In principle, the starting point for every image processing application is always the same: good images of the objects to be inspected form the basis for subsequent evaluation and reliable error detection. An important prerequisite for good, i.e. detailed, high-resolution images is, in addition to the quality of the camera and optics, optimum lighting. Nowadays, modern image processing systems mostly use light from LED sources, as this technology offers many advantages in terms of power consumption, durability and stable characteristics.
The geometric shape of the lighting, the light color and the light intensity that provide the optimal conditions for capturing the best possible images depend on the task at hand. If - as is the case in most applications of image processing systems in automated production - the objects to be inspected move on a conveyor belt or other conveyor system underneath an image processing system, for example, a great deal of light is required in order to achieve short camera exposure times and thus undistorted images of the inspection objects. However, a lot of light inevitably also means a lot of waste heat, which is fatal for any LED in the long term.

Lightning as a way out

The way out of this problem is not to operate the lighting permanently, but only to switch it on when the camera is currently exposing. In this way, the exposure of the camera is no longer controlled only by the exposure time of the camera sensor, but also by the intensity and duration of the illumination flash of the LED lighting. The duration of such illumination flashes can range from less than 1 µs to several 100 ms. The main advantage of this approach is that heat is only produced during the actual lighting period. The LED can cool down in the intermediate phases and is therefore protected, which leads to a longer service life of the lighting. In addition, flashed LED lighting can be operated with currents that are up to 100 % higher than their continuous current specification. This so-called 'over-flashing' enables a correspondingly higher light yield to be achieved during the lighting phase, which further improves the quality of the images captured.

An external flash controller is used in many image processing systems to control LED lighting. Its task is to precisely control the switch-on and switch-off processes and to ensure, among other things, that the current is stable over the entire duration of the flash and that the set lighting duration is maintained as precisely as possible. In more complex applications with several lighting units, it is also the task of the flash controller to correctly control the flash operation sequences and the required lighting duration.
The use of such lighting controllers is standard nowadays. However, the fact that these devices are operated separately from the camera has some disadvantages. Apart from the additional acquisition costs, the camera and flash controller have to be wired and require a considerable amount of additional work when integrating the software into the application via two different interfaces. The more complex overall system must also be maintained over the lifetime of the application. An alternative, more cost-effective way is to integrate the flash control into the camera.

Common control for camera and light

Flash controllers are already integrated in the current industrial camera families EXO (the picture shows the USB3 version EXO428U3) and FXO from SVS-Vistek.

© SVS-Vistek

For these reasons,SVS-Vistek initiated the development of cameras with integrated flash control over ten years ago. Up to four separately controllable channels are available in the current camera models with the company's so-called 4IO function. Each of these short-circuit-proof power outputs supplies up to 3 A of current for a short time in flash mode, which is sufficient for 95 % of all applications. The I/Os are controlled via the camera's GenICam interface. This eliminates the need to integrate and synchronize an additional software interface, which saves the user time and simplifies the path to the application. The elimination of cabling between the camera and flash controller and, above all, the entire (and usually expensive) external flash controller also reduces the acquisition costs of the image processing hardware. In this way, applications become leaner both physically and in the software, which reduces the development costs of the software in addition to the hardware savings, while precision increases: the flash control can be set to an accuracy of 15 ns.

Heat development in the camera leads to increased noise and thus to poorer image quality. To avoid this effect, SVS-Vistek uses powerful MOSFETs for the PowerOut in cameras with an integrated flash controller, which can only switch. The current and therefore the brightness of the LED lighting can be regulated using the integrated Pulse Width Modulation (PWM). The clock frequency can be set over a wide range in order to be able to react flexibly to the respective requirements. The output voltage always corresponds to the supply voltage of the camera.

The 4IO interface of the cameras can perform other tasks in addition to controlling LED lighting. For example, it includes several debounceable physical inputs, logical functions for processing input signals and an extremely flexible sequencer, which can be used to precisely control several lighting units. All inputs and outputs work with levels of up to 24 V and are therefore perfectly suited for direct communication with a PLC. Less well known, but used by some industrial customers, is the possibility of using the very flexibly programmable PWM module for control tasks using PWM signals. With PWM and sequencer, independent output signals can be programmed relatively freely on up to four lines, which can be used as a direct input for servomotors or the PLC, for example.

The camera families

Stefan Waizmann works in Technical Marketing at SVS-Vistek in Seefeld.

© SVS-Vistek

SVS-Vistek makes the 4IO functionality available to users in all its camera families. The EXO and the new FXO cameras, which are particularly suitable for the factory floor, are available in numerous variants with different CMOS and CCD sensors from Sony, ON Semiconductor and CMOSIS and cover resolutions from 2.3 to 31 megapixels. Despite the integrated flash controller, most of these cameras allow above-average operating temperatures of up to +70 °C (depending on the model). This is made possible by a very close thermal connection of the low-power-optimized electronics and the sensor to the milled unibody housing.

EXO cameras are available with GigE Vision, Camera Link and USB3 Vision interfaces and support current standards such as GenICam 3.0 and USB 3.1. The new FXO series even comes in a 'Single Line CoaXPress 12' version. In a version called Tracer, the use of an MFT bayonet makes it possible to control all lens functions such as zoom, focus and aperture electronically via the GenICam interface - here too, operation is integrated into the camera interface. The robust and precise industrial cameras in the EXO and FXO series from SVS-Vistek therefore meet a wide range of requirements and are suitable for use in a variety of industrial and non-industrial applications.

SVS-Vistek was a technological pioneer in the integration of lighting controls in industrial cameras. This approach has proven to be very successful for over ten years and is now being imitated.
Of course, it is still possible to connect the PowerOut outputs of a camera to an external controller to control LED lighting. However, the integrated solution is undoubtedly simpler and more economical from a technical point of view, meaning that the use of external flash controllers is likely to become the exception in the foreseeable future.