid surfaces such as water, etc.
  • Glass surfaces
  • smooth, reflective plastic surfaces
  • shiny metallic materials

Utilisation of polarisation effects

Example of polarised light and polarisation filter on optics

The metallic sheen of metal surfaces can be suppressed by using a polarised light source and a polarising filter. Metal objects appear very dark in the camera image. In particular, non-flat objects such as cylindrical, curved metal surfaces or crumpled foils (aluminium blisters, etc.) are generally very difficult or impossible to illuminate uniformly.

Diffuse front light illumination

Despite diffuse light, many reflections are on the cylindrical part.

Polarized light

Polarizer on lighting and optics avoids reflections on pipe and metal disk.
Depending on the position of the inspected objects in the inspection area, these reflections would ‘wander’ strongly on the surface. A trained object then quickly looks different to the inspection software, resulting in incorrect inspections. These difficulties are avoided by using this lighting and filter technology.

Unpolarised front light

Despite the diffuse lighting, there are many reflections on the crumpled plastic bag.

Polarized light

Polariser on illumination optics + avoids reflections on plastic films and metals

Tips for handling polarisation films in front of the illumination

  • If there are prefabricated polarisers and mounts for the lighting, you can save a lot of cutting and mounting work. Ring-shaped cuts with a hole in the centre are particularly difficult.
  • Keep a certain distance between hot light sources and the film. The chemical molecules used as dyes are sensitive to heat. Standard films may decompose at temperatures above about 70°C.
  • The thickness of the film has little effect on the polarising effect. However, thicker films hold their shape better and are often easier to mount in front of a light.
  • Please use only special adhesive for polarisation film.
  • Common polarising films are typically active between 400 and approx. 780 nm. They are therefore NOT suitable for infrared lighting. Buy special films for IR (very expensive)!
  • The transmission of polarisation films without elimination effect is already reduced by 35 to 40% by a second filter in front of the optics. The filter in front of the optics reduces the amount of transmitted light even more. If readjusting the exposure time and aperture does not help, use LED lighting with flash controller.
  • Laser light is normally already linearly polarised.

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