Working with the US Department of Agriculture (USDA), Zedec (Morrisville, NC, USA; www.zedec.com) has developed a machine-vision system to inspect the surface of apples for quality. To design the system, research was performed to determine a fundamental approach to the problem. Once the approach was chosen, a working prototype was manufactured. As such, the requirements for the project included software that could be integrated with a variety of imaging components and expanded to function in a complete system.
It was first necessary to understand the spectral requirements of the system. To do this, Zedec needed a system capable of capturing the individual spectral wavelengths that would best highlight the bruises or other defects on the surface of the apples. The company turned to multispectral imaging, a technique that provides images of a scene at multiple wavelengths and that can generate optical spectra at every pixel. To properly configure the system, Zedec used a VariSpec liquid-crystal tunable filter (LCTF) from CRI (Woburn, MA, USA; www.cri-inc.com) configured with an off-the-shelf CCD camera.
"If you look through an LCTF tuned to a wavelength in the visible range," says Ross Nakatsuji, marketing and sales manager at CRI, "you will see a single color. This color can be rapidly switched by computer control to any other color. How "pure" that color is depends on the bandwidth, which defines how many adjacent wavelengths are transmitted at the full width of the curve at half the maximum transmission."
In this way, researchers can analyze an object under test at specific wavelengths in the near-IR and visible spectrum to properly determine the optimum inspection wavelength. Once a specific wavelength is determined, image-processing software can extract the maximum information from the spectral image.
VariSpec LCTFs are solid-state devices that operate with no moving parts and no sound or vibration and offer large working apertures. "The VariSpec standard model filter can be used for machine vision or other applications in the visible (400 to 720 nm) or near-IR (650 to 1100 nm or 850 to 1800 nm) wavelength ranges," says Nakatsuji. To dynamically vary the wavelength transmitted through the filter, the LCTF is connected to a controller box connected to the filter's optics housing. This accepts serial commands from a host computer that controls the filter though the PC's USB port.
Several programs can be used on the host PC to control the filter and the supported digital camera to allow a number of spectral datasets to be captured. These include third-party packages such as Metamorph from Universal Imaging (Downingtown, PA, USA; www universal-imaging.com), ImagePro Plus from Media Cybernetics (Silver Spring, MD, USA; www.mediacy.com), LabVIEW from National Instruments (Austin, TX, USA; www.ni.com), IPLab from Scanalytics (Fairfax, VA, USA; www.scanalytics.com), and Openlab from Improvision (Lexington, MA, USA; www.improvision.com).
Zedec used its own analytical imaging software to capture images from multiple LCTFs and cameras covering both the visible and near-IR spectrum. With this information, researchers at the USDA were able to determine the key wavelengths that provided good contrast for defects in the surface of the apples.