Colour control and filtering
As discussed earlier, colour is often an extremely important consideration. Therefore it is extremely important to select and control the colour of wavelength of an illumination accordingly. If the light source is emitting white light, filters can be used in front of the camera to assure that only wavelengths are allowed to pass through the lens and onto the sensor, that are necessary for the image analysis. One of the most common reasons for filtering is to control ambient light pollution and the suppression of artefacts.
At either ends of the spectrum, filters can be used to block either IR or UV light from interfering with the inspection. Conversely, pass filters can be used to select these wavelengths for specific inspection scenarios.
Specular reflection from flat surfaces is most often polarised parallel to the plane of the reflection. This commonly produces unwanted 'glare' that makes the resulting image more difficult to analyse. Polarisation filters can be used to block these reflections.
The effect that a polarising filter produces can be clearly seen in the two images. Glare from the light source on the left will make checking the object for errors, or performing OCR on the text almost impossible. The image on the right shows the same object and illumination when a polarising filter is used, producing a better image quality. Glare is suppressed, all important information is contained in the image and can be analysed with software.
Polarisation filters need to be used on the illumination and the lens and need to be adjusted in regard to the relative angular position. Polarisation filters also reduce the light intensity that reaches the sensor, therefore a lighting controller should be used to overdrive the illumination to compensate for the lower intensity.
Diffusers allow direct LED lights to be converted to diffuse, indirect units. These are available in a range of different formats to suit most illuminators.