These filters are able to avoid reflections, which often inevitably arise when using imperfectly diffuse illumination sources. Reflections can appear like totally white image areas even on the darkest, shiniest materials, which can then mask inspection features and thus prevent successful inspection.
Disturbing reflections can be avoided on:
- liquid surfaces, like water etc.
- glass surfaces
- smooth reflective plastic surfaces
- metallic shiny materials
If one wants to suppress disturbing reflections in industrial image processing, usually a polarizing filter must be placed in front of the camera lens AND a polarizing foil must be placed in front of the illumination.
Required components for the use of polarization effects
Principle of polarization
The light can be considered as an electromagnetic wave, in which the electromagnetic vector oscillates transversally (at right angles) to the propagation plane. The unpolarized light itself can also oscillate in all directions. If a polarization filter is placed in the beam path, it is capable of absorbing certain vibrational directions of the light. For this purpose, chemical dye molecules in the filter material are spatially aligned in such a way that they can absorb light along their molecular structure and let through other vibrational planes. For linearly polarized light, the electromagnetic vector oscillates only in a single plane, all other directions were absorbed by the filter.
Principle of polarization of light
If two filters are placed one behind the other and rotated 90 degrees to each other, no light is transmitted; if their absorption direction is identical to each other, a vibrational plane of light is transmitted. So two polarizing filters can also replace one neutral density filter. Depending on the position of the two polarizing filters, a continuous darkening of the image can be achieved.
Polarized light can be produced in a number of ways:
- Polarization by reflection: Especially on non-metallic surfaces, such as glass, liquids and plastics (dielectric materials), the light can be naturally polarized when it strikes the surface of the materials at a certain angle. At an exit angle of 30 to 50 degrees (Brewster angle αp=arctan(n); n=refractive index material), this effect of polarization is particularly noticeable. The direction of oscillation of the resulting polarized light is aligned parallel to the surface. If the polarizing filter is placed in front of the lens in such a way that this polarized light is blocked, the reflection can be suppressed and the unpolarized background can be made more visible again: Light reflections on glass panes, water surfaces or shiny plastic surfaces disappear. By using already polarized light (polarizing filter foil additionally in front of the illumination) this effect can be intensified. The two polarizing filters have to be rotated 90 degrees to each other. In practice, the filter on the lens is simply turned until the effect is satisfactory. On metallic surfaces, the light is not polarized but mirrored. Here it is absolutely necessary to mount a polarizing filter on illumination and optics in order to suppress disturbing light reflections.
- Polarization by absorption: As just described, polymeric dye molecules can absorb light along their chemical backbone. Certain oscillation planes are thus preserved. The light is polarized. This effect is used for polarizing filters, in which such dye layers are embedded between two glasses. Plastic films can also be made into flexible polarizing films by using such organic molecules.
- Polarization by scattering: very fine dust and aerosol particles are also able to polarize the light. This is exploited in nature photography: Sky light is polarized, and a single polarizing filter on the lens is sufficient to achieve blocking of light already oscillating in one direction. As a result, the sky appears darker and thus much bluer.
Polarizing filters do not affect the colors of the inspected image. They are therefore suitable for both color and monochrome cameras.
Polarization in incident light
Reflections on plastics can be effectively suppressed. Polarized illumination and a polarizing filter must be used on the lens to suppress interfering reflections. However, the use of this method requires light in excess, since very light is absorbed by the filters. This can be compensated for with LED lighting, for example, by using an LED flash controller.
The principle of reflection suppression on smooth non-metallic surfaces such as plastics, glass and water is based on the fact that light is polarized when it strikes the surface of the materials at a certain angle. At an exit angle of 30 to 50 degrees (Brewster angle), this polarization effect is particularly noticeable. If the polarizing filter is placed in front of the optics in such a way that this polarized light is blocked, the unpolarized background can be made more visible again: Light reflections on surfaces disappear.