Custom Machine Vision Illumination
In the industrial on-line spectroscopic and machine visions measurements spectral and spatial uniformity, good efficiency and passline stability are very important parameters for target illumination. VTT has long experience in designing custom illumination and imaging optics for these applications. In addition, free-form optical surfaces give an additional degree of freedom in the design process, which can be used to achieve better efficiency and uniformity in the illumination systems with fewer optical components.
VTT (Technical Research Centre of Finland) is the biggest contract research organisation in Northern Europe. VTT provides high-end technology solutions and innovation services. From its wide knowledge base, VTT can combine different technologies, create new innovations and a substantial range of world-class technologies and applied research services, thus improving its clients’ competitiveness and competence. Through its international scientific and technology network, VTT can produce information, upgrade technology knowledge, and create business intelligence and added value for its stakeholders. We have expertise in the whole development cycle of optical instruments, ranging from measurement principles and quick prototyping for on-line feasibility studies to final product development. Our mission is to provide superior research and development services for both the process measurement and control system suppliers and the end-users. We have a successful track record working with both kinds of customers. Our resources include several on-line-capable IR, NIR and Raman instruments available for feasibility studies on the real processes. In most cases, they can be tailored to interface with the customer’s processes in a matter of weeks. VTT also has wide experience in developing advanced machine vision and camera systems for on-line measurement purposes, such as high-accuracy 3D measurement technologies for dimension and form, colour measurements, surface topography technologies, quality inspection, micro- and biotechnological measurements, and the monitoring of vehicle operators. Our areas of expertise include imaging components, illumination techniques, image processing and analysis, software algorithms, visualisation and calibration methods.
Introduction
Spectral and spatial uniformity, good efficiency and pass-line stability are usually very important parameters in the designing of the illumination optics for industrial process measurements applications. On-line spectroscopic and machine vision measurements have very demanding stability requirements because the calibration usually cannot be performed for every single measurement. Therefore, the illumination optics has to be designed in such a way that it is stable over the movement of the samples in the production line. A conventional solution for the spatial and spectral mixing is Kohler illumination, which leads to good optical performance in most of the cases. Pass-line stability can be achieved using e.g. telecentric system. However, that requires several (at least two) optical components, which can increase the size and complexity of the system. In addition, the required illumination area is not necessarily symmetrically shaped. If the power budget is limited, the illuminated area should be matched - e.g. the shape of the detector or detected area. We take advantage of the freeform optics, which can be used to perform optical functions with fewer surfaces and can be tailored to the specific illuminationproblem. Finally, we can manufacture free-form mirrors inhouse using a 5-axis CNC machining centre.
Line generation using LED source
Figure 1 shows a line generation optical system based on the LED component. The standard technology for producing line patterns in the machine vision systems is a laser source with a diffractive optical component. However, in some cases this introduces speckle-related noise to the measurement signal, which can be really difficult to get rid of. When LED’s are used the speckle noise is not a problem. In addition, the beam pattern can be modified according the specific needs, similar to using diffractive optics with the laser. In this work the target is a 1400 mm long line with a Gaussian cross-section profile. The specified distance of the module from the target is 1200 mm and the angular width is ± 23.2°. The numerical aperture of the collection optics is 0.5 and about 25 % of the total emitted optical power is collected and used for the line generation. The simulated irradiance profile of the line is shown in Figure 2. The irradiance profile at the centre of the line is quite flat (irradiance maximum is about 0.54 W/m2). The cross-section of the line should be Gaussian shaped and, as can be seen in Figure 3, this requirement is fulfilled almost along the whole line. Only at the -700 mm end of the line does the profile have a dip at the centre.
  
Custom-designed free-form optical surfaces
Figure 4 gives an example of a free-form mirror that generates four separate illumination shapes from the single LED source. The mirror NA is about 0.5 and the total collection efficiency is about 25 %. The efficiency can be increased, but this will reduce the contrast of the illuminated areas. This example is designed purely for a demonstration purposes, but it shows the real capability of free-form optics in creating custom-shaped illumination
Mirror optics manufacturing Figure 6 (left) shows the simulated irradiance at the target of the custom-designed illumination. The measured illumination profile of the mirror is shown on the right (Figure 7), which was manufactured at VTT using 5-axis CNC machining. The shape of the simulated and measured profiles is the same. The low number of rays in the simulation introduces noise to the result and is not a real effect (Figure 6, left).
  
Summary
We have developed optimization methods for generating freeform optical surfaces for illumination applications. Free-form surfaces can be used in illumination systems to reduce the number of components and the complexity of the system when the complexity of the single surface is increased. In addition, the free-form surface can perform optical functions that are not possible with conventional optical components.
|
