The goal is very simple, to create a contrast. It is the prerequisite for simple, fast and robust testing programs. Without it, the most efficient processing software cannot operate reliably.
There are numerous direct and indirect reciprocal effects between lighting and the environment: materials to be inspected, ambient light, lenses, cameras and machine environment, as well as image processing hardware and software, all have an effect on the success or failure of lighting.
The composition of the illumination must fundamentally match the sensitivity of the image sensor, so that the width of the spectral range of light is normally restricted to 300 nm for UV light and 1400 nm for NIR. The inspected material, lens and sensor must be coordinated in this range.
Depending on the technology and design, image sensors, including the human eye, have different sensitivity to different light colors. Particularly in the field of calculable but not directly comparable invisible light (UV and infrared), they apply to the recognition of illumination.
In the case of mechanical image processing, only the absolute differences located in the brightness are used and analyzed. The results for man and machine are far away and must be calculated, therefore the help of image processing technology is necessary.
Example of contrast:
Distance is an essential factor for successful lighting. The photometric distance law gives the following contexts for incident light:
If the distance from the object lighting component is doubled, only a quarter of the original lighting and the irradiation is maintained.
Unfortunately, in this case, the human eye, with its logarithmic recognition of brightness, leads to errors. Therefore, the image processing system should be consulted for the brightness drop test.
Changes in distance have no influence on the brightness of the lighting. Therefore, backlighting can be placed at a greater distance from the material to be inspected without loss of brightness.
Different implementation scenarios require different components. Thus, on the one hand, lighting that allows maximum homogeneity is required (e.g. for backlight illumination). Other forms of lighting, on the other hand, are often designed in such a way that they create a lighting profile directed at a particular individual form (for example, for darkfield lighting).
From the absolute brightness, homogeneity or lighting profile, incident and background lighting information can be provided.
The color of illumination, which is created by specific wavelength radiation, is a considerable feature of illumination, which can be externally recognized by humans. This color causes a reaction in the object (absorption, reflection and transmission). The illumination that corresponds to the color of the test object, with white light is particularly well reflected and is therefore quite effective. The complementary color, on the other hand, is effectively removed and therefore appears dark. The selective use of complementary color and color is a principle for the selection of wavelengths of light.
Objects that appear achromatic with white light (for example, metal surfaces) point to the fact that they reflect all wavelengths uniformly. The color selection of the lighting plays an important role for these objects.
However, not all white light is the same. In order for there to be even brightness in the material to be inspected, the composition of the spectrum used in the white light must be known. This is important for color image processing, where differences in light composition can be corrected by white balance.
Human perception is not effective for UV and infrared light, whose wavelength is beyond the visible field. No prediction can be made for the interaction between the "colour" of the light and the material to be inspected. They must be established through experiments.
Imagen 1: Los colores y los colores complementarios están opuestos en el círculo de color. Imagen 2: Regleta de bornes gris con contactos de bornes naranja (ambos a la izquierda en la imagen). Encendido en naranja (izquierda), e iluminado con el color complementario azul (derecha).
Changes in the position of the element or variable light radiation during the course of the day can lead to the implemented lighting not being able to deliver constant contrast, thus compromising the stability and reliability of image processing.
The following examples can be used as countermeasures for strange and interfering lighting:
● The use of lightingThe light source is much more efficient, so that the specific illumination is stronger than the interference.
● Short-term lighting in combination with strobe lighting.
● The use of lighting filters in combination with the appropriate light colour.
● Light-tight protective housing around the imaging machine.
Due to the large amount of energy of the illumination irradiated using lasers and LED's as light source, it is necessary to take measures. There is also the fact that some of the lighting used radiates into the invisible field of UV and infrared light, which is particularly dangerous, as it cannot be directly recognised by humans. Concerns of danger to both eyes and skin (particularly UV). However, in some simple procedures, the hazards caused by these light sources can be reduced:
● Do not look directly at the lighting.
● Turn on the lighting only when necessary for image processing (pulse or flash - no continuous operation).