Use of lighting in image processing
1. Introduction
The three basic pillars for good image quality, and therefore for a reliable application, include the camera with a suitable sensor, the optics and the lighting. Flash lighting ensures well-lit and sharp images. Digital lighting controllers are used to ensure consistent illumination over long periods of time. The following document shows how lighting improves image quality.
2. Starting point LEDs
The invention and widespread availability of LED technology and the presence of the three primary colors red, green and blue as LEDs marked a milestone for lighting in industrial image processing. Above all, the excellent price-performance ratio, long service life and robustness against vibrations and dust did the rest to accelerate the triumphant advance. Another advantage over fluorescent tubes and halogen lamps is the short reaction times of LEDs, which enable LED activation synchronized with image acquisition. The LEDs are so robust that they are not damaged by repeated switching on and off. Switching off prevents a rise in temperature and therefore extends the service life. "Flashing" is an operating mode that is supported by LEDs. This means that for a short time, LEDs can be supplied with ten times more power than in continuous operation. The high intensity reduces the influence of extraneous light and at the same time provides sufficient light for short exposure times. This means that motion blur in fast applications can be kept to a minimum. An illumination controller ensures that the current pulse has the correct intensity, length and duty cycle when "flashing".
3. Structure of an example application with lighting controller
Reading fonts or codes on packaging, for example in the pharmaceutical and medical sectors, is a common practical application. The following components are used to set up the example application:
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An industrial camera
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A line light, blue, directional, illuminated areas 14 mm x 100 mm
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A lighting controller
The distances between
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Camera and packaging: 370 mm
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Packaging and line lighting (hypotenuse): 190 mm
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Packaging and line lighting (opposite cathetus): 100 mm
Figure 1: Distances and structure of the example application
4. Flowchart of the example application with lighting controller
Figure 2: Flowchart of the example applications
An image taken with ambient light results in an image that looks good to the human eye. In industrial reading, most areas should also be legible, but the edges are blurred in most places and could lead to inaccuracies and thus to poor reading quality.
Figure 3: Packaging in ambient light
In this image you can see that most of the gray tones have been eliminated and the image is sharper.
Figure4: Flashed packaging
Clear differences in quality can be seen in the details. By "flashing", two parameters of the image can be adjusted. Firstly, the exposure time can be shortened, in the example to 0.2 milliseconds, which reduces external disturbing influences such as vibrations. Secondly, a smaller aperture or a larger f-number (4.0) can be selected. This means that the aperture of the lens is less open and therefore lets less light through to the sensor, resulting in a greater depth of field. This also reduces the influence of extraneous light.
Figure 5: Comparison of the images
5. Conclusion
Flashed applications provide a better source material for downstream image processing, which is impressively demonstrated by the sample images. The use of lighting controllers enables precise intensity control of the lighting. Consistent currents guarantee the same brightness and therefore a consistent image.