Correct Lighting Increases Reliable Detection in Industrial Automation
- Fig. 1: Vision sensor with integrated LED lighting
- Fig. 2: With bright field lighting reflections can even occur on black plastic surfaces. Preference is therefore generally given to dark field lighting (below)
- Fig. 3: Dark field alignment with lateral grazing incidence is recommended for embossed structures (below)
- Fig. 4: When detecting reflective surfaces, bright field lighting (above) generally delivers the best results
- Fig. 5: Use of a ring light enables the accentuation of outer contour
Vision sensors used in industrial automation demand a great deal of lighting. Both the choice of lighting and how the sensor and light source are positioned in relation to each other are decisive features to ensure reliable detection. Dependant on the application there are different choices for the best illumination set-up.
Vision sensors have conquered numerous applications in industrial automation over the past few years. The most important fields today include part detection using sample and contour detection, color detection as well as the identification of bar codes and data matrix codes. However vision sensors demand a great deal of the lighting source: the better the image quality, the greater the detection reliability. Correct lighting is essential for the success and stable, reliable functioning of each automation application with vision sensors.
Bright or Dark Field Lighting
Basically there are two main types of lighting: bright field and dark field lighting. In bright field lighting, the sensor and lighting system are aligned in a way that light is reflected from the surface directly into the sensor. On the contrary in dark field lighting, the light source and sensor are positioned to ensure that only diffuse light, e. g. from edges, arrives directly the sensor. Bright field lighting with vertical alignment of the sensor in relation to the object can cause problems due to reflections – even with a black plastic surface, see figure 2 – as it over-illuminates the white sample to be detected. Instead it is advisable to tip the sensor with its integrated lighting slightly in relation to the surface of the object (e. g. 10–20°). This avoids direct reflections from the background so that it appears dark and the white imprint, with a diffuse reflection, is simultaneously accentuated. The result is a stable, high-contrast image. Bright field lighting is not recommended either for plastic parts with embossed extruded features such as markings for example, due to reflections. Dark field alignment with a sensor at a tilted angle to the object’s surface provides better results; however the optimum solution in this case is frontal alignment of the sensor with flat incidental, lateral light from an external light source, shown in figure 3.
Also bright field lighting has advantages, e. g. by detecting strongly reflective metal surfaces (fig. 4). Even from a white background the total reflection accents the metal surfaces, so a high level of detection reliability is achieved. When using bright field lighting the image quality depends strongly on the surface and angle of alignment, so the stability of these factors must be observed in the series production.
Special Lighting Situations
Special applications need special solutions. Detecting a symmetrical round part, dark field alignment with an external ring light is recommended. With it the contour of a toothed wheel, e.g., becomes clearly visible without undesirable shadows. Different object details can be highlighted according to the angle of incidence of light adjusted by varying the lighting distance. To detect markings on cylindrical, strongly reflective metal parts, the use of diffuse dome or tunnel lighting is indispensable. This is also valid for randomly and dispersively formed – and thus randomly and dispersively reflective – series of plastic film and aluminum foil, as used for medicine blister packs and other packaging. With this type, light is not directed, but penetrates evenly from all directions, like on a cloudy day. With transmitted light, the outer contours or profile of a part are particularly accentuated. Structures presented on the part itself are on the other hand not visible with this configuration. However, transmitted light is problematic to install in many cases.
When detecting embossed or recessed structures on transparent objects – such as a laser-etched data matrix code in a glass panel – the problem of double contours can arise due to reflections from the front and rear. Therefore the sensor should be positioned vertically to the surface whilst external light shines from the side. For this purpose either a surface light or ring light can be used.
Intensity of Lighting
Intensity of lighting can be adapted with almost all vision sensors indirectly via the exposure time (shutter opening time) and sensor gain. Short shutter times, which mean very bright light, should generally be aimed at, so that fluctuations in ambient light have only little effect on the function of the application. With moving parts, short exposure times (e. g. through a flash light) eliminate movement blurs caused by too long exposure. In order to avoid a flurry of flashes in installations where people are also working, the use of non-visible infrared lighting is advisable.
By using colored lighting or color filters, it is possible to accentuate part features or render them “invisible“. For example, a black marking appears with maximum contrast on a red plastic surface when a monochrome sensor with red lighting is used, as the red background reflects a large amount of light and thus appears very bright whilst the black marking hardly reflects at all. However, if green lighting is used on the same part, the contrast between the red background and black marking virtually disappears as the red plastic background reflects almost none of the complementary color green and the black marking no longer stands out from the background which now also appears as almost black. The “black marking” characteristic is thus blanked out using this method. By combining red lighting with a red light band pass filter fitted in front of the sensor, interfering effects of ambient light can be largely blanked out to avoid shadowing the application.
The majority of applications can be solved with bright field or dark field lighting using the lighting source integrated in the vision sensor. Different positions of the sensor in relation to the object should thus first be tested. Dark field lighting allows to accentuate edges on contoured objects (e. g. structures which are embossed or recessed in relation to the object surface). This works reliably over entire part series as these edges vary less over the entire series than the surface which can often range from very shiny to oil-smeared or corroded. By using bright field lighting, the shine from very reflective component surfaces can be used for its high-contrast accentuation. It must be ensured that the object is shiny throughout the entire production series and that the angle of alignment to the part surface is easily reproducible. Due to its reliable function and low angular dependence dark field lighting is preferable wherever possible. If a satisfactory result cannot be achieved with the integrated lighting, it is recommended to disassociate the direction of detection and lighting by using an external light source. Surface lights, ring lights and transmitted light are available as alternatives with a white, red or infrared light source. By varying the measurement distance, angle of detection and lighting angle, a suitable solution can be found for almost every measurement task. However, the choice of the correct lighting depends strongly on the applicatio