Typical applications of multi-spectral cameras have been in inspections for defects not visible with a color camera while still using color information to gauge visual appearance.
The inspection of shape is based on the use of a structured near-infrared light projected on the object, which allows the height profile of the object to be measured. Figure shows the key components of the vision system required: a) the 2-CCD camera, b) a strobed ring light and c) a line pattern projector. The 2-CCD camera is GigE complaint and has dual Gigabit Ethernet ports that connect to the PC. The strobed ring light is used to provide the white light illumination needed for the color image.
The near-infrared line pattern projector is a 5 Watt 850 nm LED and projects an 8-line pattern on to the camera’s field of view. With the additional integration of a single component (the line pattern projector) to a conventional 2-CCD camera vision system configuration we can obtain the height information, which enables the shape quality measurement.
The images acquired by the camera are shown in Figure 3: color image (d), near-infrared image (e). The rendered 3-D image based on the color and height data is shown in Figure 3 (f). A simple triangulation algorithm was used to estimate the height from the displacement of the lines shown in Figure 3 (g). A calibration process establishes the height proportional to the displacement measured in the images and this height information is combined with the color information to create the 3-D rendering. Although the height information is not obtained at every pixel location, it allows for a measurement of the overall height and identification of shape deformities. The change in curvature of the projected lines allows for the detection of small height deformities. The green lines in the Figure 3 (f) show the regions where the curvature change is not large and the red lines show the regions where an unacceptable curvature change is detected corresponding to a depression in the package.
Figure 4 shows a demonstration of a sortation system built with this technology. The top row of images corresponds to the packages that were sorted by their color and the bottom row shows packages that were failed by their a) shape, b) length and c) height. The system is capable of inspecting 30 packages per second at the full image capture rate of the camera.
This novel use of the 2-CCD camera technology to obtain shape information along with color images has enabled the demonstration of a vision system that can sort by color and shape of objects. This technique can be extended to stereo imaging and further to be used with grid patterns or random patterns for enhancing the height information extracted from the near-infrared image. The simplicity of the configuration of the system allows for its use in many quality inspection applications based on machine vision.V&S
Tech TipsThe use of machine vision for the traditional task of performing quality inspection is expanding with the capabilities offered by cameras.
These include an unprecedented number of pixels, frame rates, sensitivity and range of wavelength information.
Today’s machine vision cameras can offer up to 29 megapixels of information from a single CCD sensor and image transfer rates of thousands of frames per second with high sensitivity CMOS sensors.